Ž .
Journal of Applied Geophysics 44 2000 217–236 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
b
, T.R.W. Hawkins
c
a
EnÕironmental and Industrial Geophysics Research Group, Department of Geology, UniÕersity of Leicester, UniÕersity Road, Leicester LE1 7RH, UK
b
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 .
spread-length ABr2 F 25 m dc resistivity depth soundings to accurately map the near-surface in this glacial-covered
terrain. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electromagnetic sounding; Resistivity inversion; Stratigraphy; Glacial terrain
Corresponding author. Tel.: q44-116-252-3628; fax: q44-116-252-3628.
Ž .
E-mail address: mxwle.ac.uk M.A. Meju .
1. Introduction
Geological mapping of aquifers in gently dip- ping formations is straightforward in areas with
0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 9 2 6 - 9 8 5 1 0 0 0 0 0 0 5 - 7
ample rock outcrops. In areas with little or no Ž
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
Ž
. e.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 Ž
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
Ž 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
Ž 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 Ž
. MM 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
Ž 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 Ž
. Ž
. Ž
. Ž
. CP , Bolton House Farm BH , Link Hall LH , Grange Farm-Shiptonthorpe GS and Grange Farm-Upper Helmseley
Ž .
GH .
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
Ž
. Hawkins 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 Ž
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 Ž
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
Ž
. Hawkins and Chadha, 1990 . Owing to the
difficulty of deploying large loops and the over- riding need to minimize lateral effects on the
Ž 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 V m 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
73.0 q 65
PF Sand
4.0 4.0
27 Clayey drift
22.5 26.5
19 SS
77.0 q 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
70 q 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
116.0 q 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
150.0 q 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
SS 214.0 q
3
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 .
corrected for the effect of transmitter turn-off Raiche, 1984 .
tional success on the availability of a priori geological information.
2. Field experiments and qualitative assess- ment of results
