Locating Dense Non-Aqueous Phase Liquids (DNAPLs) is often a bottleneck for the successful remediation and/or control of polluted sites. Electrical Resistivity Tomography (ERT) between boreholes can detect DNAPLs because of their high electrical resistivity. In applying ERT the definition of the 'best' measurement schedule is a poorly resolved problem. In two case studies, and with synthetic modeling, the cross-hole tripole-pole electrode configuration is compared with the more widely used circulating dipole-dipole and cross-hole dipole-dipole electrode configurations. The case studies involved ERT measurements between five boreholes at two polluted sites with unconsolidated sediments in the Netherlands. The outcome of the modeled data and the field data show that a cross-hole tripole-pole electrode configuration is more effective in locating DNAPLs than the other configurations. The tomographic image from the cross-hole tripole-pole configuration shows clear horizontal patterns that are in accordance with the sediment layers of the subsoil. The locations of the DNAPLs as indicated by the ERT measurements have been confirmed with groundwater samples. Still, the interpretation of the tomographic images should be done with care because ERT cannot distinguish DNAPLs from other high resistivity objects (e.g., massive building blocks) and in the top of some of the images highresistivity computational artifacts are present that could be mistaken for DNAPLs. An important practical advantage of the cross-hole tripole-pole configuration is that the number of measurement points, and thus the measurement time, is less than half that of the other configurations.

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Figure 6. Demonstration of model refinement. (a) Moho depth map from gravity inversion. (b) Moho depth map from satellite gradient inversion. (c) Gradient components Gyz and Gzz in MRF at 260-km elevation. (d) Observed and calculated Bouguer anomaly (Gz) at surface. (e) Profile through final 3D density model. The stippled black line shows the Moho from the gravity inversion in (a), while transition from blue to red bodies indicates Moho depth from (b). The section location is indicated in (a) and (b), white numbers (e) indicate densities in kg/m 3 .

Apparent resistivity is a useful concept for initial quickscan interpretation and quality checks in the field, because it represents the resistivity properties of the subsurface better than the raw data. For frequency‐domain soundings several apparent‐resistivity definitions exist. One definition uses an asymptote for the field of a magnetic dipole in a homogeneous half‐space and is useful only for low induction numbers. Another definition uses only the amplitude information of the total magnetic field, although this results in a non‐unique apparent resistivity. To overcome this non‐uniqueness, a complex derivation using two different source–receiver configurations and several magnetic field values for different frequencies or different offsets is derived in another definition. Using the latter theory, in practice, this means that a wide range of measurements have to be carried out, while commercial systems are not able to measure this wide range. In this paper, an apparent‐resistivity concept is applied beyond the low‐induction zone, for which the use of different source–receiver configurations is not needed. This apparent‐resistivity concept was formerly used to interpret the electromagnetic transients that are associated with the turn‐off of the transmitter current. The concept uses both amplitude and phase information and can be applied for a wide range of frequencies and offsets, resulting in a unique apparent resistivity for each individual (offset, frequency) combination. It is based on the projection of the electromagnetic field data on to the curve of the field of a magnetic dipole on a homogeneous half‐space and implemented using a non‐linear optimization scheme. This results in a fast and efficient estimation of apparent resistivity versus frequency or offset for electromagnetic sounding, and also gives a new perspective on electromagnetic profiling. Numerical results and two case studies are presented. In each case study the results are found to be comparable with those from other existing exploration systems, such as EM31 and EM34. They are obtained with a slight increase of effort in the field but contain more information, especially about the vertical resistivity distribution of the subsurface.

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