Near Surface Geophysics, 2008, 185-194
Geophysical characterization of derelict coalmine
workings and mineshaft detection:
a case study from Shrewsbury, United Kingdom
J.K. Pringle1*, I.G. Stimpson1, S.M. Toon1, S. Caunt2, V.S. Lane1,
C.R. Husband1, G.M. Jones1, N.J. Cassidy1 and P. Styles1
1
Environmental Physical Sciences and Applied Mathematics (EPSAM) Research Institute, School of Physical Sciences & Geography,
Keele University, Keele, Staffordshire, ST5 5BG, UK
2
The Coal Authority, 200, Lichfield Lane, Berry Hill, Mansfield, Nottinghamshire, NG18 4RG, UK
Received July 2007, revision accepted March 2008
ABSTRACT
A study site of derelict coalmine workings near Shrewsbury, United Kingdom was the focus for
multi-phase, near-surface geophysical investigations. Investigation objectives were: 1) site characterization for remaining relict infrastructure foundations, 2) locate an abandoned coalmine shaft, 3)
determine if the shaft was open, filled or partially filled and 4) determine if the shaft was capped
(and if possible characterize the capping material).
Phase one included a desktop study and 3D microgravity modelling of the relict coalmine shaft
thought to be on site. In phase two, electrical and electromagnetic surveys to determine site resistivity and conductivity were acquired together with fluxgate gradiometry and an initial
microgravity survey. Phase three targeted the phase two geophysical anomalies and acquired
high-resolution self potential and ground penetrating radar datasets. The phased-survey approach
minimised site activity and survey costs.
Geophysical results were compared and interpreted to characterize the site, the microgravity
models were used to validate interpretations. Relict buildings, railway track remains with associated
gravel and a partially filled coalmine shaft were located. Microgravity proved optimal to locate the
mineshaft with radar profiles showing ‘side-swipe’ effects from the mineshaft that did not directly
underlie survey lines.
Geophysical interpretations were then verified with subsequent geotechnical intrusive investigations. Comparisons of historical map records with intrusive geotechnical site investigations
show care must be taken using map data alone, as the latter mineshaft locations was found to be
inaccurate.
INTRODUCTION
The successful detection of near-surface voids and mineshafts
using geophysical techniques has been well demonstrated (see
Maxwell 1976; McCann et al. 1987; Styles et al. 2005; Branston
and Styles 2006). These methods, in comparison to geotechnical
methods, can offer a relatively inexpensive and rapidly deployable solution that can provide a more detailed site characterization
before intrusive investigations of targeted geophysical anomalies
(using conventional boreholes and trial-pits) are undertaken.
Bulk ground resistivity and conductivity surveys can be used to
actively measure the electrical properties of a volume of material
below a sample position (see Reynolds 1997) and have been
* j.k.pringle@esci.keele.ac.uk
© 2008 European Association of Geoscientists & Engineers
shown to be very effective at detecting cleared building foundations and, indeed, mineshafts in some cases (McCann et al. 1987).
Resistivity measurements determined using direct electrical methods are typically less sensitive to the cultural ‘noise’ produced
from surface objects such as metal fences and oil tanks than conductivity measurements obtained using electromagnetic (EM)
methods. Magnetic surveys are sensitive to near-surface ferrous
materials, which can be associated with building foundations,
railway tracks or other near-surface metal. Magnetometry has also
been shown to be successful at detecting mineshafts because scrap
metal was often thrown into the shaft during filling in at abandonment (McCann et al. 1987). Fluxgate gradiometry surveys measure the local magnetic field gradient over a sample position
between two vertically orientated fluxgate magnetometers. These
results are, therefore, more sensitive to near-surface survey site
185
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J.K. Pringle et al.
FIGURE 1
Annotated a) location map with b)
site map and c) site photomosaic.
b) Shows location and direction
where c) photomosaic was taken.
Images supplied by Ordnance
Survey/EDINA service. © Crown
Copyright Database 2007.
FIGURE 2
a) Map of Shrewsbury coalfield and b) Local Carboniferous Coal
Measures stratigraphy. Site location and the likely worked seam are
marked. Adapted from Hull (1873).
material but can also suffer from surface cultural ‘noise’.
