Geological appraisal of TEM models

Table 2 Summary of parameter ranges from constrained most-squares inversion for all the TEM stations. The interpreted correlation Ž . with lithology Table 1 is shown in the last column. The question marks denote unequivocal interpretations Ž . Ž . Stationrlayer number Resistivity V m Depth to base m Depth-correlated lithology LH 1 28–30 9–10 Sandrupper boulder clay ? 2 23–25 22–26 Lower boulder clay 3 34–38 43–48 SS with marl bands 4 59–72 SS PF 1 22–25 8–9 Sandrupper clay drift ? 2 18–20 25–28 Lower clay drift Ž . 3 22–26 41–46 SS muddy section ? 4 30–37 SS CP 1 28–37 9–11 Sandrclay drift ? 2 13–16 28–32 Mudstone Ž . 3 26–30 43–50 SS muddy section ? 4 42–65 SS GH 1 17–22 7–8 Sandrupper clayey drift ? 2 14–16 21–24 Lower clay drift 3 56–64 67–74 Gypsumranhydrite mudstone 4 21–30 SS BH 1 21–22 7.8–9 Sandy clay drift ? 2 19–20 45–50 Mudstone Ž . 3 40–48 110–125 Marl and mudstone gypsiferous ? 4 29–37 SS GS 1 10.5–12 7.5–8.1 Boulder clay ? 2 17–20 45–50 Marl and mudstone 3 105–141 170–192 Gypsiferousranhydritic marl 4 4.8–5.2 SS ? common interpretational approach in which the layer boundaries correspond to those stratal Ž boundaries indicated in the borehole logs Table . 1 and are held fixed while the resistivity pa- rameters are free to vary. The resulting models will be called fixed-depth models and are also presented in Figs. 6–11 for comparison; the resistivity deduced for the various lithological units from this alternative TEM inversion are given in Table 1. Except for station GH, only the inversion results for soundings with 50-m- sided Tx loops will be presented in this paper.

4. Geological appraisal of TEM models

The lithologic logs from the six water wells Ž . LH, PF, CP, GH, BH and GS are summarised in Table 1. These are very generalized well logs produced by different drilling companies. Re- Ž . cent downhole geophysical nuclear logging at Ž some of these wells Hawkins and Chadha, . 1990 show a much more complex lithological make-up than those shown in Table 1; unfortu- nately, resistivity logging was only done for the SS section at these operational wells since they are metal-cased down to approximately the top of the aquifer. We will use the lithological data in Table 1 and the incomplete resistivity well Ž . log information of Hawkins and Chadha 1990 to evaluate the interpretational accuracy or geo- logical significance of the biased estimation models. An optimal four-layer biased estimation Ž model for station LH from the western sector where drift deposits directly overlie the SS . aquifer is shown in Fig. 6 and the computed model bounds are listed in Table 2. The layer resistivities derived for the fixed-depth model Ž . see Fig. 6 and Table 1 fall within the most- squares parameter ranges for the biased estima- tion model. In Table 1, a 4.5-m-thick sand layer Ž . possibly of ca. 32 V m resistivity overlies Ž . boulder clay or clayey drift deposit that ex- tends down to a depth of 24.5 m and may have a resistivity of about 25 V m. The underlying SS contains bands of marl in its upper portion Ž . 24.5–40 m where the resistivity may be ca. 34 V m. The deeper section of the SS aquifer may have a resistivity of about 65 V m. In the biased estimation models, a cover unit of 28–30 V m resistivity extends to a depth of about 9–10 m and was required to fit the early-time segment of the interpreted sounding curve. The base of this geoelectric cover unit does not coincide Ž with any known lithological boundary Table . 1 , but its resistivity approximates the average resistivity for both the sand and boulder clay layers in the fixed-depth model. The underlying Ž . moderately resistive 23–25 V m layer with inferred base at 22–26 m coincides with the Ž . drift deposit at this location Table 2 . The underlying unit of 34–38 V m resistivity ex- tending to a depth of 43–48 m probably corre- lates with the upper part of the SS containing marl. The substratum has a resistivity of 59–72 Ž . V m see Table 2 . Downhole induction logging Ž at this station Hawkins and Chadha, 1990, p. . 