Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue2-3.2000:

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www.elsevier.nlrlocaterjappgeo

Evaluation of small-loop transient electromagnetic soundings to

locate the Sherwood Sandstone aquifer and confining formations

at well sites in the Vale of York, England

M.A. Meju

a,)

_{, P.J. Fenning}

, T.R.W. Hawkins

EnÕironmental and Industrial Geophysics Research Group, Department of Geology, UniÕersity of Leicester,

UniÕersity Road, Leicester LE1 7RH, UK

Earth Science Systems Ltd., Kimpton SG4 8HP, UK c

42 Hilltop Close, Cheshunt, Hertfordshire, EN7 6QN, UK

Received 29 September 1998; accepted 26 January 2000

Abstract

Ž .

Shallow-depth transient electromagnetic TEM soundings have been performed at six borehole locations in an intensively farmed area in northern England to evaluate their usefulness in mapping geological formations under a thick

Ž .

cover of glacial drift deposits. The regionally important Triassic Sherwood Sandstone SS Group aquifer is directly overlain by Triassic Mercia Mudstone in the eastern two-thirds of the study area and by drift deposits in the west. Owing to the difficulty of deploying large loops and the overriding need to minimize lateral effects on the depth probes, square transmitter loops of 20, 40 and 50-m side-lengths were deployed in the central-loop configuration with the Geonics EM47 and PROTEM47r57 field equipment. Using a two-stage data interpretation technique, it is found that the effective depth of mapping ranged from about 8 to 150 m at most sounding locations. Comparison of inversion models with borehole data shows that the SS and some overlying sedimentary rocks may be discerned from the TEM soundings; there is a consistent pattern of resistivity distribution within each geological formation at all the borehole sites enabling a realistic identification of the key stratal units. However, a 7–11-m-thick upper layer is found in all the constructed models, which does not correlate with any known formation boundaries, but appears to be justified by comparison with sample dc resistivity

Ž .

soundings at two locations; it would also appear that the earliest time windows -0.016 ms are somewhat distorted by the band-limitation operation of the TEM instrumentation. This pilot study demonstrates that the TEM method is a potent tool for stratigraphic mapping in the region, but the upper 5–8 m remains largely inaccessible to the method using state-of-the-art equipment and conventional data processing techniques. It may therefore be necessary to combine TEM and short

Ž .

Keywords: Electromagnetic sounding; Resistivity inversion; Stratigraphy; Glacial terrain

)_{Corresponding author. Tel.:}

q44-116-252-3628; fax: q44-116-252-3628.

Ž .

E-mail address: [email protected] M.A. Meju .

1. Introduction

Geological mapping of aquifers in gently dip-ping formations is straightforward in areas with

Ž .

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ample rock outcrops. In areas with little or no

outcrops such as regions with extensive glacial

cover deposits , direct drilling or remote sensing methods may be the only source of useful infor-mation. Although boreholes allow accurate defi-nition of aquifer boundaries, correlation of hori-zons between well sites, and direct assessment of any variations in groundwater quality in an aquifer, they are expensive and do not provide a continuous picture of the structure across an area. Also, in some contaminated land investiga-tions, borehole exploratory drilling may not be a safe option. Cost-effective, non-invasive geo-physical methods can provide continuous sub-surface structural information to guide geologi-cal mapping or geoenvironmental investigations

Žand only a few judiciously located boreholes may be necessary to resolve any interpretational

ambiguities . Groundwater aquifers range from alluvial sand and gravel deposits to weathered fracture-zones in crystalline rocks. In many field situations, a confined aquifer lends itself to remote detection using geophysical methods that measure physical property distributions related to porosity and fluid content in the subsurface

Že.g. electrical resistivity, dielectric permittivity ..

Thus, the inductive and galvanic resistivity depth sounding methods are the central tools in groundwater and geoenvironmental

investiga-Ž

tions e.g. Patra, 1970; Worthington and Grif-fiths, 1975; Bugg and Lloyd, 1976; Koefoed and Biewinga, 1976; Palacky et al., 1981; Fit-terman and Stewart, 1986; Lindqvist, 1987; Mc-Neill, 1987; Buselli et al., 1988; Hazell et al., 1988; Hawkins and Chadha, 1990; Goldman et al., 1991, 1994a; Sandberg, 1993; Christensen and Soresen, 1994; Sorensen, 1996; Meju et al.,

1999 although the seismic reflection and ground probing radar methods currently have the best

potential for stratigraphic mapping see, e.g. Beres and Haeni, 1991; Meekes and van Will,

1991 .