Although relatively slow to acquire, micro-gravimetric surveys have been shown by many authors to produce high-resolution maps of near-surface voids or cavities (Fajklewicz 1976;
Butler 1984; Emsley and Bishop 1997; Styles et al. 2005;
Branston and Styles 2006). Two- (and three-) dimensional
microgravity profiles can also be modelled and inverted; there-
fore, allowing the size and depth of voids below ground level
(bgl) to be estimated.
The self potential (SP) method measures surface electrical
current differences from their associated voltage (potential) signals and have been recently used to locate subsurface voids
(Quarto and Schiavone 1996; Jardani et al. 2006). Subsurface
voids have been found to create both positive and negative SP
anomalies; the polarity of the SP anomaly is believed to be due
to the composition of cavity fill. The exact mechanisms responsible for creating SP anomalies over cavities are subject to debate
(Green 2000; Lange 2000), but the general consensus is that the
SP anomalies can be attributed to the preferential percolation of
water within or around the cavity. SP has been successfully
shown by other authors to detect mineshafts (BGS Report 2004;
Wilkinson et al. 2005).
The ground-penetrating radar (GPR) method is commonly
used in geotechnical engineering, for example, to determine soil
and rock stratigraphy (Davis and Annan 1989), detect pavement
voids (Nichol and Reynolds 2001) and detecting cleared building
foundations, cellars and utility services (Grandjean et al. 2000).
This is due to both the comparatively high-resolution data
recorded from higher frequency (225 MHz) near-surface GPR
systems and their relative ease of use. GPR has also been used
successfully for mineshaft detection (Kulessa et al. 2004).
This paper presents multi-disciplinary research over derelict
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
Characterization of coalmine workings and mineshaft detection
FIGURE 3
Historical site records a) 1882, b) 1902, c) 1954 and d) 1976, showing
the differing positions of the mineshaft, spoil heap, onsite buildings and
a rail system. © Crown Copyright and Land Information Group Ltd
2007, all rights reserved.
187
coal mine workings located close to the village of Hanwood,
near Shrewsbury, Shropshire, UK (Fig. 1). Geological records
indicate that Devonian glacial-till drift deposits overlie Keele
Formation sandstones, which themselves overlie Carboniferous
Upper Coal Measures of the Hanwood Coalfield (Fig. 2a), part
of the larger Shrewsbury Coalfield (Hull 1873). The three main
seams worked in the area were the ‘Yard’, ‘½ Yard’ and ‘Best
Coal’ seams (Fig 2b), with records (Brown 1976) which showed
that the site was locally worked by the Arscott Colliery. All commercial mining ceased in the area in the 1940s (Trueman 1954).
Today, the site lies within the garden of a private residence
(Fig. 1c) and has been landscaped with clay-rich topsoil,
although there is estimated to be (8 m+) thickness of mine spoil
still present below ground level. An approximately 2 m high,
irregularly-shaped raised area to the East was presumably former
mine spoil, as shown on historical records (Fig. 3).
Geophysical results of bulk ground resistivity, micro-gravity, SP, magnetic and GPR geophysical surveys conducted
within the site grounds in 2006 are detailed together with the
subsequent 2007 documented intrusive investigation of the
targeted geophysical anomalies. The objectives for the investigation were to: 1) characterize the site for remaining relict
coalmine infrastructure foundations, 2) locate the abandoned
coalmine shaft, 3) determine if the shaft was open, filled or
partially filled and 4) determine if the shaft was capped (and if
possible characterize the capping material).
FIGURE 4
3D MATLAB™ microgravity
models of an a) open, b) partial
and c) filled mineshaft with d) 2D
profile comparisons. The modelled mineshaft is based on historical records and is 2.5 m in
diameter and 110 m deep.
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
188
J.K. Pringle et al.
METHODOLOGY
A three-phase survey approach was used during this case study.
