Field experiments and qualitative assess- ment of results

tional success on the availability of a priori geological information.

2. Field experiments and qualitative assess- ment of results

Six borehole locations representing different Ž geological environments in terms of types and . thicknesses of SS cover rocks have been care- Ž fully selected for the TEM evaluation study see . Fig. 1 and Table 1 . Stations LH and PF are located in areas where glacial drift deposits rest directly on the SS aquifer. Station CP is located near the subcrop zone of the SS–MM contact, while stations BH and GH are located in areas with thicker MM deposits. Station GS is located in the eastern sector with much thicker confin- ing rock formations over the target SS aquifer. The first TEM soundings were made in Novem- Ž ber 1994 at four borehole locations PF, LH, . BH and GH in Fig. 1 using the Geonics EM47 Fig. 3. Consistency and quality assurance check of TEM instrumentation. Shown are the 50-m loop data recorded for a Ž specific time-band using different transmitter equipment triangular symbols for PROTEM47 and round symbols for . PROTEM57 . Ž . field equipment and square transmitter Tx Ž loops of two different sizes 20 and 40 m on a . side . The recorded data were of good quality and highly repeatable over the sounding band- width 6.9 ms to 10 ms. The field data for the deeper-probing time windows were of variable quality. Field measurements were made in June 1995 Ž at stations LH, PF, CP, GH, BH and GS see . Fig. 1 using a 50 m = 50 m Tx loop. The 1995 survey employed another TEM equipment, the Geonics PROTEM47r57 instrument. For data quality assurance at each station, recordings were made for different values of Tx current and gain settings over three overlapping instru- ment time-bands using separate TEM transmit- Ž ters PROTEM47 with 1–3 A; the PROTEM57 . with 12–13 A . For these measurements, a small dipole receiver with an effective area of 31.4 m 2 was placed at the centre of the Tx loop. In the 1995 measurements, the TEM data recording instrument was placed outside the Tx loop, unlike the more common arrangement used in 1994 where the operator and data logging equipment are situated within the Tx loop, only about 5 m away from the centrally placed multi-turn receiver; deploying the PRO- TEM47r57 equipment in this mode was found to yield concordant results with the Sirotem equipment in a comparative study elsewhere Ž . M.A. Meju, unpublished report, 1995 . Despite the intensive agricultural activities in the region during the 1995 survey, effort was made to locate the stations within ca. 100 m of the Ž respective boreholes except at station GS, which had to be located about 400 m east of the . borehole away from crop farms . Note that the 1995 sounding locations were offset by several metres from the 1994 sounding points except at station GH; the 1995 experiment took place Fig. 4. Illustration of optimised biased estimation technique using TEM data from station GH. The apparent resistivity data are shown in the top left-hand panel, while the TEM voltage decay data are shown in the lower left-hand panel. The Ž . Ž . right-hand plot shows the simple resistivity–depth transformation of the data crosses , an optimal 1-D model solid lines for which the initial mdel was constructed using the simple transform model, and the depths to lithologic boundaries in the Ž nearby borehole horizontal dashed lines with numbers refering to the relevant sequence of boundaries given in Table 1; . Ž . EOH s end of hole . The computed response curves for the 1-D model solid lines are superimposed on the field data in the left-hand panels. during the local cropping season and the trans- mitter loop positions differed inevitably from Ž those of 1994 recorded when the fields have . been harvested . 2.1. Comparison of sounding curÕes for differ- ent loop sizes and data quality assurance The TEM sounding curves obtained with dif- ferent Tx loop sizes at representative sites are shown in Fig. 2. The sounding curves from a given location are identical for the various Tx loop sizes with coincident loop centres; all the 20- and 40-m loop sounding curves are similar in shape. The 40- and 50-m loop sounding curves are coincident for station GH. However, the curves are different where the 50-m loop centres are offset from the smaller loop centres Ž . see, e.g. PF and BH , suggesting a possibly complex glaciated terrain. The 1994 soundings Ž . at LH see Fig. 2 are deemed to be affected by a metallic fence or pipe-line. In general, the data from the 20-m-sided Tx loop become noisy earlier than the larger loop data as expected and will thus have limited potential for resolving deep stratigraphic targets in the region. For the 1994 data, the first three time-windows appear to be somewhat biased or distorted relative to the rest of the recording band. In the 1995 data, however, the first and the tenth time-windows Ž . 0.0069 and 0.069 ms of the shallowest probing Ž . band 0.0069–0.704 ms appear to be slightly Ž distorted see, e.g. 50-m loop data for GH and . LH . These bias effects may be due to the Ž . band-limitation band-pass filtering operation Ž of the respective instruments cf. Efferso et al., . 1998 . Sample apparent resistivity sounding curves recorded for the same sampling bands using the PROTEM47 and PROTEM57 transmitters are shown in Fig. 3 for comparison. The data are practically identical for the time-band 0.0069– Fig. 5. Illustration of method of equivalence analysis of a biased estimation model. The right-hand plot shows the 2M Ž . most-squares models solid lines for M model parameters that fit the TEM field data to a specified threshold misfit. The apparent resistivity data are shown in the top left-hand panel, while the voltage decay data are shown in the lower left-hand panel; the solid curves are the computed response curves. Fig. 6. Comparison of borehole stratigrahic data and TEM inversion results for station LH. The apparent resistivity data are shown in the top left-hand panel, while the TEM voltage decay data are shown in the lower left-hand panel. The right-hand Ž . Ž . plot shows the simple resistivity–depth transformation of the data crosses , an optimal 1-D model solid lines for which the initial mdel was constructed using the simple transform model, and the depths to lithologic boundaries in the nearby Ž borehole horizontal dashed lines with numbers refering to the sequence of boundaries given in Table 1; EOH s end of . Ž . hole . The computed response curves for the 1-D model solid lines are superimposed on the field data in the left-hand panels. 0.6 ms, but different beyond 1 ms; the PRO- TEM57 data may be preferred at such late measurement times. Notice from Figs. 2 and 3 that except for stations GH and GS, the forms of the sounding curves range from near-invariant to slowly ris- ing with time due to the small contrasts in resistivity between the various subsurface for- mations; accurate formational boundaries may thus be difficult to retrieve from such data using Ž the commonest TEM inversion approach e.g. . Anderson, 1982; Meju, 1992 without adequate geological control.

3. Quantitative data modelling approach