Ž .

The transient electromagnetic TEM method has good potential for subsurface mapping and is of interest in this paper. Stratigraphic or aquifer mapping using the TEM method is not

Ž .

new see, e.g. Sinha, 1990 , but is of interest for two reasons. First, recent advances in digital technology have led to newer TEM

instrumenta-Ž

tion e.g. Geonics PROTEM47, Sirotem Mk3,

ARTEMIS, Zonge NanoTEM, Bison TD2000 with improved capability for mapping the

near-Ž .

surface ca. 10–200 m below the surface at geometrically constrained sites such as typifying many built-up or intensively cultivated regions where it may not always be possible to deploy large loops. Second, the development of novel schemes for processing TEM data to yield geo-logically significant resistivity–depth profiles or

geoenvironmentally meaningful models Chris-tensen, 1995a,b; Meju, 1995, 1998; Zhdanov et

al., 1995 would suggest that it is possible to produce a realistic interpretive model without much reliance on the availability of a priori subsurface information. This paper describes a recent evaluation study of the usefulness of

portable TEM instrumentation the Geonics EM47 and PROTEM47r57 equipment with

20–50-m-sided transmitter loops in shallow-de-pth stratigraphic mapping in an intensively cul-tivated area in northern England. The EM47 equipment and the equivalent PROTEM47 mod-ule have capability for sampling from about 6 m

Ž .

depth see Goldman et al., 1994a to over 100 m depth depending on the subsurface conductivity distribution. The PROTEM57 module features a more powerful transmitter and offers an in-creased capability for deeper penetration.

Ž .

In the Vale of York Fig. 1 , the geological formations dip eastwards under a cover of Pleis-tocene fluvioglacial drift of variable thickness and the present geological maps derive solely from borehole data. The regionally important

Ž .

Triassic-aged Sherwood Sandstone SS aquifer rests on Upper Permian Marl and is directly overlain by Triassic-aged Mercia Mudstone

ŽMM sequence in the eastern two-thirds of the.

study area and by drift deposits in the west. This is a complex glaciated terrain and deeply concealed paleo-channels are known to be com-mon in a similar terrain further south of the study area. The glacial deposits consist of a

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Fig. 1. Site location map and sketch cross-section through line AB showing the broad solid geology of the area after

. Ž .

Hawkins and Chadha,1990, Fig. 1 . The geological section shown in the lower diagram not to scale is based on sparse

Ž .

borehole data. The borehole sites used for the TEM experiments are shown — Providence Farm PF , Calton Park Farm

ŽCP , Bolton House Farm BH , Link Hall LH , Grange Farm-Shiptonthorpe GS and Grange Farm-Upper Helmseley. Ž . Ž . Ž . ŽGH ..

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downward sequence of silty, locally sandy or gravelly clay, silty sand with local clay horizons and basal sand and clay. Sporadic perched water tables occur in sand materials within the drift deposits, but most of the drift is clayey in the study area and the SS aquifer is confined

ŽHawkins and Chadha, 1990 . The quality of the.

groundwater in the SS aquifer varies and is considerably more saline where the aquifer is confined by thick MM sequence than where it is

covered by drift alone Hawkins and Chadha,

1990 .

Previous dc resistivity investigations in the

Ž .

area Hawkins and Chadha, 1990 suggest some difficulty in distinguishing between low-contrast drift deposits and MM or SS, and between MM

and SS whose upper parts may contain

mud-.

stone interbeds . Also, some of the resistivity values determined from surface electrical sound-ings for the MM sequence and the SS aquifer are anomalously high; the reconstructed SS re-sistivities were not in agreement with the values determined by down-hole induction logging

ŽHawkins and Chadha, 1990 . Owing to the.

difficulty of deploying large loops and the over-riding need to minimize lateral effects on the

depth probes in this faulted Hawkins and

Aldrick, 1994 farming region, the central-loop TEM technique was selected for this pilot study. The specific practical issues to evaluate in this

Ž .

study are: 1 the possibility of accurately locat-ing the SS aquifer in the western part where it is

Table 1

Ž .

A summary of drillers’ lithological logs for six boreholes PF, LH, CP, GH, BH and GS in the study area. The last column contains the interval resistivities deduced from fixed-depth inversion of the TEM soundings with 50-m-sided Tx loops

Ž . Ž . Ž .

Borehole name Lithology Thickness m Depth to base m Inferred resistivity Vm

LH Top soil 0.5 0.5

Sand 4.5 5.0 32

Boulder clay 19.5 24.5 25

Ž .