Phase one entailed a ‘desk-study’ investigating the historical
records to assess both the likely position of relict mine infrastructures and the possible size of the coalmine shaft. Phase two
entailed acquiring geophysical datasets over the part of the survey area identified by the phase one desk study, whilst phase
three collected high-resolution data over any anomalous geophysical areas identified during phase two. Bulk ground resistivity, conductivity, magnetic fluxgate gradiometry, microgravity,
Self Potential and Ground Penetrating Radar techniques were all
employed.
PHASE I DESK STUDY
The desktop study initially sourced historical records of the site,
archive maps (Fig. 3) showing various onsite mine buildings and
related infrastructure within the last 150 years. The buildings,
railway and other features, however, have differing positions on
the various map series, partly due to different map projections,
and therefore cannot be relied upon to indicate the true location
of a feature. This is not uncommon, as transference between successive versions of mine abandonment plans often leads to misplotting, sometimes duplication and even omission of data. The
records do, however, show the site was locally worked by the
Arscott Colliery.
A 1930 Coal Mine Abandonment Plan (7227) held by The
Coal Authority also showed that a vertical access mineshaft
(No. 3: 843308-001), was present onsite. Specific information
about the shaft was not available, although the plan does show
that the ½ Yard (0.5 m) coal seam was located 110 m
(120 yards) below ground level; this was the seam presumably
mined at the time of abandonment. Records from the nearby
Hanwood Mine (Brown 1976) suggest that the shaft may be
approximately 110 m deep and 2.5 m in diameter. It was not
known whether the shaft had been filled and/or capped and if
so, what material was used. Prior to 1945, shafts were generally filled with unregulated materials and caps were of a poor
standard. The exact shaft location cannot be determined from
historical maps alone; although several of these suggest that it
lies approximately 10 m to the southeast of a now demolished
building (the footprint of which lies within the present-day
property). A ~50 m by ~20 m irregularly-shaped area to the
south of the present house was therefore decided upon for the
initial acquisition of the geophysical data.
3D microgravity modelling of a 2.5 m diameter, 110 m deep
mineshaft was also undertaken using MATLAB™ software.
Both the likely horizontal and vertical microgravity gradient
responses were modelled to assess the likely anomaly size and
extent that may be produced from an open, partially filled and
completely filled mineshaft (Fig. 4). The modelled likely anomalies for these variations would be approximately 100 μGal,
80 μGal and 10 μGal respectively, compared to background values (see Fig. 4d).
PHASES II AND III GEOPHYSICAL SURVEYING AND
DATA PROCESSING
Following the initial site inspection and topographic survey
using Leica 1200 total station equipment, a northeast-southwest
orientated, 1 m-spaced, survey grid was created on the site,
located to the south and east of the present-day building (see
Fig. 5). The initial grid provided the framework over which the
phase II bulk ground resistivity, conductivity, SP and magnetics
geophysical techniques could be deployed. Each of these techniques took ~1 day each, with a two-man field team. These data
were acquired at 0.5 m-spaced sampling positions on each survey line in a south to north, one-way pattern. All geophysical
data sampling positions were also accurately topographically
surveyed using a Leica 1200 System total station theodolite.
For the bulk ground resistivity survey, a RM4 Geoscan resistance meter, mounted on a custom-built, twin-probe array on a
mobile frame that featured two 0.1 m-long steel probes set 0.5 m
apart, was used. Reference probes were placed 0.75 m apart and
0.1 m into the soil, positioned 20 m from the survey grid following standard methodologies (Milsom 2001). For the conductivity
survey, a Geonics EM38B™ instrument was carefully zeroed
over a conductively quiet area of the site for site calibration.
In-phase and quadrature readings were obtained for both vertical
(VMD) and horizontal (HMD) component orientations. Typical
maximum penetration depths using this device in VMD and
HMD mode is ~1.5 m and ~0.75 m below ground level, respectively. VMD and HMD conductivity surveys were separately
acquired to avoid any potential interference from the different
EM fields. For the magnetic survey, a Geoscan FM18™ fluxgate
gradiometer was carefully zeroed over a magnetically quiet area
of the site to remove any potential reading differences that may
result from positional variations in instrument orientation relative to magnetic north when acquiring the data (see Reynolds
1997). Magnetic data were then acquired on a south to north,
one-way pattern. SP Lead-Chloride based, non-polarizing probe
electrodes were used to collect the SP surveys with a reference
probe being placed 5 m outside the grid, whilst the mobile probe
was placed ~5 cm into the ground at each sampling position.