238 showed that the SS aquifer has a resistivity ranging from 42 to 70 V m; this is in accord with the result of TEM inversion. It would thus appear that a boundary corresponding to the boulder clay–SS interface may be determined by TEM sounding, but the deeper geoelectric interface between the marl-rich top portion and the cleaner aquiferous SS section could be wrongly selected as the top of the target forma- tion without some geological control. A four-layer biased estimation model for sta- tion PF is shown in Fig. 7 and the computed most-squares model bounds are given in Table 2. The resistivities obtained for the fixed-depth Ž . model Fig. 7 and Table 1 are comparable to those of the biased estimation models at depths greater than 8 m. In the biased estimation mod- Ž . els, a moderately resistive 22–25 V m , 8–9- m-thick cover unit is underlain by a conductive layer of 18–20 V m that extends to a depth of 25–28 m. The bottom of the cover layer does not coincide with any known lithological boundary, but its resistivity approximates the average resistivity for both the upper sand and clayey drift layers in the fixed-depth model Ž . Fig. 7 . The second geoelectric horizon coin- cides with the clayey drift deposits in the litholog shown in Table 1. The third layer of about 22–26 V m resistivity extends to a depth of Ž . 41–46 m in the most-squares models Table 2 . The substratum has a resistivity of 30–37 V m. Note that a three-layer fixed-depth inversion Ž model for part of the PF data sounding band- . width 0.0069–1 ms yielded a resistivity about 32 V m for the SS aquifer. Previous downhole Ž investigation at this site see Hawkins and . Chadha, 1990, Fig. 5 suggests that the upper Ž part of SS coinciding with the zone of 22–26 . V m resistivity in the biased estimation model contains bands of mudstone, and the induction well log for the aquiferous interval 32–71 m shows a resistivity of 25–30 V m, which is in good agreement with the TEM models. Overall, there are similar patterns of resistivity distribu- tion within the clayey drift and the SS for both stations LH and PF. Recognizing such a trend may be of exploration significance when there are only marginal contrasts in formation resis- tivities. The inversion results for station CP are pre- sented in Fig. 8 and Tables 1 and 2. The TEM station is located about 60 m from the water well. As shown in Table 1, a 3.1-m-thick super- ficial sandy drift exists at this site and may have a resistivity of ca. 43 V m. It is underlain by 8.9 m of clay drift deposit whose resistivity may be about 23 V m. The MM occurs as a 19-m-thick wedge between the clay drift and the SS aquifer at this site and may have a resistivity of about 13.5 V m. The SS aquifer appears to have a resistivity of ca. 50 V m. The biased estimation Ž . models Fig. 8 and Table 1 suggest a 9–11-m- thick uppermost layer of 28–37 V m. It is un- derlain by a layer of 13–16 V m that extends to 28–32 m depth and is in accord with the fixed- depth result. A zone of 26–30 V m with bottom at 43–50 m depth appears to be required by the data. The substratum has a resistivity of about Ž . 42–65 V m see Table 2 . The base of the top layer in the biased estimation models coincides with that of the clay drift, but its resistive nature and similarity to those seen at the other stations precludes any straightforward geological corre- lation; its resistivity is a good approximation to the average resistivity for the combined top sand and clay drift in the fixed-depth model, but it could also be the result of data distortion arising from the recording instrument’s band- Ž . limitation low-pass filtering of the field re- Ž . sponses cf. Efferso et al., 1998 as will be tested in a later section. The second layer coin- cides with the depth location of MM. The third layer and the substratum are respectively inter- preted as the upper muddy and lower cleaner sections of the SS aquifer; we have made the geological assumption that the upper part of the SS aquifer is muddy as known from stations LH and PF. Station GH is located near the inferred sub- Ž . drift position of an east–west fault see Fig. 1 in the northwest part of the region. The use of layered-earth TEM inversion may not be appro- Ž priate for this faulted area see, e.g. Wilt and . Williams, 1989; Goldman et al., 1994b , but the Tx loop was positioned more than 200 m away from the inferred subcropping fault for this trial sounding. The data in Table 1 show the pres- ence of 3.5-m-thick surficial sandy drift deposit Ž . possibly of ca. 37 V m resistivity , an underly- Ž . ing clayey drift of 14 V m resistivity extend- Ž . ing down to 20.1 m, and a resistive 51 V m gypsiferous mudstone down to 68 m depth with the SS as the substrate of about 30 V m resistiv- ity. The biased estimation models for station GH for the 50-m-sided Tx loop were presented Ž . in Figs. 4 and 5 see also Table 2 . A relatively Ž . resistive 20–23 V m cover unit extends to about 7–8 m depth in the models; its base does