Sandstone SS with some marl bands 15.5 40.0 34

SS 73.0q 65

PF Sand 4.0 4.0 27

Clayey drift 22.5 26.5 19

SS 77.0q 23–35

CP Sandy drift 3.1 3.1 43

Clay drift 8.9 12.0 23

Mudstone 19.0 31.0 13.5

SS 70q 50

GH Sandy drift 3.5 3.5 35.6

Clayey drift 16.6 20.1 14.4

Mudstone with beds of gypsumranhydrite 47.9 68.0 51

SS 116.0q 30

BH Sandy clay drift 8.0 8.0 21.5

Mudstone 38.0 46.0 20

Marl 35.0 81.0 38

Mudstone 28.0 109.0 46

SS 150.0q 34

GS Top soil 0.5 0.5

Sand and gravel 2.0 2.5 12 ?

Ž .

Red clay with chalk gravel till 5.5 8.0 11

Marl and mudstone 38.0 46.0 18

Marl with gypsum and anhydrite beds 141.0 187.0 137

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directly overlain by glacial drift deposits and in the eastern part where it is covered by both the

Ž .

MM Group and drift deposits, and 2 the poten-tial of TEM soundings to distinguish between clayey drift and mudstone rocks. Soundings near boreholes would serve to gauge the stratigraphic

mapping capability of TEM in this complex glaciated terrain. For quantitative data interpre-tation, a two-stage approach involving direct

Ž .

data transformation Meju, 1998 and optimised

Ž .

biased parameter estimation Meju, 1994 is adopted to reduce the dependence of

interpreta-Fig. 2. Comparison of TEM sounding curves for different transmitter loop sizes. The data from 20, 40 and 50-m-sided transmitter loops are represented by round, cross and triangular symbols, respectively. All the sounding curves have been

Ž .

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tional success on the availability of a priori geological information.

2. Field experiments and qualitative assess-ment of results

Six borehole locations representing different

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

specific time-band using different transmitter equipment triangular symbols for PROTEM47 and round symbols for

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Ž .

field equipment and square transmitter Tx

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

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 m2

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

ŽM.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

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

nearby borehole horizontal dashed lines with numbers refering to the relevant sequence of boundaries given in Table 1;

. Ž .

EOHsend of hole . The computed response curves for the 1-D model solid lines are superimposed on the field data in the left-hand panels.

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during the local cropping season and the trans-mitter loop positions differed inevitably from

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

Žsee, 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

Ž0.0069 and 0.069 ms of the shallowest probing.

Ž .

band 0.0069–0.704 ms appear to be slightly

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

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.

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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

borehole horizontal dashed lines with numbers refering to the sequence of boundaries given in Table 1; EOHsend 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

the commonest TEM inversion approach e.g.

Anderson, 1982; Meju, 1992 without adequate geological control.

3. Quantitative data modelling approach

A practical strategy in geophysical inversion is to bias the result towards any available geo-logical information about the sounding site. If there is no quantified site information, the best

option may be to seek smooth models that are

consistent with the field data e.g. Constable et

al., 1987 . In this paper, we adopt a two-stage approach. At the first step, the TEM apparent resistivity curve is transformed into an approxi-mately continuous resistivity–depth profile

ŽMeju, 1998, Eqs. 1 and 6 . The positions of.

significant changes in trend in this resistivity vs. depth profile are assumed to point to possible

stratal boundaries assuming that there are no

strong variations in salinity in the subsurface . In the second step, this first-pass interpretation is optimised using a constrained iterative model refinement procedure based on the biased

esti-Ž .

mation algorithm Meju, 1994

mest

y1 T

T T

₄

s A EAqD BD

Ž

.

dc qD B h

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Fig. 7. Comparison of TEM inversion result and borehole data for station PF. The symbols are as for Fig. 6. Downhole

Ž .

logging Hawkins and Chadha, 1990; Fig. 5 suggested that the upper part of SS aquifer contains mudstone bands, hence the four-layer approach in TEM inversion.

where the matrix EsWT_{W, W is a diagonal}

weighting matrix containing the reciprocals of

Ž . 4

the observed data errors si , the quantity dc

0₄

s W yqWAm is a kind of data, D is a

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Fig. 9. Comparison of TEM inversion result and borehole data for station GH. The TEM sounding employed a 20-m-sided Tx loop. The symbols are as for Fig. 6.

derivative regularisation matrix, BsbTb is a

matrix of undetermined multipliers, ys dy

Ž 0_._x

f m is the discrepancy vector and As

Ž 0_. 0

Ef m rEm is the matrix of partial derivatives

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evaluated at an initial model, m0_{. The field}