For each of the geophysical datasets obtained, sample position recorded readings were initially transferred to digital spreadsheets and converted to x,y, value formats. A median filter (using
a block size of 1 m × 1 m) was applied, before a minimum-curvature gridding surface algorithm (using a block size of 0.5 m ×
0.5 m) was created. Any linear site trends were then removed,
before both full site and anomaly subset areas were gridded and
contoured as x,y, value data.
Phase III gravity data were acquired using a Scintrex CG5™
gravimeter on each 2 m-spaced sample position using three,
45-second sampling periods for data quality control. A base station reference was collected at the beginning and end of each day
and at approximately 1 hour intervals. Two sets of three 45-second data periods were acquired at each base station measurement
and the relative instrument elevations determined from the total
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
Characterization of coalmine workings and mineshaft detection
189
FIGURE 5
Plan (or map) view plots of processed: a) micro-gravity, b) bulk
ground resistivity, c) Fluxgate
gradiometry and d) SP data collected at the survey site.
Interpreted and numbered anomalies are marked (see text). The
property’s structure is shown in
red in the top-left part of the figure. Background map is from the
1902 Ordnance Survey (Fig. 3b).
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
190
J.K. Pringle et al.
FIGURE 6
2D GPR profiles L1–4 (location on map inset) with interpreted site features and anomalies highlighted. Note the ‘side-swipe’ feature evident in L4 was
probably related to the mine-shaft.
station measurements. In total, 124 gravity readings were made
(with a 0.0012 Standard Deviation or SD) with 57 re-observations and 31 base station observations (with a 0.0019 SD). Data
were reduced to Bouguer Anomalies, using an initial site ground
density of 2.2 Mg m-3, based on the typical subsurface conditions
anticipated at the site. Elevation corrections were obtained from
the accurately surveyed total station positions (with an average
positional error of 0.5 cm). Ideally, the adjacent ~2 m-high, mine
spoil heap should have also been surveyed, as this extra mass
could have interfered with micro-gravity results, but it could not
be accessed due to the dense, deciduous woods and vegetation
cover. The CG5 was also able to correct for the majority of temporal variations using in-built software, whilst any residual temporal effects due to instrument drift and earth tides were removed
using the base station values. The instrument raw files were
edited and reduced using in-house software. The base station
residuals were always less than 5 μGal. A regional gravity gradi-
ent across the study site was also removed using in-house
processing procedures and software.
Phase III GPR data were also acquired following initial tests
with PulseEKKO™ 1000 225 MHz and 450 MHz antennae to
determine the optimum frequency. 225 MHz antennae were
subsequently used to acquire the 2D, fixed-offset (0.25 m) bistatic, co-planar, broadside profiles over the survey lines
(Fig. 6) with 5 cm trace spacing and an 80 ns time window.
Thirty-two repeat pulse stacks were used on all profiles to
improve the signal-to-noise ratio (see Milsom 2001). GPR
PulseEKKO™ raw data files were imported into REFLEX-W™
processing software. For each 2D profile, the first break arrival
was picked and flattened to 0 ns. A time-cut was applied to
remove blank data at the base of each profile before standard
‘dewow’ and DC_shift’ filters were applied. A manual gain
function was also applied to boost deeper reflection event
amplitudes. A site average velocity of 0.11 m/ns, obtained by a
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
Characterization of coalmine workings and mineshaft detection
common-mid-point (CMP) profile, was used to convert the 2D
profiles from time (nanoseconds) to depth (metres). The resulting profiles were then exported as images (Fig. 6).
