Capabilities and Methods for DC Electrical Resistivity Surveys
Equipment and Software
SuperSting R8/IP/SP/WiFi Meter, 112 electrodes and covers, 8 70-m long passive cables each with 14 electrode take-outs spaced by 5 m, 200 m of extension cable, 2 56-electrode Switchboxes, all made by Advanced Geosciences Inc. (Fig. 1). Power from two 12V deep-cycle batteries or two AGI 120/220V AC-DC field power converters and a Honda 1000i generator. Software includes AGI’s SuperSting Administrator, SuperSting Manager App on dedicated Samsung TabPro, EarthImager 2D, and EarthImager3D.
Linear Distance Coverage, Penetration Depth, and Spatial Resolution
With electrodes spaced at 5 m, our equipment spans 555 m (1820 ft) and is capable of penetrating to a depth of ~120 m (385 ft). Penetration depth is ~15-25% of the length of the electrode string.
There is a trade-off between penetration depth and spatial resolution. The greater the penetration depth, the wider the electrodes must be spaced to achieve it and so the lower the subsurface spatial resolution. The spatial resolution of a resistivity image is not only a function of electrode spacing but also varies with depth in the image, resistivity contrast between target and surrounding material, target shape, and inversion methods. Roughly, spatial resolution is about 0.5*electrode spacing near surface and double the electrode spacing near the maximum penetration depth. If the electrode spacing is 5 m, the spatial resolution ranges from ~2.5 m near the surface to ~10 m at a depth of 100 m.
Using various leap-frogging techniques with the electrode cables such as ”roll-along”, we can extend the ERI image to significantly greater lengths.
Before going into the field, we complete some forward modeling to determine whether ERI can answer key questions posed by the project objectives and also to design the optimal measurement program. The first step is to create simple 2D synthetic models with approximate resistivities for the materials we think are in the subsurface based on prior knowledge (Fig. 2a). We then simulate the apparent resistivity values that would result from each synthetic subsurface model using several electrode arrangements and measurement schemes (called arrays) (Fig. 2b, c). Finally, we invert the synthetic data sets to obtain a synthetic subsurface resistivity distribution (Fig. 3).
Fig. 2. (a) Synthetic geologic model for a shallow pit (2.5 x 3 m) of conductive material (10 Ωm) in more resistive material (100 Ωm). Surface electrodes (filled circles) are spaced every 2.5 m. (b) Pseudo-section showing the contoured apparent resistivity values that would be collected by surface electrodes following the “Wenner“ survey scheme. This electrode array exaggerates lateral layering. (c) Same as (b) following the measurement scheme of the “dipole-dipole” array. *
Fig. 3. Inverted synthetic resistivity sections for the pseudo-sections in Fig. 2. The shape of the conductive pit in the synthetic geologic model is shown for reference. Note that ERI gives a smoothed image of subsurface resistivity and that different electrode arrays yield different smoothed images. To draw boundaries on an inverted resistivity section requires ground truth.*
Field Data Collection
Our field program currently involves: laying out the lines on which electrodes will be planted, driving the electrodes into the ground, connecting the electrodes, switchboxes and SuperSting with cables (Figs. 4-6), running contact resistance tests to insure proper ground connections, collection and rapid evaluation of the resistivity data, surveying the electrodes, and repacking the equipment. Initial results are available in field using Wifi. Electrodes are either hammered into sediment, coupled to hard-surfaces using salty wet diapers, or drilled into hard surfaces.
Fig. 4. Laying out electrode line, driving 18” long electrodes 7-10” into the ground and hooking up cables.
Fig. 5. Attaching cables to a switchbox.
Fig. 6. Attaching cables and power to the SuperSting.
To complete an ERI survey involving 56 electrodes requires about 6-8 person hours depending on the terrain. Typically, two different measurement schemes are run in automated mode for the 56 electrodes resulting in collection of ~1400 data pts. Data acquisition time is ~ 1 hour.
*Binley, A., and Kemna, A., 2005, DC resistivity and induced polarization methods, in Rubin, Y., and Hubbard, S. S., eds., Hydrogeophysics: Dordrecht, Springer, p. 129-156.