D5777 Guide for Using the Seismic Refraction .pdf

Designation: D 5777 – 00Standard Guide for Using the Seismic Refraction Method for Subsurface Investigation1 This standard is issued under the fixed designation D 5777; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. 1. Scope proven; however, other approaches or modifications to the 1.1 Purpose and Application—This guide summarizes the seismic refraction method that are technically sound may be equipment, field procedures, and interpretation methods for the substituted. assessment of subsurface conditions using the seismic refrac- 1.2.5 Technical limitations and interferences of the seismic tion method. Seismic refraction measurements as described in refraction method are discussed in D 420, D 653, D 2845, this guide are applicable in mapping subsurface conditions for D 4428, D 5088, D 5730, D 5753, D 6235, and D 6429.. various uses including geologic, geotechnical, hydrologic, 1.3 Precautions: environmental (1), mineral exploration, petroleum exploration, 1.3.1 It is the responsibility of the user of this guide to and archaeological investigations. The seismic refraction follow any precautions within the equipment manufacturer’s method is used to map geologic conditions including depth to recommendations, establish appropriate health and safety prac- bedrock, or to water table, stratigraphy, lithology, structure, tices, and consider the safety and regulatory implications when and fractures or all of these. The calculated seismic wave explosives are used. velocity is related to mechanical material properties. Therefore, 1.3.2 If the method is applied at sites with hazardous characterization of the material (type of rock, degree of materials, operations, or equipment, it is the responsibility of weathering, and rippability) is made on the basis of seismic the user of this guide to establish appropriate safety and health velocity and other geologic information. practices and determine the applicability of any regulations 1.2 Limitations: prior to use. 1.2.1 This guide provides an overview of the seismic 1.4 This standard does not purport to address all of the refraction method using compressional (P) waves. It does not safety concerns, if any, associated with its use. It is the address the details of the seismic refraction theory, field responsibility of the user of this standard to establish appro- procedures, or interpretation of the data. Numerous references priate safety and health practices and determine the applica- are included for that purpose and are considered an essential bility of regulatory limitations prior to use. part of this guide. It is recommended that the user of the 1.5 This guide offers an organized collection of information seismic refraction method be familiar with the relevant mate- or a series of options and does not recommend a specific rial in this guide and the references cited in the text and with course of action. This document cannot replace education or appropriate ASTM standards cited in 2.1. experience and should be used in conjunction with professional 1.2.2 This guide is limited to the commonly used approach judgment. Not all aspects of this guide may be applicable in all to seismic refraction measurements made on land. The seismic circumstances. This guide is not intended to represent or refraction method can be adapted for a number of special uses, replace the standard of care by which the adequacy of a given on land, within a borehole and on water. However, a discussion professional service must be judged, nor should this document of these other adaptations of seismic refraction measurements be applied without consideration of a project’s many unique is not included in this guide. aspects. The word “Standard” in the title of this guide means 1.2.3 There are certain cases in which shear waves need to only that the document has been approved through the ASTM be measured to satisfy project requirements. The measurement consensus process. of seismic shear waves is a subset of seismic refraction. This 2. Referenced Documents guide is not intended to include this topic and focuses only on P wave measurements. 2.1 ASTM Standards: 1.2.4 The approaches suggested in this guide for the seismic D 420 Guide to Site Characterization for Engineering, De- refraction method are commonly used, widely accepted, and sign and Construction Purposes2 D 653 Terminology Relating to Soil, Rock, and Contained 1 Fluids2 This guide is under the jurisdiction of ASTM Committee D-18 on Soil and Rock and is the direct responsibility of Subcommittee D18.01 on Surface and D 2845 Test Method for Laboratory Determination of Pulse Subsurface Characterization. Current edition approved Feb. 10, 2000. Published May 2000. Originally published as D 5777 – 95. Last previous edition D 5777 – 95e1. 2 Annual Book of ASTM Standards, Vol 04.08. Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States. 1 1. thickness and the seismic velocity of subsurface soil Used at Low Level Radioactive Waste Sites3 and rock or engineered materials. When the angle of incidence equals the to characterize some of the properties of natural or man-made critical angle at the interface. 3).1 This guide summarizes the equipment.5 A number of elastic waves are produced by a seismic cal methods may be necessary to properly interpret subsurface energy source.1. Vol 04. trigger D 5753 Guide for Planning and Conducting Borehole Geo.1. it is the first wave to arrive at each geophone (see Fig. When the along with distance between the source and geophone(s).4 Also see Terminology D 653.2 Complementary Data—Geologic and water table data to the surface (Fig. Explosives are used for deeper refractors or 3. geophones. Fig. travel through the soil or rock from the source. the shear modulus and the density in the following 4 The boldface numbers given in parentheses refer to a list of references at the end of the text. geophone(s) are made from the land surface and are used to 5. This are defined in Refs (2) and (3).4 The seismic energy source generates elastic waves that interpret subsurface conditions and materials. Purposes with Emphasis on Soil. the Vadose Zone 5.6 The P-wave velocity Vp is dependent upon the bulk Annual Book of ASTM Standards. highest seismic velocity. 2 4.1. the waves are refracted according layers). 1 Field Layout of a Twelve-Channel Seismograph Showing the Path of Direct and Refracted Seismic Waves in a Two-Layer Soil/ Rock System (ac 5 Critical Angle) 2 . modulus. 5. This travel time. a mechanical device that strikes the ground. Summary of Guide geophone) is determined from the seismic wave form. The geophones are Waste Contaminated Sites3 usually located in a line. The seismic source may be a sledge hammer.1 The majority of the technical terms used in this guide convert the ground vibrations into an electrical signal. 2 and Fig. The calculated seismic velocities of the layers are used to Snell’s Law (4. or some other type 3. 1). 8). sometimes referred to as a geophone D 6429 Guide for Selecting Surface Geophysical Methods3 spread. and a physical Logging3 seismograph (see Fig. refractor.1.2 Measurement of subsurface conditions by the seismic and Ground Water3 refraction method requires a seismic energy source. and interpretation methods used for the determination of D 5608 Practice for Decontamination of Field Equipment the depth. 1). is seismic wave reaches the interface between two materials of interpreted to yield the depth to refractors refractors (refracting different seismic velocities. D 6235 Guide for Expedited Site Characterization of Va. transmitting energy back 4.1 Concepts: ing2 D 5088 Practice for Decontamination of Field Equipment 5. electrical signal is recorded and processed by the seismograph.1. Geophones 3.09. D 5777 Velocities and Ultrasonic Elastic Constants of Rock2 5. Significance and Use D 4428/D 4428M Test Methods for Crosshole Seismic Test- 5. using the seismic refraction D 5730 Guide to Site Characterization for Environmental method. geophone cable. the interface between two materials. Because the compressional P-wave has the conditions from seismic refraction data. geologic maps. Terminology of impulse source. cable (or radio link). 3 5. 