RESULTS AND INTERPRETATION
The interpreted microgravity results identified four anomaly lows
with respect to background values (G1-G4 on Fig. 5a). Comparisons
of these anomalies with the modelled profiles (Fig. 4) indicated
that anomaly G1 was the most likely position for a mineshaft, due
to its size and low anomaly amplitude (approximately –55 μGal
with respect to background values). The G1 low anomaly value
was probably accentuated by the digital surface gridding algorithm, due to its edge position. The mineshaft was theorized to be
partially filled (Fig. 4). Gravity anomaly low G2 (approximately
–20 μGal with respect to background values) was bounded to the
north by an old railway track shown on the historical records and
may, therefore, be attributed to different ground material from the
surrounding area, perhaps related to the presence of the railway.
Gravity anomaly G3 (approximately –10 μGal with respect to
background values) correlated with a now-demolished building
shown on the 1903 historical map and was likely to be related to
the foundations or demolished remains of this feature. The origin
of Gravity anomaly G4 (approximately –10 μGal with respect to
background values) was unknown but was of similar amplitude as
anomaly G2 and may therefore be due to the presence of similar
demolition material.
The resistivity results showed a high (400 Ωm) anomaly (R1
on Fig. 5b), with a SE–NW orientation. A high resistivity
anomaly would be expected over an open shaft (as air is highly
resistive) but the anomaly was relatively dispersed and showed a
preferred orientation that was inconsistent with the presence of a
vertical shaft. A comparison of the R1 anomaly to the 1903 maps
shows it was bordered to the north by a railway track. It was also
noted that during the resistivity acquisition the electrodes penetrated a coarse ‘gravel’ layer in this area that may be railway
ballast (or similar, higher-resistivity material), which may be
responsible for this anomaly. A resistivity (200 Ωm) anomaly
(R2 on Fig. 5b) was also observed in the western part of the study
area. This was also interpreted to be a similar high-resistivity
material, probably a gravel/ballast layer.
In contrast to the resistivity, the conductivity results were
generally poor. This could be due to the limited penetration depth
of the instrument used. Two strong high isolated anomalies
(25 mS/m with respect to background values) that could be correlated to an onsite household oil tank and an observed, protruding metal spike in the ground that was most probably masking
out the other site features. Attempts to filter these values out were
unsuccessful; these data are therefore not shown for brevity.
The fluxgate gradiometry results (Fig. 5c) showed a low magnetic anomaly (–80nT compared to background values) M1 that
was rectangular in shape. This anomaly was interpreted as being
man-made due to its highly regular shape but could not be correlated to any features in the historical records. This was there-
191
fore assumed to be a modern feature, possibly a septic tank. A
magnetic high, anomaly M2 (200 nT) was not explained, whilst
a similar anomaly (150 nT) to the north of the grid (M3) was
interpreted as ferrous-rich, near-surface ground fill. Anomaly M4
was interpreted as being associated with the septic tank (possibly
a pipe?) whilst the anomalies M5–8 correlated to the railway
tracks shown in the 1903 map. These were interpreted as relict
railway anchor pins that were used to tie the sleepers to the
ground and left in place after the track was removed.
The SP results (Fig. 5d) showed a clear band of large, high
readings (anomaly S1 – 10 mVolts compared to background
values), which was also bounded by the old railway track to the
north. This was therefore interpreted to be associated with the
gravel/ballast area as it correlated with the resistively high
FIGURE 7
Summary plot showing the positions of interpreted geophysical anomalies.
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
192
J.K. Pringle et al.
FIGURE 8
Site investigation: a) discovered mineshaft, b) partial excavation, c) schematic of the shaft and remediation solution. Note feature 1 in b) shows a loose
infill area that may be associated with location of shaft equipment (e.g. winch foundations, etc).
anomaly R1. SP anomaly S2, however, seemed to be comprised
of three low areas in a triangular pattern that correlated with the
magnetic (M1) anomaly and was, therefore, interpreted as being
associated with this modern feature.
The 225 MHz 2D GPR profiles, L1–4, have been interpreted
and sub-divided into different areas, based on their GPR characteristics (Fig. 6). These differences could be correlated across the
profiles, shown schematically in the map-view interpretation of
Fig. 6.
