Seismic Reference Datum TheoryThis page explains why an SRD is used. The surface of the earth cannot be used as a reference datum for seismic as it is not level, whether on land or ocean bottom. Therefore, a Seismic Reference Datum (SRD) is used. For ocean surveys, the sea level is usually used, as it is level and also is where the source and the geophones are (except for ocean bottom surveys). While this should be "mean sea level", there is little or no difference between the current sea level and "msl" except for surveys in tidal basins and immediate shorelines (where drilling is usually not allowed). For land surveys, topography can be a serious problem. 1 an SRD is selected.To correct this problem. This allows the data to be set up as if the sources and receivers were at a constant elevation. 2 . This requires the use of a replacement velocity for the rock layers above the SRD and for the air below the SRD. This correction lines the reflections at zero offsets for the raw field gathers. 3 . they should be applied before applying the SRD corrections. not the KB). The problem with this is that the problem is caused by a lack of data at the very top. NMO corrections are applied first. Log Data and SRD's For practical reasons. zero time will match zero depth even after the SRD correction is applied. we create synthetics to approximate the equivalent raw field gather of that well location.Note that surveyors commonly can select an SRD that always or almost always underlies the surface. before SRD corrections are applied. SRD corrections performed after this step will reflect which method was used. Filtering. It may result in time zero not matching depth zero (which makes no logical sense). deconvolution and stacking can be applied before or after. Therefore. Check Shot Surveys Check shots are depths and 2-way times measured from the surface. The log above the first check shot will be distorted to achieve this. but this method spreads the error over the entire above-check-shot section. NMO corrections cannot be applied to data corrected to an SRD because the Dix equation measures time from the surface. This method avoids distorting the log above the first check shot. If you add a depth 0: time 0 data point to this checkshot survey (value 0. 0 as it takes no time to go no distance). only the measured check shot values are used. even if it results in events at negative times. time zero will match depth zero after the correction. But this requires log data for the well right up to the surface. log data is never available for this section. so if an origin point were used. Therefore. HRS compensates for that by extending the first (uppermost) log value up to the surface of the well (note: the surface. as must angle gather and prestack migration. Therefore we must apply an SRD correction to the synthetic to match the seismic trace for that location. They are more accurate than log-based synthetics (both because they are measured from the surface and because sonic logs can have errors and inaccuracies). this is not a problem. distorting valid data. By default. Modeling with SRD's For AVO modeling. unlike the example above. with no "origin" data point (0 time. 0 depth). so the replacement velocity usually does not have to be applied to air. However. 4 .