3 time of a compressional (P) wave from a seismic source to a shows a seismograph record using twelve geophones. Rock.1 Definitions: special conditions that require greater energy.1. Fig. data from out.3 The geophone(s) and the seismic source must be dose Zone and Ground Water Contamination at Hazardous placed in firm contact with the soil or rock. The travel time of the seismic wave (from the source to the 4. field proce- Used at Nonradioactive Waste Sites3 dures. This interface is referred to as a obtained from borehole logs.1 Summary of the Method—Measurements of the travel shows a seismograph record using a single geophone. 5. manner (4): FIG. the refracted wave moves along man subsurface materials. crops or other complementary surface and borehole geophysi. 4 shows the source and geophone layout and the sufficient velocity contrast. K 5 bulk modulus. Intercept time and crossover distance-depth µ 5 shear modulus. (3) each layer is homoge- 5. 5 0. the velocity in each layer is geophone is recorded by the seismograph (Fig. Reference (9) provides an excellent summary of these layered earth. The travel calculated from the time-distance plot. FIG. derivations are straightforward inasmuch as the travel time of 5. Fig. The formulas for a 5. 2 A Typical Seismic Waveform from a Single Geophone FIG. (2) there is no land-surface relief. and formulas have been derived in the literature (6-8).8 The travel times are plotted against the distance neous and isotropic. 4). seismic energy source and a geophone(s) is a function of the 5. equations for two and three layer cases.1. thickness and lateral extent to be resulting idealized time distance plot for a horizontal two. (4) the seismic velocity of the layers between the source and the geophone to make a time distance increases with depth. D 5777 NOTE 1—Arrow marks arrival of first compressional wave.1. the depth to the refractor and the seismic velocities of the materials through which the wave passes. The unit of time is usually milliseconds (1 ms planes that are either horizontal or dipping at a constant angle. 4 (a) Seismic Raypaths and (b) Time-Distance Plot for a Two-Layer Earth With Parallel Boundaries (9) distance between them.9 The travel time of the seismic wave between the two-layered case (see Fig. 3). (see Fig. and (5) intermediate layers must be of plot.1 Intercept-time formula: 3 . FIG. and the raypath geom- time (the time it takes for the seismic P-wave to travel from the etry is known.7 The arrival of energy from the seismic source at each the seismic wave is measured. determining the Vp 5 =@~K 1 4/3µ!/r# (1) apparent seismic velocities (which are the reciprocals of the slopes of the plotted lines in the time distance plot). detected.10 The depth to a refractor is calculated using the source to geophone geometry (spacing and elevation). These interpretation formulas are based on the seismic energy source to the geophone(s)) is determined from following assumptions: (1) the boundaries between layers are each waveform. 4) are given below. These r 5 density.1. 3 Twelve-Channel Analog Seismograph Record Showing Good First Breaks Produced by an Explosive Sound Source (9) 5. and the where: intercept time or crossover distances on the time distance plot Vp 5 compressional wave velocity.1.001 s).10.1. FIG.11 Three to four layers are usually the most that can be resolved by seismic refraction measurements. a refraction measurement is indicate a horizontal layered earth.2 Crossover distance formula: xc z52 Œ V2 2 V 1 V2 1 V 1 (3) where: z. and V1 5 seismic velocity in layer one.1. V2 and V1 are as defined above and xc 5 crossover distance. Fig. depth to water table or bedrock. These are referred to as forward and measurements and the intercept time or crossover distance are reverse measurements. ti 5 intercept time. from which separate time distance plots are refractor. These two apparent velocity made in each direction.1. 5. 7 Time Distance Plot (a) and Interpreted Seismic Section (b) Three-Layer Model With Parallel Boundaries (9) (29) 4 . 6 shows the source and geophone layout and the obtained using this approach (see Fig. Both measurements are necessary to resolve the true seismic veloc- FIG. D 5777 ti V2V1 z52 (2) =~V2! 2 2 ~V1! 2 where: z 5 depth to refractor two. Fig. sometimes incorrectly called reciprocal used to calculate the true velocity.12 The refraction method is used to define the depth to Two-Layer Model With A Dipping Boundary (9) or profile of the top of one or more refractors. depth and dip of the measurements. V2 5 seismic velocity in layer two.10. The velocity obtained for the refractor from either of these two measure- ments alone is the apparent velocity of the refractor.1. Depth to the refractor resulting time distance plot for a dipping refractor. 5 (a) Seismic Raypaths and (b) Time-Distance Plot for a FIG. for example. 6 (a) Seismic Raypaths and (b) Time-Distance Plot for a 5. 5. 5. 5 shows the source and geophone layout and the resulting time distance plot for an idealized three-layer case.13 The source of energy is usually located at or near ity and the dip of layers (9) unless other data are available that each end of the geophone spread.1. 7). Note that only two depths of the planar refractor are made. or both. 3 Igneous rocks than sedimentary rocks.2.3. bilities of the seismograph will usually be sufficient.1 Denser rocks than lighter rocks.2.1. The geophone is data collection and interpretation approach. with sufficient energy. D 5777 is obtained under each geophone by using a more sophisticated is normally used with a single geophone. Metamorphic rock 10 000 to 23 000 3050 to 7000 Three to five hammer blows using signal enhancement capa- A Depending on temperature and salt content.4 Signal Enhancement—Signal enhancement using 5. Computer programs are available that help the 5. With a it is not a unique indicator. a 4 to 7 kg (10 to 15 lb) sledge hammer may be used.2 For shallow depths of investigation.7 Water-saturated unconsolidated sediments than dry while reducing the amplitude of the random noise by averag- unconsolidated sediments.3. First hydrologic and environmental applications are carried out to seismic wave arrival times (Fig. number of impacts.1 The selection of seismic refraction energy sources is Gravel or dry sand 1500 to 3000 460 to 915 Sand (saturated) 4000 to 6000 1220 to 1830 dependent upon the depth of investigation and geologic con- Clay (saturated) 3000 to 9000 915 to 2750 ditions. Geophone 5.6 Consolidated sediments than unconsolidated sedi. Four types of energy sources are commonly used in WaterA 4700 to 5500 1430 to 1665 seismic refraction surveys: sledge hammers.3.2.3 Equipment—Geophysical equipment used for surface 5. However.1 Single Channel Seismograph—A single channel velocity information in the shallow subsurface. This is useful in areas that the local conditions and other data.3. Single channel systems are also used to 5. A wide variety of seismic geophysical equipment The geophones are connected to a geophone cable that is is available and the choice of equipment for a seismic connected to the seismograph (see Fig. 3).2 If connections between geophones and cables are not waterproof. It is an aid when working in 5. While the seismic refraction 5.2.3. geophone cable.3. the hammer at increasing distances from the geophone.5 Unweathered rocks than weathered rocks.2. and corded simultaneously for all geophones (see Fig. This process increases the signal to noise ments.3. 5.2.2. 48 or more geophones. an energy source and a trigger cable work. tives of the survey.3.3. Signal stacking is 5.2.2 Geophone and Cable: 5. compressional P-waves in subsurface materials. For some 300 ft). must interpret the seismic are seismically noisy and in areas with complex geologic refraction data and arrive at a geologically feasible solution. etc. based on knowledge of reliable picks of first arrival times.2. seismograph is the simplest seismic refraction instrument and 5. adjusted at the shot end of a cable to provide additional seismic 5. the frequency of the geophones varies from 8 to 14 Hz. mographs use 6.1 A geophone transforms the P-wave energy into a seismic refraction measurement includes a seismograph.8 Wet soils than dry soils. 5 to 10 m (15 to Granite 15 000 to 19 000 4575 to 5800 30 ft).2.2. Although the 5.1.2. and Shale 9000 to 14 000 2750 to 4270 explosives.2. The geophone intervals may be and display the seismic wave digitally. mechanical Sea waterA 4800 to 5000 1460 to 1525 Sandstone 6000 to 13 000 1830 to 3960 weight drop or impact devices.3 Energy Sources: Weathered surface material 800 to 2000 240 to 610 5.3.1. unit is satisfactory.2. record depth of the refractor(s). 