On the L1 2D profile, a weak, chaotic reflective area (located
between 2 – 9 m along the profile) could be correlated with the
1902 building plan and was interpreted as either building foundations or infill. Several deep, half-parabola point-source reflectors
(at 12 and 14 m) could be correlated with the magnetic low (M1)
anomaly and was thus interpreted as the modern, possibly septic
tank, feature. Strong reflectors were evident (18 – 22 m) in the
region where gravel/ballast has been interpreted from the resistivity surveys, whilst planar reflections were encountered where
railway tracks were located on the 1902 map (22 – 36 m). Lastly,
an area of chaotic reflectors (36 – 43 m) was interpreted to be
disturbed ground.
On the L2 2D profile, an area (4 – 14 m) showing strong
planar reflectors was interpreted as natural ground before being
succeeded by an area (14 – 19 m) that had deep parabolas present
and was similar to features observed on the L1 2D profile. These
deep parabolas were therefore interpreted as being the septic
tank. Northward dipping reflectors were then observed (19 –
24 m) that were interpreted to be the gravel area, before more
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
Characterization of coalmine workings and mineshaft detection
planar reflectors were observed (24 – 36 m) that were interpreted
as the railway track area. The northerly part of the profile was
then composed of chaotic reflectors that were interpreted as
being made ground.
On the L3 2D profile, an area (0 – 13 m) showed strong, planar reflectors and was interpreted as natural ground with several
deep, parabolic, point-source reflectors (at 15 and 12 m) being
located at the start of the profile that may indicate isolated, buried features. The dipping reflectors present between 13 – 27 m
were interpreted as being associated with the gravel/ballast layer,
together with ‘side-swipe’ effects from the modern house foundations (this can be seen near the bottom of the profile). Poor
data quality complicates the data interpretation in the old railway
track area (now a rockery – 27 – 40 m), before interpreted made
ground was encountered at the end of the profile.
On the short L4 2D profile, chaotic reflectors were initially
observed (probably due to acquiring data over the rockery area)
before an area (6 – 11 m) of chaotic reflectors was observed. This
latter area was very close to the presumed mine-shaft location, and
had a strong amplitude, deep, westward dipping reflector/quasiparabolic reflection near the base of the profile. This feature was
most probably related to a sub-surface object that did not lie
directly underneath the profile, as there were no likely aboveground structures in this area that could be the other cause. This
parabola was located close to a gravity anomaly (G1) that was
suggested to be the mineshaft and thus added further geophysical
evidence for the mineshaft to be located at this position.
GEOTECHNICAL INTRUSIVE INVESTIGATIONS AND
SUBSEQUENT REMEDIATION
Subsequent intrusive site investigations undertaken by geotechnical engineers found the mineshaft to be centered approximately 8 m south-east of the property garage (Fig. 8a). The
mineshaft diameter was also confirmed to be 2.5 m, based upon
probes drilled around this position. The shaft was filled with
superficial deposits of rubble, and with no open entry into the
actual mine workings. It was calculated that there was only 15 m3
of material missing from the shaft, as 30 tonnes of grout were
needed to fill the small voids in the mineshaft fill.
During excavations, supporting wall brickwork was found to
the north of the shaft (Fig. 8b) with quite different fill than the
mineshaft, with the floor of this feature having an apparent 20º
dip to the north. It was difficult to interpret the original use of
this feature but it could be an access walkway to the shaft
entrance or foundations for winding equipment, but these interpretations are speculative. If this feature continues to the north of
the shaft, then it could be correlated with the magnetic (M3)
anomaly (Fig. 5c).
It is commonly considered best practice to remediate old UK
mineshafts by drilling to the rock head, stabilizing the shaft, filling
it with concrete and then capping with reinforced concrete from
the rock head to the surface. However, it was not possible to do this
on this site, due to the thick superficial mixture of mine spoil and
193
soil deposits (8 m thick) overlying the rock head and the proximity of both the spoil mound and site boundary to the shaft. A concrete ‘plug’ was therefore installed from 0 – 2 m below ground
level with grouting around this plug to stabilize it (see Fig. 8c). It
was also recommended that no development should take place
within 5 m of the mineshaft (Edmondson, pers. comm.).