Ž .

responses apparent resistivities are contained in d and those predicted by forward modelling

Ž 0_.

are contained in f m . The vector h contains

the a priori parameter estimates inferred stratal

boundaries and resistivities towards which we wish to bias our solution. The above algorithm is applied in an iterative fashion. The logarithms of the TEM responses are considered in the

Ž 0_.4

inversion scheme, i.e. ys ln dyln f m

Ž 0_.4 0

and AsE ln f m rEm as in standard

prac-Ž .

tice e.g. Meju, 1992, 1996 . The components of

m are also taken to be the logarithms of the

resistivities and interface depths of the sought subsurface model. The above algorithm can be interpreted as incorporating some kind of a priori constraints in the inversion process as a partial remedy to the problem of

non-unique-Ž .

ness in inversion cf. Jackson, 1979 especially since we seek only a particular class of interpre-tive resistivity models; an illustrainterpre-tive example

of this model construction approach is shown in Fig. 4. Model bounds are estimated using the non-linear most-squares technique described in

Ž .

Meju 1994 ; the solution envelopes were com-puted for a threshold misfit set equal to 1.1

Ž 2_.

times the least-squares x misfit for each

Ž .

station see Meju and Hutton, 1992 as in the example shown in Fig. 5 for the optimal model presented above in Fig. 4.

The TEM data from all the borehole sites have been inverted using the approach discussed above. The resulting models are shown in Figs. 6–11. In each figure, the optimal least-squares model and the initial transformation result are shown together with the depth locations of the main lithological boundaries from Table 1. The interpretative most-squares model bounds for the least-squares models are given in Table 2. For convenience, these models will be referred to as the biased estimation models in this paper. All the TEM data were also inverted using a

Fig. 11. Comparison of TEM inversion result and borehole data for station GS. The symbols are as for Fig. 6. Note the

Ž .

resistivity undershoot depressed apparent resistivity curve around 0.07–0.2 ms whose effect might be misinterpreted as a separate conductive layer around 20 m depth on the simple resistivity–depth transformation.

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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 Vm Depth to base m Depth-correlated lithology

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

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

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

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

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

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

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

ŽLH, PF, CP, GH, BH and GS are summarised.

in Table 1. These are very generalized well logs produced by different drilling companies.

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Re-Ž .

cent downhole geophysical nuclear logging at

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

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

Žsee 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

Žpossibly of ca. 32 Vm 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 25Vm. The underlying SS contains bands of marl in its upper portion

Ž24.5–40 m where the resistivity may be ca. 34. Vm. The deeper section of the SS aquifer may have a resistivity of about 65Vm. In the biased estimation models, a cover unit of 28–30 Vm 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

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 Vm 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 Vm 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 .

Vm see Table 2 . Downhole induction logging

at this station Hawkins and Chadha, 1990, p.

238 showed that the SS aquifer has a resistivity ranging from 42 to 70 Vm; 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 Vm , 8–9-m-thick cover unit is underlain by a conductive layer of 18–20 Vm 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

ŽFig. 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 Vm 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 Vm. Note that a three-layer fixed-depth inversion

model for part of the PF data sounding

band-.

width 0.0069–1 ms yielded a resistivity about 32 Vm for the SS aquifer. Previous downhole

investigation at this site see Hawkins and

Chadha, 1990, Fig. 5 suggests that the upper

part of SS coinciding with the zone of 22–26

. Vm 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 Vm, which is in good agreement with the TEM models. Overall, there are similar patterns of resistivity

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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. 43Vm. It is underlain by 8.9 m of clay drift deposit whose resistivity may be about 23 Vm. 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 Vm. The SS aquifer appears to have a resistivity of ca. 50 Vm. The biased estimation

Ž .

models Fig. 8 and Table 1 suggest a 9–11-m-thick uppermost layer of 28–37 Vm. It is un-derlain by a layer of 13–16 Vm that extends to 28–32 m depth and is in accord with the fixed-depth result. A zone of 26–30 Vm with bottom at 43–50 m depth appears to be required by the data. The substratum has a resistivity of about

Ž .

42–65 Vm 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

Žpossibly of ca. 37 Vm resistivity , an underly-.