24. simple geologic conditions and small projects a single-channel ments can be made to depths of 300 m (1000 ft) and more (6).2.2 Older rocks than younger rocks.2 Parameter Measured and Representative Values: measure the seismic velocity of rock samples or engineered 5.3. geo- voltage that is recorded by the seismograph. Most engineering seismographs sample.2. Natural Soil and Rock ft/s m/s 5.2. A strike 5 . the seismic wave forms are re- sediment or rock has a wide range of seismic velocities. single-channel units to very sophisticated mul- detail needed to describe the surface of the refractor and the tichannel units.4 Solid rocks than rocks with cracks or fractures. it is the interpreter who. 12. They range from rela- or 3 ft to hundreds of feet) apart depending upon the level of tively simple.2 Multi-Channel Seismograph—Multi-channel seis- P-wave velocity is a good indicator of the type of soil or rock.2. ratio by summing the amplitude of the coherent seismic signals 5. rain. noisy areas or with small energy sources. The geophone cable refraction survey should be made in order to meet the objec- has electrical connection points (take outs) for each geophone. engineering. many of these ranges overlap. refraction measure. For refraction phones. conditions. Table 1 shows that each type of multi-channel seismograph. or radio link. and ing. Chalk 6000 to 13 000 1830 to 3960 Limestone 7000 to 20 000 2134 to 6100 5. Special waterproof geo- and Rock (4) phones (marsh geophones).1.3. 2 and Fig. accomplished by adding the refracted seismic signals for a 5.2.3 The simultaneous display of waveforms enables the technique measures the seismic velocity of seismic waves in operator to observe trends in the data and helps in making earth materials. 5.3. available in most seismographs.1.2 P-wave velocities are generally greater for: interpreter pick the first arrival time.1 Seismographs—A wide variety of seismographs are placements are spaced from about 1 m to hundreds of meters (2 available from different manufacturers. projectile (gun) sources. 5. geophone cables and connectors Materials Velocity are required for areas covered with shallow water. usually placed at a fixed location and the ground is struck with 5.14 Most refraction surveys for geologic.2.1 The seismic refraction method provides the velocity of materials. 3) are identified on determine depths of refractors that are less than 100 m (about the instrument display of the seismic waveform. 5. 1). care must be taken to assure they will not be TABLE 1 Range of Velocities For Compressional Waves in Soil shorted out by wet grass. usually located at uniform intervals along the cable. filtering and stacking that improve the signal to noise ratio is 5. 3 A layer must also have a sufficient thickness in order more seismic energy is required. Some significant geologic or hydrogeologic 6. and noise should be considered and its time of occurrence and some additional information.5 kg) at 1 to 2 m (3 5.3.5 Alternative Methods—The limitations discussed above information.2. 5. larger explosives 5. as with most geophysical techniques.3. Cultural 5. investigate subsurface conditions (see Guide D 5753). The error is a function of dip angle and the understanding of the theory.3. there are situations in which seismic 5.3 For deeper investigations in dry and loose materials. In most situations. Mechanical seismic the layer above it (a velocity reversal).2.4. location noted.5 kg) are required and usually any vibration of the ground due to wind. vibration due to movement of the field crew.3. piezoelectric devices vertical variations in seismic velocity of subsurface layers (for or a geophone (or accelerometer). 5. Use of explosives requires example.4.1 General Limitations Inherent to Geophysical Meth. and cost-effective method of obtaining subsurface infor. be detected. investigation (below 300 ft or 100 m). One of the most important interpretation also deviates from the ideal.4. There survey. is layers deviate significantly from this idealized model. and cultural cal measurements alone cannot resolve all ambiguities. waves breaking on a nearby beach).1. carrying out the survey.3. the seismic velocity of the layers are assumed to 6.1 The seismic refraction method is sensitive to ground to 6 ft) is effective for shallow depths of investigation (less than vibrations (time-variable noise) from a variety of sources. Explosive charges are usually buried 5.2 All surface geophysical methods are inherently lim. accu.4.4 Limitations and Interferences: and construction equipment. projectile (gun) source may be selected. blasting unit. and methods for velocity contrast between dipping layers (10.2 Limitations Specific to the Seismic Refraction Method: cation.4 If a layer has a seismic velocity lower than that of discharged at or below the ground surface.4. technique.2. increase in energy levels. An increasing error factors is the competence of the person(s) responsible for is introduced in the depth calculations as the angle of dip of the planning. and interpretation of seismic refraction data.3.2 Ambient Sources—Ambient sources of noise include charges (greater than 1 lb or 0.5 During the course of designing and carrying out a of subsurface conditions.4 Timing—A timing signal at the time of impact (t 5 0) 5. 12). Procedure ited by decreasing resolution with depth.3.4. 300 ft or 100 m) if backfilled and tamped.4.4. water movement (for are buried 2 m (6 ft) deep or more. If actual conditions in the subsurface refraction survey.4. nearby vehicles. Special seismic blasting caps should be used for 5. Personnel not having specialized training and experi- must be a sufficient contrast between the seismic velocity of the ence. field procedures. may prevent the use of the seismic refraction method. because it depends upon the magnitude of the noises and the a seismic refraction survey is not a complete assessment of geometry and spacing of the geophones and source. such as borehole data. For greater depths of Geologic and cultural factors also produce unwanted noise.3. D 5777 plate on the ground is used to improve the coupling of energy across them and consequently cannot be detected with this from the hammer to the soil. sources of ambient. interpretation of seismic refraction data and an understanding 5.3 Interferences Caused by Natural and by Cultural to reduce energy losses and for safety reasons. 11). velocity with depth. then any dependent upon many factors. activity.3. factors such as buried structures under or near the survey line ods: also may lead to unsuspected variations in travel time. Burial of small Conditions: amounts of explosives (less than 1 lb or 0.3 Geologic Sources—Geologic sources of noise in- is sent to the seismograph (see Fig. An layer increases.4. geologic. geophysical or non-geophysical methods may be required to mation. 5. or blasting. 5. 9. 5.4 A small amount of explosives provides a substantial velocity reversals occur). or by rainfall on the geophones. seismic refraction surveying is an effective. is that a given set of data cannot be associated with a unique set 5. Projectile sources are 5. aircraft.4. Nearby 5.1 A fundamental limitation of all geophysical methods powerlines may induce noise in long geophone cables. natural seismic specially-trained personnel and special procedures. and a mechanized or a to be detected (12).4. and other rate.4. deeper layers are greater than the actual depths (although the cal weight drops are usually trailer mounted because of their most common geologic condition is that of increasing seismic size. is required. and interpreting the data. Properly integrated with other geologic 5.4 Cultural Sources—Cultural sources of noise include accurate timing. should be cautious about using this technique and solicit overlying material and that of the refractor for the refractor to assistance from qualified practitioners. Interpretation methods are available to address this problem in some instances (13). As a result.1. subsurface conditions. 6.1 Qualification of Personnel—The success of a seismic be uniform and isotropic. Mechani.4. planning and implementing the seismic refraction 5.1. The time of impact (t 5 clude unsuspected variations in travel time due to lateral and 0) is detected with mechanical switches. 1).3. or with a signal from a example. layered earth. 