GEOL 335.3 Natural-source (passive) electrical methods Spontaneous Polarization (Self-Potential, SP) Method Occurrence and origin of self-potentials; Measurement Interpretation Case Histories Magnetotelluric (MT) method Origin and characteristics Measurement Interpretation Reading: Reynolds, Chapter 8, Telford et al., Chapter 6. GEOL 335.3 Self-potential anomalies Mineralization potentials: Graphite, coal, sulphite ores (pyrite, chalcopyrite, sphalerite, galena), magnetite and other electronically conductive materials] Negative potentials, 100's of mV. Quartz veins, pegmatites Positive, 10's of mV Background potentials (although of primary interest in, e.g., hydrogeological investigations) Fluid streaming, geochemical reactions Positive or negative, < 100 mV. Bioelectric (plants) Negative potentials, < 300 mV. Topography Negative, <2V. GEOL 335.3 Origin of self-potentials Oxidation-Reduction Reaction Groundwater is responsible for the various processes leading to self-potentials: water acting as electrolyte; water acting as a solvent for various materials; water flow. Electrochemical potentials Reduction (gain electronscathodic) Oxidation (electrons are lost from the solution anodic reaction) Electrokinetic potentials GEOL 335.3 Measurement of self-potentials Two electrodes... Should be non-polarizing; metal electrode in its own salt (e.g., Cu in Cu2SO4); porous pots that allow solution to leak slowly through and make contact to the ground. ...and a millivoltmeter Digital DC meter with input impedance ~108 Ω. Field arrangement: Check the electrodes periodically for < 2mV differences side by side; Use one fixed base station or leap-frog across the strike of the measured anomaly 30 m to several km apart. Create SP profiles or maps GEOL 335.3 SP Interpretation Primarily qualitative; May need corrections for telluric (large-scale surveys) and bioelectric potentials. Depth to the source body is about ½ of the width of SP anomaly; Attitude of the body can be judged from the asymmetry of the anomaly. GEOL 335.3 Effect of overburden on SP Clay cover may remove surface SP effects... Sand overburden Clay overburden GEOL 335.3 SP case history (Meiser, 1962) Graphite bodies in folded gneisses GEOL 335.3 SP case history (Sennetere area, Quebec, from Telford et al.) Very strong SP anomalies caused by massive sulfides ? GEOL 335.3 Telluric currents Electromagnetic waves generated by the ionospheric currents, electric storms EM waves bounce back and forth between it and Earth's surface Frequencies of 10-5-105 Hz (~10-3-103 used in TM); Substantial vertical component of wave propagation GEOL 335.3 Telluric current Variations throughout the day Note that high-frequency variations are below ~50% within 6-8-hr periods around 2pm. Telluric current Azimuthal variations throughout the day Note that azimuthal variations are also low around 2pm. This provides a convenient time window for TM measurements. GEOL 335.3 GEOL 335.3 Telluric current measurements Currents cannot be measured directly So, mesuring the corresponding potential gradients near the surface Using pairs of non-polarising electrodes on the surface or boreholes. Spacing < 30 m for mineral exploration, 100-600 m for structural studies or oil exploration; Lead plates, or steel or brass stakes are also commonly used for frequencies above 1 Hz; At each station, using two orthogonal electrode pairs (to capture the direction of telluric current). Using two electrode spreads (one moving and one as a base station monitor) Because telluric currents vary with time! However, in a simplified scheme (<100 Hz used in mineral exploration), when working around 2pm, base station may be dispensed with. GEOL 335.3 Magnetotelluric measurements Magnetic coils should by buried in a trench to avoid shaking. Audio-range MT (AMT) can also use controlled grounded electric dipole source. Airloop vertical magnetic sensor for stony grounds (where it is difficult to dig a hole Magnetic sensors for 0.0002-400 Hz Receiver with A/D converter AMT sensors for 0.1-10000 Hz MT system by Phoenix Geophysics GEOL 335.3 Telluric interpretation For a profile (axis X) across the strike of a linear structure, H y , that is Hy = const along the surface. x =0 Therefore, |Ex|2 ∝ ρ, and ρapparent can be estimated by simply measuring Ex at various frequencies. Also, numerical modeling of Maxwell's equations can be used possible distributions of ρ(x,z). E E|| GEOL 335.3 Magnetotelluric interpretation Measured quantity is the frequency-dependent apparent resistivity (T is the EM period): a Ey Hx = 2 E 0.2T y Hx E in [mV/km], H in [nT]; T in [sec] 2 For a single layer over a half-space (Cagniard, 1953): 2 a = 1 where: 2 e e 2 2 = = 2z 2 e cos 1 2 e cos 1 2 1 2 1 2 = 1 ρ1 z , ρ2 , 2z zs 0.004z f . 1 Use precomputed master curves of ρa/ρ1: Plot f and ρa in log scales; Find ρ1 and ρ2/ρ1; Find fs for which γ=π/2, and: z 250 2 1 fs 400 2 1 fs GEOL 335.3 AMT master curves (ρ2<ρ1) γ=π/2 GEOL 335.3 AMT master curves (ρ2>ρ1) γ=π/2 MT example GEOL 335.3 Uranium mineralization, Coxwell Dome, Northern Saskatchewan