Ž .

ing clayey drift of 14 Vm resistivity

extend-Ž .

ing down to 20.1 m, and a resistive 51 Vm gypsiferous mudstone down to 68 m depth with the SS as the substrate of about 30 Vm 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 Vm 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 Vm resistivity extends to a depth of 21–24 m in the models and coincides

Ž .

with the clayey drift layer Table 1 . The third

layer in the model is very resistive ca. 55–64

Vm 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 Vm. 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

section to be of relatively low resistivity 30–36

Vm . 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 Vm in the depth interval 72–116 m see

(16)

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 Vm Table 1 . The biased estimation models

ŽFig. 10 and Table 2 have a 7.8–9-m-thick. Ž

topmost layer of 21–22 Vm resistivity coincid-ing fortuitously with the sandy clay drift in

Table 1 . The second layer has a resistivity of 19–20 Vm 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 Vm 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 Vm 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

downhole resistivity for station BH Hawkins

and Chadha, 1990, Fig. 8 range from 42 to 50

Vm 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 dede-posit

Žtill . 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 Vm 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 Vm 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

Ž105–141 Vm. 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 Vm 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 m2 _{are required for improved model}

identification in this locality.

5. Discussions

Ž .

From Tables 1 and 2, it is apparent that: 1 in the western part, typified by stations LH and

Ž .

PF, the clay drift or boulder clay deposit is more conductive than the confining sand drift

Ž .

and SS aquifer; 2 in the area of thin MM group subcrop such as station CP, the mudstone is more conductive than the confining clayey

Ž .

TEM method. Also, Hawkins and Chadha 1990 observed that the groundwater salinity in the SS aquifer is considerably higher where the aquifer

(17)

is confined by thick MM sequence than where drift alone covers the Sandstone; this is in ac-cord with the inference from the TEM sound-ings presented in this paper.

Ž .

Efferso et al. 1998 recently showed that the band-pass filtering of TEM responses, which is common in most commercial field equipment, affects the integrity of part of the recorded data since the low-pass filtering operation leads to damping of the high frequency content of the TEM responses; the attendant signal distortion affects model resolution and increases at lower cut-off frequencies and higher ground resistivi-ties. All the biased estimation models presented in this paper contain a 7–11-m-thick surface layer that does not match any known strati-graphic boundaries. It is tempting to interpret it as a fictitious or erroneous layer and attribute

this to the distortion of the earliest time -0.016

ms TEM response by the band-limitation of the TEM equipment. We can test the geoelectrical validity of this surficial layer by examining the

Ž .

dc resistivity vertical electrical sounding VES data from coincident locations. Fig. 12 shows the result of joint inversion of the TEM and VES data from station LH. In this figure, an interpretative layered model is shown together with the simple resistivity vs. depth

transforma-Ž

tions Meju, 1995, Eqs. 4 and 5 for VES; Meju,

1998, Eqs. 1 and 6 for TEM used to guide its construction; the depth locations of the main lithological boundaries given in Table 1 are also shown for comparison. Notice the good fit of the model responses to the VES field data and much of the TEM data. The 1-D model suggests that the depth interval 5–25 m contains a

rela-Fig. 12. Consistency analysis of coincidentally located Schlumberger VES and central-loop TEM soundings at station LH.

Ž .

The field apparent resistivity data round ornaments for VES, crosses for TEM are shown in the top left-hand panel. The

Ž . Ž .

lower left-hand panel shows the VES resistance data in ohms lower curve and the TEM voltage decay data upper curve .

Ž .

The right-hand plot shows simple resistivity–depth transformations of the data squares for VES, crosses for TEM , an

Ž .

interpretive 1-D model solid lines constructed using the simple transform model, and the depths to lithologic boundaries in

Ž .

the nearby borehole. The computed response curves for the 1-D model solid lines for VES, dashed lines for TEM are superimposed on the field data in the left-hand panels.

(18)

tively resistive unit in the upper part and a basal conductive unit in accord with the original bi-ased estimation TEM model. The same agree-ment was noted in the joint TEM and VES model for PF. However, the first three time-windows of the TEM data could not be matched in the joint interpretation process at LH and PF. As can be gleaned from Fig. 12, the early-time apparent resistivity curve is depressed relative to the synthetic response of the joint 1-D model

Žsuggesting possible damping of high frequency

content . Thus, while the presence of a resistive geoelectric horizon at around 8–10 m depth may be justified in the biased estimation mod-els, its resistivity and thickness are not well constrained by the TEM data in their present form in which the effect of the instrument’s

band-pass filtering is unaccounted for cf.

Ef-.

ferso et al., 1998 .