6.2 Planning the Survey—Successful use of the surface boundaries have no field-measurable seismic velocity contrast seismic refraction method depends to a great extent on careful 6 . the computed depths of kg) that is dropped or driven downward under power.1 This section includes a discussion of personnel qualifi- 5. The interference is not always predictable Because of this inherent limitation in the geophysical methods.1 When refraction measurements are made over a survey. surface geophysi. the presence of large boulders within a soil).2. refraction survey. the low seismic velocity sources use a large weight (of about 100 to 500 lb or 45 to 225 layer cannot be detected.2 Another limitation inherent to seismic refraction of the site geology is necessary to complete a seismic refraction surveys is referred to as a blind-zone problem (4. whereby the depth to the refractor is 7 .1 The desired level of detail and geologic complexity topographic maps or aerial photos. and satisfactorily approximated by a plane (horizontal or dipping).4. the spacing method uses simple field and interpretation procedures. topography.3. (over the length of the geophone spread) can be neglected.1 Objective(s) of the Seismic Refraction Survey: method at the site is assessed. the surface of the refractor can be ences containing the seismic velocities of earth materials. the cost of obtaining seismic exists in the overburden and in the refractor within a single refraction data is relatively low. depth used interpretation methods are classified into two general and type of rock. information. and 6. 8. istics of the site.1 Planning and design of a seismic refraction survey 6. These factors determine the survey design.4 Methods Used To Define Planar Refractors: 6.1. 6. a pre-survey test is desirable at a control point. It is also common to resulting subsurface data is more detailed. 6.5 Methods Used To Define Nonplanar Refractors—A that provide an indication of subsurface velocity layering.2 Numerous approaches are used to quantitatively ditions at the site should be developed early in the design phase interpret seismic refraction data. In reconnaissance surveys. knowledge of the geology. the Given the thickness and the seismic velocity of the subsurface equipment used. depth to water table and a stratigraphic groups: methods that are used to define planar refractors and section with the horizons to be mapped with the seismic methods that are used to define nonplanar refractors.4 Forward modeling using mathematical equations (7. 13-15). Shots off 6. and number of methods can be viewed as an extension of the a test refraction line near a known point of control. ing a seismic refraction survey and to determine the geometry Important considerations include site geology. 9) can be used to develop theoretical time distance plots.2. 6. They can be described as the rigorous application nary engineering studies. D 5777 and detailed planning.3 When there is doubt that sufficient seismic velocity thin intermediate seismic velocity layers and seismic velocity contrast exists.3 The objective of the survey may be a reconnaissance 6. or both) as possible about will determine the interpretation method to be used for a the site prior to designing a survey and mobilization to the refraction survey. end of the geophone spread). such as a borehole or well.2.2 Information from previous seismic refraction sur.2.1 The intercept time method (ITM) and crossover of subsurface conditions or it may provide the most detailed distance method are the simplest and probably the best known subsurface information possible.2. The intercept time data are not very detailed.2.2. or geophone spacing. single geophone spread. 11). inversions can be neglected. is an adequate seismic velocity contrast between the two 6. 6).4. of the field-survey. The used. but the resulting subsurface geophone spread (between the shot points). Under these conditions.2. Mea- between the geophone spreads. Sufficient information about layer thickness tigation. published reports of seismic refraction studies performed under lateral variations in seismic velocity of the subsurface layers similar conditions should be used. The results obtained using this Under these conditions.2. and operational constraints (for example.1 One of the most critical elements in planning a additional data. the feasibility of using the seismic refraction 6. From this intercept time method.2 A geologic/hydrologic model of the subsurface con.2. and topographic maps or hand-level elevations are intercept time method requires that a constant seismic velocity sufficient. the spacing between the geophone of Snell’s law to a subsurface model consisting of homoge- spreads.2. which in turn will determine the field field. be used under the following conditions: where a limited 6. initial field structures). data from any previous seismic refraction work. depth of inves. however.3 Additional discussion of survey design and field the seismic velocities can be determined. restric. It is seismic velocity contrast exists between the subsurface layers good practice to obtain as much relevant information (for of interest. 6. place one shot in the middle of the geophone spread. number of refractor depth determinations are required within a veys in the area. published refer.2. must also be considered.2. or geophone spacing.1. man-made a site before field work is carried out. the most commonly and should include the thickness and type of soil cover. Guide D 5753) 6. The presence of noise. on-site utilities.2.2. a few shot-points are neous layers and horizontal or dipping planar interfaces. procedures to be followed (4. these plots are used to assess the feasibility of conduct- selected. surements are usually made from each end of the seismic multiple shot-points are used. Three types of tests considerations for the intercept-time method are given by Refs may be considered: a vertical seismic profile (VSP) (8) (4 and 9). of all the methods for the interpretation of seismic refraction such as regional geologic or ground water studies and prelimi. and budget necessary to achieve the desired results. In a detailed survey. the interpretation method layers. and elevations and locations of refraction line (a minimum of one off-end shot-point on each geophones and shot-points are more accurately determined. In this case. 9.2 Assess Seismic Velocity Contrast: of each end of the spread may also be made to provide 6. is small. geologic and geophysical logs in the study area. but can still be cost-effective because the the refractor at two points (see Fig.3.2. the cost of obtaining seismic refraction method include the thickness of the overburden and the dip of data is higher. and seismic velocities may not be available to accurately model generating activities (for example.2 The intercept time or crossover distance method can geologic or hydrologic units of interest. measurements should be taken to assess whether an adequate tions on the use of explosives). 6.2.3 Selection of the Approach: boring. should consider the objectives of the survey and the character. the level of effort.2. is large. example.2. data (8. where the stratigraphy is known and 6. 6. borehole log (such as a density log or sonic log.4.2. Additional shot-points increase the number of seismic refraction survey is the determination of whether there points along the refractor where depth can be determined.1.2. and access.2. refraction method. 8. 6.1 A group of methods (referred to as the common refractor as a plane with a limited number of points. strate the application of the generalized reciprocal method to The Generalized Reciprocal Method (GRM) shallow targets of geotechnical significance. 6. geophones and the number of shot-points. and the Hagiwaras Method methods and can provide the most detailed profile of a These CRM methods require additional field and interpretation effort and are refractor.5 The choice of interpretation method may vary from sion of survey design and field considerations for these site to site and depends upon the detail required from the methods are given in Refs (4) and (10). then the intercept time or crossover distance meth- tors. inversions are neglected.2.8. The resolution of the surface of the refractor obtained 6. therefore. spread.3 Additional discussions of survey design and field 8 . Additional discus.2.2. time and space) to achieve the necessary resolution. as de- scribed by Palmer (12. The generalized reciprocal method survey Methods Used for Defining Non-Planar Refractors The Common Reciprocal Method (CRM) Including Plus-Minus Method. These methods seismic velocity changes within a geophone spread can be can provide a more detailed interpretation of nonplanar refrac.8. has still to achieve routine • Lateral variations in seismic velocity within a single acceptance in engineering geophysics because it requires a geophone spread are neglected. considerations for this method are given by Palmer (17).2. • Thin intermediate velocity layers and velocity Geophone spacing for this method is smaller to provide inversions.2. The CRM has many variations including the plus-minus shape of a non-planar refractor at each geophone location. neglected. 