DISCUSSION
Of the geophysical surveys trialled, microgravity was deemed
the optimal location technique, since although the subsequent
intrusive investigations confirmed that the mineshaft was located
at the very edge of the survey area, microgravity still identified
its location. The 3D microgravity modelling of the coalmine
shaft prior to data acquisition was highly useful, as it helped to
identify its likely location when interpreting the phase two geophysical results. This approach allowed the detailed phase three
geophysical investigations to be undertaken in the geophysically
anomalous areas. Modelling also allowed the likely characteristics of the shaft fill to be ascertained, which was important information for the geotechnical engineers before initiating site
remediation. The microgravity and GPR survey also delineated
the building located at the south of the survey site that was on the
1902 historical map.
The railway track that was shown on the 1902 historical map
had a significant effect on the site, although no surface evidence
remains. Various magnetic anomalies (M5–8) could be correlated
to the track position that are speculated as being potential relict
sleeper anchor tie pins or associated metallic remains. The southerly railway track also provided a sub-site border with an interpreted gravel/ballast area located directly to the south that was
identified by microgravity (G2), SP (S1) and bulk ground resistivity (R1) techniques.
A magnetic (M1) and SP (S2) low anomaly area was identified that did not correlate with the historical features, but this was
interpreted as post-dating the 1902 maps and probably associated
with the building phase of the present house. It was also delineated in high resolution by the GPR profiles during the phase
three investigations and was interpreted as being related to a
septic tank.
Generally, the GPR results provided the high-resolution,
stratigraphic information necessary to confirm the other geophysically identified target anomalies, and most importantly,
observed ‘side-swipe’ effects on the L4 profile added weight to
the microgravity anomaly G1 being the coalmine shaft. GPR
results showed that even if GPR profiles were not directly over
targets, they can still be detected. Numerous, relatively deep,
parabola point-sources were also identified over the site, which
probably indicated relict buried objects associated with the
workings.
Due to the confined site access conditions, several of the
important geophysical anomalies were located close to survey
boundaries and this had a bearing on the processing and interpretation of the geophysical datasets. Despite this apparent limita-
© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
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J.K. Pringle et al.
tion, it has still been possible to identify and locate the key features of the site and provide accurate information for the process
of engineering an appropriate remediation strategy.
It should also be noted that in the investigation of former
industrial sites, historical map records should be used with caution due to their lack of accuracy. The 1976 map, for example,
showed features that had a spatial footprint of over 10 m different
from both other map projections and the true location of the
modern site (determined by both GPS and modern maps).
CONCLUSIONS
For this complex, derelict coalmine site of limited survey extent,
the multi-technique geophysical investigations have been found to
be very useful for site characterization and it was possible to
extract the maximum amount of relevant site information in order
to aid the subsequent targeted geotechnical investigations. Using
multiple geophysical techniques allowed a three-phased survey
approach to be adopted, minimizing site activity, surveying and
potential costs. Undertaking 3D microgravity modelling prior to
surveying also allowed the most likely geophysical anomalies to
be further investigated, which subsequently reduced survey time
onsite. Due to the presence of ‘side-swipe’ effects from the subsurface features, GPR was shown to assist in target detection even
when survey lines were not directly coincident with the casual
features. The results of this study have shown that despite the
apparent complexity of the site, near-surface geophysics can benefit geotechnical engineers in confined, derelict industrial sites by
rapidly characterizing the ground conditions and identifying
anomalous areas for subsequent intrusive investigations.
ACKNOWLEDGEMENTS
Colin Barnett is acknowledged for access to the site. We are
grateful to Keith Edmondson for geotechnical borehole information and for Figure 8(b). The Keele University geophysical and
survey equipment used during this research were obtained from
a 2003 UK HEFCE equipment bid. Geomatrix Ltd is acknowledged for the loan of Geonics EM38B™ equipment for the
project. This manuscript benefited from reviews by Reinhard
Kirsch, Ugur Yaramanci and an anonymous reviewer.
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© 2008 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2008, 6, 185-194
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