6. Conclusion and recommendations

The main goal of this study is to assess the usefulness of small-loop TEM soundings in mapping the top of the Sherwood Sandstone

ŽSS aquifer and for distinguishing between the. Ž

drift deposits and the Mercia Mudstones which are useful parameters for water resource deve-lopment and environmental planning in this

region . The TEM models suggest that the boundaries between the key formations may be deciphered once a resistivity trend had been judiciously determined at a few control stations. Overall, the TEM results from all the six bore-hole sites are very encouraging and suggest that the central-loop technique and the two-stage data interpretation strategy adopted in this study can be fruitfully applied to stratigraphic map-ping in this glaciated terrain. It was originally thought that the early-time windows of the PROTEM47 equipment would allow the deduc-tion of useful stratigraphic structure at shallow

Ž .

depths ca. 6–8 m in this glaciated terrain, but the field response for the first few windows

appear to be distorted by the instrument’s band-pass filtering especially in areas of

resis-Ž

tive cover materials as shown elsewhere by

Efferso et al., 1998 thus limiting their useful-ness and affecting model resolution in our adopted data processing schemes. It will there-fore be necessary to augment the TEM

sound-Ž .

ings with small spread-length ABr2F25 m dc resistivity soundings to accurately map the upper 8 m of the formations overlying the SS aquifer. It is recommended that the effect of TEM band-limitation be incorporated in routine data processing for improved near-surface

map-Ž

ping. Large transmitter loops G75 m on a

side and deeper probing TEM equipment such as the Geonics EM37 or Sirotem Mk3 will be required to map the SS aquifer in the eastern sector of the region where the confining materi-als are over 150 m thick.

Acknowledgements

The field study in 1995 was supported by a research grant from the Geology Department of

Ž .

Leicester University UK . The Geonics EM47 and PROTEM47r57 systems used in this study

Ž .

were provided by Earth Science Systems UK . The authors are grateful to the various land owners in the region and thank Emin Ulugerg-erli and C. Hatzichristodoulu of Leicester Uni-versity for assistance with the 1995 fieldwork. We thank the reviewers for the very construc-tive criticisms and in particular, Dr. N.B. Chris-tensen for pointing out the recent development in the analysis of band-limited TEM responses by Dr. Efferso and co-workers at Aarhus

Uni-Ž .

versity Denmark .

References

Anderson, W.L., 1982, Nonlinear least-squares inversion of transient soundings for a central induction loop

Ž .

system program NLSTCI . USGS Geol. Surv. Open File Report 82-1129.

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Beres, M., Haeni, F.P., 1991. Application of ground-penetrating-radar methods in hydrogeologic studies.

Ž .

Ground Water 29 3 , 375–386.

Bugg, S.F., Lloyd, J.W., 1976. Study of fresh-water lens configuration in the Cayman Islands using resistivity methods. Q. J. Eng. Geol. 9, 291–302.

Buselli, G., Barber, C., Zerilli, A., 1988. The mapping of groundwater contamination with TEM and DC meth-ods. Explor. Geophys. 19, 240–248.

Christensen, N.B., 1995a. Imaging and inversion of tran-sient electromagnetic soundings. In: EEGS Proc. Symp. Application of Geophysics to Engineering and Environ-mental Problems, Orlando, April 1995. pp. 511–517. Christensen, N.B., 1995b. 1D imaging of central loop

transient electromagnetic soundings. J. Environ. Eng. Geophys., 53–66.

Christensen, N.B., Soresen, K.I., 1994. Integrated use of electromagnetic methods for hydrogeological investiga-tions. In: EEGS Proc. Symp. Application of Geo-physics to Engineering and Environmental Problems, Boston, March 1994. pp. 163–176.

Constable, S.C., Parker, R.L., Constable, C.G., 1987. Oc-cam’s inversion: a practical algorithm for generating smooth models from electromagnetic sounding data. Geophysics 52, 289–300.

Efferso, F., Auken, E., Sorensen, K.I., 1998. Inversion of band-limited TEM responses. Geophys. Prospect., in press.

Fitterman, D.V., Stewart, M.T., 1986. Transient electro-magnetic sounding for groundwater. Geophysics 51, 995–1005.

Goldman, M., Du Plooy, D., Eckard, M., 1994a. On reducing ambiguity in the interpretation of transient electromagnetic sounding data. Geophys. Prospect. 42, 3–25.

Goldman, M., Gilad, D., Ronen, A., Melloul, A., 1991. Mapping of seawater intrusion into the coastal aquifer of Israel by the transient electromagnetic method. Geo-exploration 28, 153–174.

Goldman, M., Tabarovsky, L., Rabinovich, M., 1994b. On the influence of 3-D structures in the interpretation of transient electromagnetic sounding data. Geophysics 59, 889–901.