7 shows an can be used. The full use of the generalized They can be applied where: • Depth computations are provided at geophones. 6. This method geophone spread are neglected. Fig.2. Lankston and Lankston (20). and • Thin intermediate velocity layers and velocity greater field effort.7.8. • Lateral variations in seismic velocity within a single a relatively small geophone spacing is required. directions from three to seven shot-points per single geophone then the generalized reciprocal method can be used. 6.2.2. It is modified from Palmer (11). can aid Include the Time Intercept and Crossover Distance Methods in resolving complex conditions including undetected layers. refractor has some relief. All these methods usually require velocity changes in subsurface layers and account for interme- that travel times be measured in both forward and reverse diate seismic velocity layers and seismic velocity inversions.6 Common Reciprocal Methods: 6.6. thereby ac. These methods require the least field and interpretation effort and are. The GRM requires the greatest level of field and interpretation effort and is the sufficient spatial data.7.2 The generalized reciprocal method can sometimes In addition to all the features of the CRM methods.2. D 5777 calculated at the shot-points and at each geophone location. but requires considerably more effort in field data intermediate in cost.8. 16).2 These methods can be applied where depths to the the site. the lowest cost.7 Generalized Reciprocal Method: Methods Used For Defining Planar Refractors 6. model data and case histories. and thin intermediate seismic velocity layers and seismic velocity inversions can be neglected. 20). reciprocal method. sition. most costly. method (11). which has been demonstrated by Palmer for • The refractor has some relief. and Lankston (14. thin intermediate seismic velocity geophone spread. seismic refraction survey and the complexity of the geology at 6.8. The Delay Time Method and Hales Method are special cases of the GRM 6. processing. and interpretation. and seismic velocity inversions cannot be neglected. The GRM method requires a large data set (in • The refractor is approximated by a plane (horizontal or dipping). the Generalized Reciprocal be used where lateral variations in seismic velocity within a Method (GRM) may account for: • Lateral variation in seismic velocity within a single single geophone spread. The case histories in Palmer (19) demon. counting for irregular refracting surfaces (nonplanar refrac. These methods require a greater level of effort in data acqui.2. Depths are obtained under each geophone. 17-19) and Lankston (14. and lateral reciprocal methods (CRM) by Palmer (11)). and usually requires that travel times be measured in both forward • Thin intermediate velocity layers and velocity inversions are neglected.6 When selecting the approach for data acquisition the of the subsurface layers (over the length of the spread) can be specific processing and interpretation method that is used must neglected. lateral changes in seismic velocity and anisotropy. and reverse directions from five to seven shot-points per geophone spread. of the methods are based on the assumption that within geophone spread can be neglected. collection and interpretation. The interpretation method in turn determines the refractor are required at each geophone. 6. the ABC Method and Hagiwaras Method. then one of the many a single geophone spread. interpreted seismic refraction section of an irregular rock 6. Most.1 The generalized reciprocal method (GRM). The GRM They can be applied where: includes as special cases the delay time method and Hales • Depth computations are provided near shot-points.3 If there is a need to account for lateral seismic surface using this approach. and method.2. ods may be sufficient. TABLE 2 Features and Limitations of Methods (Modified from Ref (11)) 6. therefore.2. seismic velocity in the overlying common reciprocal methods that define nonplanar refractors units and in the refractor do not vary laterally.2.4 Table 2 summarizes the features and limitations of by the survey is dependent on the spacing between the each of these methods. the surface of the approach and level of effort required in the field.8 Summary of Two Approaches: 6. layers.1 If it is acceptable to describe the surface of a 6.2 If there is a need to define the depth and approximate tors).8. the ABC incorporates the strengths of most other seismic refraction Method.7. lateral variations in seismic velocity 6. but the lateral seismic velocity in subsurface layers within a not all.2. 3. buried channels.3.3.2.2 Layout the Survey Lines—Locate the best position for detailed mapping of the top of the refractor. Examples of geophone spacing and shot distance them to the cable. the refraction survey line made upon arrival to determine if the initial survey plan is should cross over the channel so that its boundaries can be feasible.3. precluded by this guide.2.3 Lay out the geophone cable. Geophone spacing is determined by ducted.7 There are many field and interpretation methods that spread and the source to geophone offset are not sufficient to fall under the broad categories listed above.3 Coverage—Survey coverage and orientation of sur. etc. data can be obtained by interpretation methods not specifically mentioned are not using multiple geophone spreads.3.9. alized reciprocal method. such as. geophones.4 and the on-site visit in 6. Greater overlaps may be necessary priate geometric corrections are applied to the data. The environment written under the guidance of the Society of area of survey should be larger than the area of interest so that Exploration Geophysicists (SEG)—Engineering and Ground measurements are taken in both “background” conditions and Water Geophysics Committee given by Pullan (20). then the source to been made to list all of the individual field and interpretation geophone offset distance must be increased until a sufficient methods. Since the sources of cultural noise that will prevent measurements from common reciprocal method and generalized reciprocal method being made.1. The geo- the accessibility of the area. or introduce noise into the data (see section 5. common problem resulting in poor detection of the seismic geophone distance of up to three to five times the required P-wave. Each one has strengths and weaknesses and must be depth is obtained. the geophone spreads must be overlapped if continuous coverage of the refractor is desired. faults. To 6. For recon- 6. are used to obtain depth to a refractor under individual and adequate space for the refraction line. reconnaissance measurements that do not require extensive 6.2 If a feasibility test has not been previously con- seismic refraction data. The number and locations of shot-points will depend upon the method chosen to collect and interpret the 6. buried structures and utilities and other continuity required to map the desired refractor. the refractor and lateral resolution needed to define the top of 6. widely spaced the refraction lines based on the survey design described in geophones may be used. The time-distance plots for the seismic location of the line will be determined by topography. Modifications to the survey plan may be required. refractor.4 Place geophones firmly in the ground and connect a refractor.3. determined. Phantoming is discussed by Lank- interpretation method should be selected that accounts for ston and Lankston (13).2 Locate the position of the first geophone. depth of investigation. or fractures.3. the proximity of wells or test holes phone spacing and the amount of overlap of the geophones for control data. The geophone to shot-point separation will be larger Results of these initial measurements may require that changes for deeper refractors and smaller for shallow refractors.1 Location of Survey Lines—Preliminary location of naissance surveys. continuous profile of the refractor being mapped.3). adequate space for the ods have specific requirements