CE 320Environmental Measurements in Natural Systems: GeophysicsLecture 1DC resistivity SurveysDirect current (DC) Resistivity• 1. Introduction • 2. Schlumberger, Wenner, dipole-dipole, pole-dipole arrays • 3. Current flow in the ground • 4. Field methods and instrumentation • 5. Data interpretation • 6. Field ExamplesGeorg Simon Ohm (1787-1854)IRV=Ohm’s Law (discovered in 1827)It's Resistivity, NOT ResistanceLRAALR==ρρSo the unit for resistivity is ohm-meterMono-pole (pole) DipolerIVπρ2=Pole-Dipole arrayIVnnnla∆+−= )1)(1(2πρ2l2l(n-1)2lDipole-dipole array• Instrumentation• Geology– Granite with small amounts of chist– Intruded by pegmatite and aplite dikes – Poorly connected, deeply dipping fractures– Two hydraulically conductive zones (upper: 10-7—10-6 m/s; lower: 10-6—10-5m/s)Field MethodsResistivity SurveysAGI Sting R-1 and the Swift automotive switchboxCCR: Field Deployment• OhmMapper console and antenna arraySting/Swift prg: DIP-DIP title2 unit electrode spacing3 array No. dip-dip=3193 No. of data points1 1-middle point used0 0-no IP1st: apparent rho-location 2nd: P1P2 spacing3rd: dipole separation factor n4th: apparent resistivity3.000 2.000 1 2961.0005.000 2.000 1 2769.0007.000 2.000 1 1040.3009.000 2.000 1 2994.30011.000 2.000 1 779.580……….45.000 2.000 1 10305.00047.000 2.000 1 6955.20049.000 2.000 1 5515.00051.000 2.000 1 4435.9004.000 2.000 2 2168.8006.000 2.000 2 1696.4008.000 2.000 2 1233.200DC Resistivity Interpretationpath gpropagatin :couplingreceiver pattern,radiation :,spreading lgeometrica :signals source received, :,LPPGSRRSThe current flow in the layered media deviates from that observed in the homogeneous media. In particular, notice that in the layered media the current flow lines are distorted in such a way that current preferentially seems to be attracted to the lower-resistivity portion of the layered media. In the model on the left, current appears to be pulled downward into the 50 ohm-m layer. In the model on the right, current appears to be bent upward, trying to remain within the lower resistivity layer at the top of the model. This shouldn't be surprising. What we are observing is the current's preference toward flowing through the path of least resistance. For the model on the left, that path is through the deep layer. For the model on the right, that path is through the shallow layer. Electric current in layered mediapath gpropagatin :couplingreceiver pattern,radiation :,spreading lgeometrica :signals source received, :,LPPGSRRS∫⋅⋅⋅⋅=−LdsRSePPGSR)()()(ωαωωcrystalline rock can lead to low resistivitiesif they are filled with fluids. The resistivities of various earth materials are shown below. Material Resistivity (Ohm-meter)Air∞Pyrite 3 x 10^-1Galena 2 x 10^-3Quartz 4 x 10^10 - 2 x 10^14Calcite 1 x 10^12 - 1 x 10^13Rock Salt 30 - 1 x 10^13Mica 9 x 10^12 - 1 x 10^14Granite 100 - 1 x 10^6Gabbro 1 x 10^3 - 1 x 10^6Basalt 10 - 1 x 10^7Limestones 50 - 1 x 10^7Sandstones 1 - 1 x 10^8Shales 20 - 2 x 10^3Dolomite 100 - 10,000Sand 1 - 1,000Clay 1 - 100Ground Water 0.5 - 300Sea Water 0.2In shallow sediments, the averaged electric conductivity of the groundwater is the determining factor of the formation conductivity. For clay-free soil, the empirical relationship is the well-known Archie’s law. For clayey soil, the empirical formulation is the Maxwell-Smitsrelationship is used to describe the relation between groundwater conductivity, clay cation exchange capacity, and the formation conductivity.wnmSbσφσ=σ−effective formation resistivity;σw−pore water resistivity;φ – porosity;S – saturation;b – 0.4-2.0;m – 1.3-2.5;n ~2.Archie’s law (for clay-free soil)σ−effective formation conductivity;σw−pore water conductivity;Β – constant coefficient;F – Formation factor;Qv – Cation exchange capacity;Maxwell-Smits relationship (for shaly-clayey soil))(1vwBQF+=σσDC Resistivity Case Studies∫⋅⋅⋅⋅=−LdsRSePPGSR)()()(ωαωωpath gpropagatin :couplingreceiver pattern,radiation :,spreading lgeometrica :signals source received, :,LPPGSRRSEC Variation with Depth051015202550 100 150EC (micromhos/cm )Depth (ft)Cr Variation w ith Depth05101520250 5 10 15 20Cr (mg/l)De pth (ft)EC v s C ry = 0.2827x - 19.9R2 = 0.8969051015200 50 100 150EC ( micromhos/cm)C r ( m g /l)Groundwater conductivity and Chromium concentration at a contamination site, East CT.EC Var iati on with Depth051015202550 100 150E C ( mi c r omho s / c m)The averaged electric conductivity of the groundwater found at the National Chromium site is about 100 microSiemensper centimeter.It is equivalent to 10 milliSiemens per meter.The averaged formation conductivity found by the DC resistivity tomography is 1milliSiemens per meter.By assuming the porosity is 35%, 100 % saturation, and a = 1.0, m=2.0 and by Archie’s law we can estimate that the formation conducitvity is about 1.2 milliSiemens per meter.Detection of Saltwater Intrusion along the Noyo River, CaliforniaResistivity and Seismic Survey ResultsReadings:John M. Reynolds, Wiley and Sons, 1998, An Introduction to Applied and Environmental Geophysics, Chapter 7.Russell, Gary M., and Higer, Aaron L., 1988, Assessment of Ground Water Contamination Near Lantana Landfill, Southeast Florida: Ground Water, v. 26, No. 2. p.