Hawkins, T.R.W., Aldrick, R.J., 1994. The pattern of faulting across the western sector of the Market Weighton Block, Vale of York. Proc. Yorks. Geol. Soc. 50, 125–128, Part 2.

Hawkins, T.R.W., Chadha, D.S., 1990. Locating the Sher-wood Sandstone aquifer with the aid of resistivity surveying in the Vale of York. Q. J. Eng. Geol. 23, 229–241.

Hazell, J.R.T., Cratchley, C.R., Preston, A.M., 1988. The

location of aquifers in crystalline rocks and alluvium in northern Nigeria using combined electromagnetic and resistivity techniques. Q. J. Eng. Geol. 21, 159–175. Jackson, D.D., 1979. The use of a priori data to resolve

non-uniqueness in linear inversion. Geophys. J. R. As-tron. Soc. 57, 137–157.

Koefoed, P., Biewinga, D.T., 1976. The application of electromagnetic frequency sounding to groundwater problems. Geoexploration 14, 229–241.

Lindqvist, J.G., 1987. Use of electromagnetic techniques for groundwater exploration in Africa. Geophysics 52, 456–458.

McNeill, J.D., 1987. Advances in electromagnetic methods for groundwater studies. In: Proceedings of Exploration ’87: Applications of Geophysics and Geochemistry. pp. 678–702, Geol. Surv. Can. Special Publication. Meekes, J.A.C., van Will, M.F.P., 1991. Comparison of

seismic reflection and combined TEMrVES methods for hydrogeological mapping. First Break 9, 543–551. Meju, M.A., 1992. An effective ridge regression procedure for resistivity data inversion. Comput. Geosci. 18, 99– 118.

Meju, M.A., 1994. Biased estimation: a simple framework for inversion and uncertainty analysis with prior infor-mation. Geophys. J. Int. 119, 521–528.

Meju, M.A., 1995. Simple effective resistivity–depth transformations for infield or real-time data processing. Comput. Geosci. 21, 985–992.

Meju, M.A., 1996. Joint inversion of TEM and distorted MT soundings: some effective practical considerations. Geophysics 61, 56–65.

Meju, M.A., 1998. A simple method of transient electro-magnetic data analysis. Geophysics 63, 405–410. Meju, M.A., Fontes, S.L., Oliveira, M.F.B., Lima, J.P.R.,

Ulugergerli, E.U., Carrasquilla, A.A., 1999. Regional aquifer mapping using combined VES-TEM-AMTrEMAP methods in the semiarid eastern margin of Parnaiba Basin, Brazil. Geophysics 64, 337–356. Meju, M.A., Hutton, V.R.S., 1992. Iterative most-squares

inversion: application to magnetotelluric data. Geophys. J. Int. 108, 758–766.

Palacky, G.J., Ritsema, I.L., De Jong, S.J., 1981. Electro-magnetic prospecting for groundwater in Precambrian terrains in the Republic of Upper Volta. Geophys. Prospect. 29, 932–955.

Patra, H.P., 1970. Central frequency sounding in shallow engineering and hydro-geological problems. Geophys. Prospect. 18, 236–254.

Raiche, A.P., 1984. The effect of ramp function turn-off on the TEM response of a layered ground. Explor. Geophys. 15, 37–41.

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improve-ment in geoelectrical soundings applied to groundwater investigations. Geophys. Prospect. 41, 207–227. Sinha, A.K., 1990. Stratigraphic mapping of sedimentary

formations in southern Ontario by ground electromag-netic methods. Geophysics 55, 1148–1157.

Sorensen, K.I., 1996. Pulled array continuous electric pro-filing. First Break 14, 85–90.

Wilt, M.J., Williams, J.P., 1989. Layered model inversion

of central-loop TEM soundings near a geological con-tact. Explor. Geophys. 20, 71–73.

Worthington, P.F., Griffiths, D.H., 1975. The application of geophysical methods in the exploration and develop-ment of sandstone aquifers. Q. J. Eng. Geol. 8, 73–102. Zhdanov, M.S., Traynin, P.N., Portniaguine, O., 1995. Resistivity imaging by time-domain electromagnetic migration. Explor. Geophys. 26, 186–194.

(1)

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.

Ž .

band-Ž .

limitation low-pass filtering of the field

re-Ž .

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 .

Ž .

ing clayey drift of 14 Vm resistivity extend-Ž . ing down to 20.1 m, and a resistive 51 Vm gypsiferous mudstone down to 68 m depth with the SS as the substrate of about 30 Vm 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 .

with the clayey drift layer Table 1 . The third Ž

layer in the model is very resistive ca. 55–64 .