for data acquisition. the gap and aerial photos if an on-site visit is not possible.9. Geophone spacing can be varied from less than 1 m 6.1 refractor. If the length of the geophone 6.3.3. Line refraction measurements can be constructed by combining and locations should be selected so that the ground surface along plotting together the data from each geophone spread by a each geophone spread (cable) is as flat as possible or an process called phantoming. If the length of the line to be surveyed is selected to meet the project needs.1 Check for adequate space to lay out as straight a line must be smaller than the size of the spatial changes in the as possible. The use of other field and longer than a single geophone spread. refractor.3.9 Survey Design: phone spreads may be reconnaissance or detailed.2. over any anomalous conditions.8. the geophone spacing 6. Therefore.3 Implementation of Survey: to the orientation of lines with respect to geologic features of 6. for Seismic data files used in the personal computer (PC) vey lines should be designed to meet survey objectives. the results of initial measurements can be used to two factors: the expected depth of the refractor(s) and desired confirm the existence of an adequate seismic velocity contrast degree of definition (lateral resolution) of the surface of the and can also be used to assess signal to noise ratio at the site.7.1 On Site Check of Survey Plan: interest. will decrease until the geophone spreads overlap and provide a ation should be given to: the need for data at a given location.9. refraction line is a consideration. Consider.10 Data Acquisition Format—A recommended standard 6.3 Conducting the Survey: define the surface of a refractor in detail.2.9. 6. a gap may be left between the ends of survey lines is usually done with the aid of topographic maps successive spreads.3.1. As more detailed data is required. A refraction survey line may require a source-to.3.2 The geophone stations should lie along as straight a will commonly range from one to two geophones for common line as possible. For detailed mapping of the top of a 6. The geophone must be vertical and in contact needed to define various geologic conditions are given by with the soil or rock.1 A systematic visual inspection of the site should be When mapping a buried channel. Improper placement of geophones is a Haeni (9).2. 6. 6. For be made to the original survey plan.2. Often the for deeper refractors.2. more closely-spaced geophones are required.3.3.4 Refraction surveys along a line with multiple geo- 6. Consideration should be given 6. topography. (3 ft) to more than 100 m (300 ft) depending upon the depth to 6. D 5777 be considered since most processing and interpretation meth. the extent and location of any asphalt or from each cable spread will depend upon the detail and concrete surface. No attempt has reach the maximum depth of investigation.2. Deviations from a straight path may result in reciprocal method and from two to five geophones for gener- inaccuracies unless the line is carefully surveyed and appro. Each geophone spike should be pushed firmly into the 9 . The overlap 6. 1.8 During or after data acquisition. the process can be labor intensive data. cases.1 In some limited cases. possible to check their quality.3. If 6.4 Quality Control (QC)—Quality control can be applied provided. 6.10 In addition to the time-distance curves. Examples of qualitative and semi- limit the method of interpretation. If there curves may have an error in data entry or plotting. 6. plotting.3.3. A to seismic refraction measurements in the field. noise in the data will make picking the first arrival times 10 . quantitative interpretation may include the lateral location of a 6.3 A problem inherent in all geophysical studies is the 6.3.1.3.1 Method of Interpretation: tation method that are planned to be used in the study. this the refractor of interest.6 Set up the source at the first shot-point or a test highly variable.4.4.3. In many 6. then the seismograph instruction manual). The 6.4.5 Calibration and Standardization—In general. the sure that an adequate signal-to-noise ratio exists so that the first manufacturer’s recommendation should be followed for cali- arrivals can be determined.4. If the slopes between source and geophone measured with a tape measure of the two curves are sufficiently different.1.11 The irregularity test checks for travel time consis- the ground and the geophones should be assured.3.8 Test for noise level and set gains and filters (see differences between reciprocal times are excessive. one of the sets of data may be in error or the time distance to 20-cm (about 0.7 Test the seismic source and trigger cable. etc.3.4. Quality-control check should also be made after each equipment problem and procedures require that standard procedures be followed and repair.3. however.14 Finally.3.1 Documentation of the field procedures and interpre. time differences between forward and reverse profile curves. circuits and open circuits if possible (see seismograph instruc. the data may not be required and a simple qualitative interpre- and the field procedures as well as site conditions used may tation may be sufficient.3. then time picks for will be sufficient. D 5777 ground to make the contact between the soil and the geophone sary to properly resolve dipping layers.) that in turn will determine the method of interpretation. a good coupling between 6.13 The parallelism test is used to check the relative tives of the project. refraction method make geologic sense.10 Proceed with the refraction measurements.4 Conditions that could reduce the quality of the data depend upon the objectives of the survey and the detail desired (weather conditions.3 Changes to the planned field procedures should be interpretation will be necessary. to rock calculations.9 Both forward and reverse measurements are neces. the reciprocal time surface the geophone spike may be replaced by a tripod base on test. able for interpretation. time-distance plots determine the time interval from the impact of the seismic should be made to assure that the data are of adequate quality source to the first arrival of energy at each geophone.2 A field log in which field operational procedures buried channel without concern for its depth or minimum depth used for the project are recorded.4. sources of natural and cultural noise. mended to provide QC of field operations and data acquisition: 6. shot-point and 6. a single set of field data. point. making 6.3. 6.4. are considerable changes in surface elevation. The following items are recom.3. documented.5 Test the geophones and geophone cable for short the straight line slope.9 The required degree of accuracy of the position and have an error in data entry or plotting. A should be documented. or geologic conditions may be 6. tency along the refraction profile.4. In most cases.3. If the ground is relatively flat or the parallelism between selected forward or reverse time distance accuracy of the refraction survey is not critical. If there are deviations from 6.4. quantitative interpretation of method of interpretation will often dictate the field procedures.3.3.4.4. as tight as possible. 2 and Fig.3.3.4.3. Measurements (made by tape) to within 15. time-distance curves may have an error in data entry or tion manual). If no such recommendations are 6.5 If data are being recorded (by a computer or digital.4. a check should be made to determine if the geophone elevations and their horizontal locations must be depths and seismic velocities obtained using the seismic surveyed and referenced to the project datum. the geophone. procedure is straightforward (see Fig. and the parallelism test.4.2 The level of effort involved in the interpretation will 6.4.7 Ensure that a uniform method of picking first arrival through the use of geologic data and an experienced interpreter. 6. 6. number of manual methods and computer programs are avail- 6.4.5 ft) are adequate for most purposes. it is recommended that the data be reviewed as soon as for the more sophisticated methods.4 The first step in the interpretation process is to 6.3.12 The reciprocal time test is used to check reciprocal 6.3. When the and quantity to support the method of interpretation and define first arrivals are sharp and there is no ambient noise.6 Care should be taken to maintain accurate timing of non-unique correlation between possible geologic models and the seismograph.3. While the solutions for these methods acquisition system) with no visible means of observing the can be carried out manually. In both soil and rock.4 Interpretation of Seismic Refraction Data: 6. 