not match any known geological boundary, but the inferred resistivity approaches the average of those determined for both the sandy and clayey drift deposits in Table 1. The second layer of 14–16 V m resistivity extends to a depth of 21–24 m in the models and coincides Ž . with the clayey drift layer Table 1 . The third Ž layer in the model is very resistive ca. 55–64 . V m and extends to a depth of 67–74 m. This correlates with the gypsiferousranhydritic mud- stone sequence intersected in the borehole and it would thus appear that the top of the MM sequence can be discerned from the TEM depth probe especially if they contain gypsum or an- hydrite bands. The substratum in the model has a resistivity of 24–31 V m. The result of inver- sion of the TEM data from soundings with a Ž . 20-m-sided Tx loop Fig. 9 also showed the SS Ž section to be of relatively low resistivity 30–36 . V m . These results suggest that the top of the SS can be inferred to within 10 error from TEM soundings at this locality. However, the inferred resistivity of the aquifer from TEM inversion is not in excellent agreement with the published borehole resistivity log for this sta- tion, which shows a resistivity of about 40–50 Ž V m in the depth interval 72–116 m see . Hawkins and Chadha, 1990, Fig. 7 . Further east, the MM Group becomes more Ž . heterogeneous Hawkins and Chadha, 1990 with thick beds of marl as at station BH or gypsiferous and anhydritic marl as at station GS. The inversion results for station BH are shown in Fig. 10 and Tables 1 and 2. The Ž . litholog Table 1 identifies an 8-m-thick cover of sandy clay drift over a mudstone–marl–mud- stone sequence with intervening boundaries at depths of 46 and 81 m; note that neither the top soil nor the 3–5-m-thick surficial sand seen at stations LH, PF and CP were separately identi- fied in the well-driller’s log for BH. The upper mudstone unit may have a resistivity of about Ž . 20 V m Table 1 . The biased estimation models Ž . Fig. 10 and Table 2 have a 7.8–9-m-thick Ž topmost layer of 21–22 V m resistivity coincid- ing fortuitously with the sandy clay drift in . Table 1 . The second layer has a resistivity of 19–20 V m and extends to 45–50 m depth coinciding somewhat with the mudstone horizon in the borehole log. The third layer has a resis- tivity of 40–48 V m and extends to a depth of 110–125 m, thus correlating with the combined marl and lowermost mudstone member of the MM Group; the high resistivity of this unit may be indicative of the presence of gypsiferous and anhydritic bands as at station GH. A resistivity of 29–37 V m is obtained for the substratum at this station and suggests that the top of the SS aquifer may be discerned from TEM soundings because of the higher resistivity of the overly- ing, possibly gypsiferous, MM sequence. The Ž downhole resistivity for station BH Hawkins . and Chadha, 1990, Fig. 8 range from 42 to 50 V m in the aquiferous depth interval 112–152 m which is not in excellent agreement with the TEM models. The TEM inversion results for station GS are shown in Fig. 11 and Tables 1 and 2. The biased estimation and fixed-depth models are Ž . almost identical. The litholog Table 1 identi- fies a 2-m-thick surficial sand and gravel de- posit overlying a 5.5-m-thick red clayey deposit Ž . till . The underlying MM sequence consists of a 38-m-thick upper horizon of marl and mud- stone and a basal gypsiferous–anhydritic marl layer that extends to a depth of 187 m beyond which occurs the SS aquifer. In the biased Ž . estimation models Fig. 11 and Table 2 , a Ž . 7.5–8.1-m-thick, conductive 10.5–12 V m top layer was required to fit the early-time segment of the TEM sounding curve and coincides with the glacial till. The second layer is less conduc- Ž . tive 17–20 V m and extends to 45–50 m depth; this geoelectric unit roughly coincides with the marl and mudstone horizon in the Ž . driller’s log see Table 1 . A highly resistive Ž . 105–141 V m unit underlies this sequence down to a depth of 170–192 m and is correlated with the gypsiferousranhydritic marl in the Ž . driller’s log Table 1 . The substratum has a Ž . resistivity of about 5 V m see Table 2 . The SS may therefore be more conductive here than elsewhere and could contain marl bands in its upper part, but the late-time TEM data are of reduced quality, thus limiting model resolution. It would appear that a larger transmitter loop and a receiver coil of much larger effective area than 31.4 m 2 are required for improved model identification in this locality.

5. Discussions