Ž .

20-m-sided Tx loop Fig. 9 also showed the SS Ž section to be of relatively low resistivity 30–36

Vm in the depth interval 72–116 m see

. Hawkins and Chadha, 1990, Fig. 7 .

(2)

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 .

20 Vm Table 1 . The biased estimation models ŽFig. 10 and Table 2 have a 7.8–9-m-thick.

topmost layer of 21–22 Vm resistivity coincid-ing fortuitously with the sandy clay drift in

Table 1 . The second layer has a resistivity of

19–20 Vm 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 Vm 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 Vm 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

downhole resistivity for station BH Hawkins .

and Chadha, 1990, Fig. 8 range from 42 to 50 Vm 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 dede-posit Žtill . 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 Vm 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 Vm 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 Ž105–141 Vm. 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 .

identification in this locality.

5. Discussions

Ž . From Tables 1 and 2, it is apparent that: 1 in the western part, typified by stations LH and

Ž .

PF, the clay drift or boulder clay deposit is more conductive than the confining sand drift

Ž .

and SS aquifer; 2 in the area of thin MM group subcrop such as station CP, the mudstone is more conductive than the confining clayey

Ž .

drift and SS aquifer; and 3 the gypsiferous or anhydritic MM units are more resistive than both the overlying glacial drift and the underly-ing SS aquifer. It would thus appear that some of the problematical issues of subsurface map-ping in this region can be addressed using the Ž . TEM method. Also, Hawkins and Chadha 1990 observed that the groundwater salinity in the SS aquifer is considerably higher where the aquifer

(3)

is confined by thick MM sequence than where drift alone covers the Sandstone; this is in ac-cord with the inference from the TEM sound-ings presented in this paper.

Ž .

this to the distortion of the earliest time -0.016 .

ms TEM response by the band-limitation of the TEM equipment. We can test the geoelectrical validity of this surficial layer by examining the Ž . dc resistivity vertical electrical sounding VES data from coincident locations. Fig. 12 shows the result of joint inversion of the TEM and VES data from station LH. In this figure, an interpretative layered model is shown together with the simple resistivity vs. depth

transforma-Ž

tions Meju, 1995, Eqs. 4 and 5 for VES; Meju, .

rela-Fig. 12. Consistency analysis of coincidentally located Schlumberger VES and central-loop TEM soundings at station LH.

Ž .

The field apparent resistivity data round ornaments for VES, crosses for TEM are shown in the top left-hand panel. The

Ž . Ž .

lower left-hand panel shows the VES resistance data in ohms lower curve and the TEM voltage decay data upper curve .

Ž .

The right-hand plot shows simple resistivity–depth transformations of the data squares for VES, crosses for TEM , an

Ž .

interpretive 1-D model solid lines constructed using the simple transform model, and the depths to lithologic boundaries in

Ž .

the nearby borehole. The computed response curves for the 1-D model solid lines for VES, dashed lines for TEM are superimposed on the field data in the left-hand panels.

(4)

Ž band-pass filtering is unaccounted for cf.

Ef-. ferso et al., 1998 .

6. Conclusion and recommendations

The main goal of this study is to assess the usefulness of small-loop TEM soundings in mapping the top of the Sherwood Sandstone ŽSS aquifer and for distinguishing between the.

Ž drift deposits and the Mercia Mudstones which are useful parameters for water resource deve-lopment and environmental planning in this

Ž .

depths ca. 6–8 m in this glaciated terrain, but the field response for the first few windows

appear to be distorted by the instrument’s band-pass filtering especially in areas of

resis-Ž

tive cover materials as shown elsewhere by .

Efferso et al., 1998 thus limiting their useful-ness and affecting model resolution in our adopted data processing schemes. It will there-fore be necessary to augment the TEM

sound-Ž .

map-Ž

ping. Large transmitter loops G75 m on a .

Acknowledgements

The field study in 1995 was supported by a research grant from the Geology Department of

Ž .

Leicester University UK . The Geonics EM47 and PROTEM47r57 systems used in this study Ž . were provided by Earth Science Systems UK . The authors are grateful to the various land owners in the region and thank Emin Ulugerg-erli and C. Hatzichristodoulu of Leicester Uni-versity for assistance with the 1995 fieldwork. We thank the reviewers for the very construc-tive criticisms and in particular, Dr. N.B. Chris-tensen for pointing out the recent development in the analysis of band-limited TEM responses by Dr. Efferso and co-workers at Aarhus