6. out before each project and before starting field work each day. bration and standardization.4. elevation of shot-points and geophones varies with the objec. a quantitative 6.3.1.3. Where rock is exposed at the seismic refraction data: the irregularity test.3. three addi- very loose and should be scraped off so that the geophone can tional tools can be used as a means of quality control for be implanted into firm soil. the time picks may be in error. Often the top few inches (10 cm) of soil is 6.4. This ambiguity can be resolved only 6.4. data may be noisy. the distance curves and another curve from the same refractor. time is employed. 3). time picks may be in error or the time distance curves may 6.3.3.3. 6. An operational check of equipment should be carried documentation be made. a periodic check of equipment should be made. 1.1.1. 6.4.1.1. Redpath (4).6.6.4.1. copies of typical puter programs are available for solving seismic refraction raw records.4.1. survey and data interpretation.14 If conditions occurred where a variance from this 6. If such data process.1. complete and thorough interpretation. In some cases.2 For manual interpretation techniques. or soft- 6.2 The final seismic refraction interpretation is used to picks.3. a time-distance 7. Such a by the individual(s) doing the processing and interpretation.3 Programs for Interpreting Planar Refractors: their use. Analysis in the field is 7. there is no need for an extensive formal report: a time-distance plot. In be constructed. These time-distance plots are the founda.1 The purpose and scope of the seismic refraction distance plots and their relationships to geologic models are survey. Both are used to adjust field-derived travel times to some hydrogeologic conditions and any anomalous conditions at a selected datum. problems using the intercept time (or the crossover distance 7.4. 7.1. note- cially available that are based on intercept time method. it is generally good practice to have the entire seismic usually carried out on a computer. including the report. map. Care should be taken reciprocal method. et al (25) and are discussed in Scott. and com.2 Preliminary Interpretation—Preliminary interpreta.1. due Report—To provide quality assurance of the seismic refraction to the volume of data required for the method.4.3 Limitations of the seismic refraction survey.1.1.7 The shot-point/geophone layout. reviewed by a person 11 .1.1 A wide variety of formulas. tion of field data should be labeled as draft or preliminary.2 Quality Assurance of the Seismic Refraction Work and reciprocal method are described by Palmer (16).4.4. including a description of the 6. and 7.5 Verification of Seismic Refraction Interpretation— first point of movement or the point of maximum curvature. 6. seismic refraction measurements should be familiar with time. book.6 The location of the seismic refraction line(s) on a treated with caution because it is easy to make errors in an site map. to ensure that each trace is picked at the same point either at the 6.4. interpretation is work. there may be little need for a formal experience of the interpreter.11 The method of interpretation used (intercept time method).1 Manual interpretation techniques are given by Pa. 7. common reciprocal method or generalized reciprocal method). If this is done.4 Assumptions made.1 Components of the Report—The following is a list of plot of arrival times versus shotpoint-to-geophone distance can the key items that should be contained within most reports.16 Identify the person(s) responsible for the refraction available that are based on the common reciprocal method. as the data will be consistent from one trace to the is not available. were used. D 5777 difficult. equipment and the data acquisition parameters used.1.8 The approach used to pick first arrivals. a first arrival pick from one or more 6.1. (11) and Haeni (9). Anyone undertaking 7.1. (21). Lines are then fitted to these points to complete some cases. recognize the lack of a unique interpretation of these plots.4.1. given. To minimize errors. 7.4. these picks must be checked (and re-adjusted as needed) refine or confirm a geologic or hydrologic site model.1. distance plot can be associated with subsurface refractors. Seismic refraction interpretation can be verified by comparison This procedure will make the interpretation a more uniform with drilling data or other subsurface information.15 Provide appropriate references for any supporting Haeni. and Dobrin and Savit (7).4. shown by Zohdy (6) and Crice (22). 7.1.4.2 Manual-interpretation techniques for the generalized 7. Computer programs are refraction work. Computer-assisted interpretation techniques are presented by 7. These corrections can be applied manually (7) or by computer the data and related survey grid maps must be labeled.1 The report should include a discussion of: tion of seismic refraction interpretation. ASTM guide is necessary. Hand-held programmable calculator pro- 7. The two main types of This model is usually represented as a cross-section. distance plots over a variety of geologic conditions and 7.4. method.1.6 With the corrected travel-time data.1.1. If a computer program is used to make first arrival 6.3 If the original data are to be provided to the client. the reason for the variance should be khiser and Black (24).10 The results of field measurements. However.1.1.1. these picks should presentation of data or interpreted results. be noted. A number of computer programs are commercially 7.4. a consistent approach to the commercially available that are based on the generalized picking of the arrival times must be used. and time-distance plots. 6. site.1.1. and Haeni (9).5 Corrections to travel time for elevation or other all the essential features of the physical system under study.9 Corrections applied to field data. digital format.3. this fact should be mentioned within the report.6. A number of computer programs are commer- 7. 7. Examples of time. other). 26) and data used in the interpretation. nomograms.4 Programs for Interpreting Non-Planar Refractors: 7. a contour corrections are elevation corrections and weathering correc. so that straight-line segments on the time. then. 6.6 Presentation of Data: geophones may be uncertain. 7. geometric factors are then made.1.1. one must rely upon the 6.1.1.2 The geologic setting.1.1. or other drawings that illustrate the general geologic and tions. hardcopy analog recorder. SEG. 7.13 The format of recording data (for example.1 In some cases. equations (23).12 The interpreted results and any qualifications and grams are available for solving the various seismic refraction alternate interpretations. and specifically what analytical method(s).1. initial field interpretation and a preliminary analysis is never a 7.5 The field approach. Report 6. done mostly as a means of QC.4. and justification for 6. see Palmer ware program(s).1. et al (21. next. model is a simplified characterization of a site that incorporates 6.4. A 1 ms error could translate involved in picking arrival times.2. it is not always possible 8. accurately mapped without smoothing or errors in depth 8. soil conditions.1.4.4 Resolution: 8. It is important that the user of seismic refraction how small a change in depth can be determined by the results be aware of these concepts and understand that the refraction method. of rock is highly variable (for example. along a survey line. such as dip as well as the degree of weathering depends on many factors.1 Bias—Bias is defined as a measure of the closeness to 8. point.2 Published references (5. In order to measure refractors and the seismic velocity contrasts and near surface depth to a refractor. 6.3 to 3 m) depending upon of the noise impacting the measurements.2. 28).4 Positioning Differences—The drilling location and the used in the field and interpretation procedure. corrections to data.2. the depth obtained by refraction shot-point spacing.1.2 The Fundamental Differences Between Refraction and determination.2. same location with the same equipment match one another. to have exact agreement between refraction and boring data line location.4. the interpreted results may disagree with a depth obtained from drilling for the reasons discussed in 8.1. recording.1. to an error in interpretation. Keywords number of blow counts with a split-spoon sampler. surface geo- evidence of rock fragments. is repeated under identical conditions.1 Vertical resolution can be thought of in three ways: through 8.3 Lateral Geologic Variability—Agreement between re- the truth. While a refraction measurement may be quite 8. the degree to true depth. Drilling Measurements: 8. account for up to 10 m (30 ft) of difference in depth where top preter.5 Site-specific geologic limitations. of a water table near the bedrock surface can in some cases lead 8. and how much relief or dip can be drilling data.1. joints.1 geophysics. 27. such as a soil-to-rock interface. In many cases. and the level and variations to a depth error of 1 to 10 ft (0.6 Ability and experience of the field crew and inter.1.2. 8.2.2. refraction measurement may not be made at exactly the same 8.1. there will be higher lateral resolution. Precision of a seismic refraction measurement will be affected 8. 8. processing and provide an average depth over them. for example.1 The bias of a seismic refraction survey is commonly 8. 8. topography.1. boring data.2. the 9. the repeatability of the trigger signal millisecond.2. Larger errors are usually due to difficult field which the travel times from two identical measurements in the situations or improper interpretation due to blind zone prob. a signifi. fraction and boring measurements may vary considerably 8.1 Lateral Resolution—Lateral resolution of a seismic thought of as how well the refraction results agree with refraction survey is determined by geophone spacing and borehole data. tables. such as dip. lems.4. and noise. that is. D 5777 knowledgeable with the seismic refraction method and the site with each other or the top of the rock surface measured by the geology but not directly involved with the project.2 Differences Between Depths Determined Using Seismic be expected to have a high level of precision. relating to geophone spacing. In other cases. indicate that the 8.4 Variation of the earth from simplifying assumptions 8.1 The seismic refraction method is based upon a complex function of the geophone spacing. Refraction measurements may not 8. Precision and Bias differences in depth even when the top of rock is relatively flat. None of these necessarily agree physics 12 .2.2 8.1. record-keeping.3 Arrival times must be picked with an accuracy of a by the sources used. the depth to the measure of travel time of the P-wave. It is common to find that the boreholes are located on the fractures and highly weathered rock with gradual changes in basis of drill-rig access and may not be located along the seismic velocities with depth. Therefore. Refraction and Those Determined by Drilling: 8.2 When the top of rock is defined by drilling it is often based upon refusal of the drill bit to continue to penetrate. Differences in position can easily 8. and seismic refraction line. precision is depth to a refractor can be determined to within6 10 % of the the repeatability between measurements. account for small lateral geologic changes and may only picking of first arrivals. karst). or the first 9.1.1. 2 and Fig. how thin a layer can be detected by the results of a seismic refraction survey will not always agree with seismic refraction method. seismic refraction method.3 Geometry limitations.2 Instrument errors in measuring.1. 8. changes in materials on which cant change in seismic velocity must exist between the two sources and receivers are placed and fluctuating of water layers.1 The bias with which the depth and the shape of a along the seismic refraction line depending upon lateral geo- refractor can be determined by seismic refraction methods logic changes.2.1 Human errors in field procedures. placement of geophones. conditions such as freezing.4.2.4. the measurements would 8. Some of these factors are: and fracturing in the rock. The differences between seismic refraction and drilling interpretation can yield considerable 8. If a refraction survey geometry and seismic velocities of the subsurface layers.3 Precision—For the purposes of this guide. the care the pulse (see Fig.1. greater definition of the considerable disagreement between the refraction results and shape of the top of the refractor.1. 9. This is done using what appears to be the onset of timing.1.2 The answers to all three of these questions is a 8. Close spacing of geophones will provide agrees with the borehole data. the presence interpretation.2 Vertical Resolution: accurate. seismic refraction. refraction. In addition. 3). (10) Sjogren. E. (28) Wallace. R. OK. Jr. D.. Helbig and S. B. 1988.” Geophysics. 1991.. No.. H.. 1990. S. Geological Survey of Canada Economic Geology Report 26. Introduction to Geophysical Pros. New York. of Explt. A. Environmental Protection Agency... (3) Bates. Bureau of Mines Report of (7) Dobrin. tion Interpretation. Sheriff. 1–6. H. The American Society for Testing and Materials takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this standard...“ Application of Surface Files in the Personal Computer Environment. 1990. 1959. 3rd edition. Tulsa... Vol 38.“ Microcomputer- Section 1 Seismic Refraction. “The GRM an Integrated Approach to Shallow Refraction ics. Book 2.. Geophysicists. D. Tibbetts. Book 2. Chapman and Hall.. 2.). 1989.S. L.” Handbook of Geophysical Exploration Vol 13 (25) Haeni. Pankratz. with the Exploration Seismograph.. 2. p. (11) Palmer. H. Explosive Excavation Research Lab.. Army Engineering 1991. physicists. J. and Ellefsen. Bengl. (22) Crice. “Exploration for Ancient Channels New York. Bulletin of the Australian Society of Borehole Geophysical Techniques at Contaminated Sites. 175–200. and Sawl. F. and Keys. (19) Lankston. “The Use of Seismic Refraction (14) Ackermann. Soc. Soc.. Geophysics. A.S. Geological Survey. Geological Time Data Using Interactive Computer Methods. 1. 1990. G. and Marldewicz. 1987. Investigations. A.. No. 1993 (17) Palmer. 1990. D. Golden. Shallow Refraction Seismics. No. or through the ASTM website (www. In: Geotechnical and Environmental Geophysics.S.” Proceedings of the Symposium and Interpretation. No..” Geophys- (16) Palmer.. pp.. ics. 87-183-A. M.. and Dansereau. pp. Your comments will receive careful consideration at a meeting of the responsible technical committee. P. Robert E. (26) Scott. Robert and Lankston. D.astm. Explor. and King. and Mabey. 1972. (20) Pullan. Pergamon Press. Investigations: Technical Report E-73-4. Vol 24. A. Seismic Refraction Exploration for Engineering Site Generalized Reciprocal Method. W. 1970. Society of Engineering and Mineral Exploration Geo- Geophysics No. “Obtaining Multi-Layer Recip- Livermore. Cambridge University Press. 1981. 1986. J. S. West Conshohocken.. Geophysics. Northwest Mining Association. (24) Pakhiser. R. New York. Oklahoma. H. No. Geological Survey Techniques of Water Resources Wiggins. No. Vol 51. 544–568. L.. on the Application of Geophysics to Environmental and Engineering Vol 1. Tredel (eds. Geophys. Manual of Geophysical Hand-Calculator Programs.. or service@astm. 265. 610-832-9555 (fax). P. Grantham.. Encyclopedic Dictionary of Exploration Geophys.” In: Mining Comprehensive System for Interpreting Seismic Refraction Arrival. (9) Haeni. PA 19428-2959.. W. pp. This standard is copyrighted by ASTM. (27) Eaton. Vol 55.... 1970. of Seismic Refraction Data. U. Chapter D1. Vol 21. Oklahoma. G. 1980. New York. Geophysics to Ground Water Investigations. Based Version of SiPT—A Program for the Interpretation of Seismic- Geophysical Press. 100 Barr Harbor Drive. 274–284. Vol 8. Washington.” Geophysics.Use of Airborne. Tulsa. P.” Ground Water. pp. “The Resolution of Narrow Low Velocity Zones with the (4) Redpath. are entirely their own responsibility. Robert. (5) Griffiths. Your comments are invited either for revision of this standard or for additional standards and should be addressed to ASTM Headquarters. California. pp. Applied graphs..” U. and logic Studies. A. and Groundwater Geophysics 1967. This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years and if not revised. 13 .). Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the above address or at 610-832-9585 (phone). Survey Open File Report 82-1065. “Refraction Seismics. C. Spokane. 1973. 1980. Open File Report (12) Saska. pp. J. 1981. L. Geophysics.” U.. pp. B.. rocal Times Through Phantoming. “High Resolution Refraction Seismic Data Acquisition Analyzing Seismic Refraction Data. of Expl. R. R. 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