environmental forensic geophysical survey for a buried canal … › sites › default › files ›...

Keele University Nov. 2010

1

Environmental Forensic Geophysical Survey for a buried canal barge, Burslem, Stoke-on-Trent, Staffs

© Jamie Pringle, 2010

November 2010

Confidential Report


2

Contents Page Section Page no. 1 Project Aims 3 2 Project Background 3 3 Method 3 4. Results & Interpretation 4 5. Conclusions 5 6. Acknowledgements 5 7. References 5 Figures 6-13 Disclaimer 13


3

1. Project Aim To undertake geophysical surveys, in order to determine the location of the buried coal-barge ‘Elizabeth’. 2. Background The Burslem Port Volunteers approached Keele with a view to assist them to locate a buried coal barge in part of a filled-in artificial canal, the Burslem Branch Canal, that opened in 1805 and closed in 1961 after a major breach (Fig. 1). If the barge is subsequently found to be in a good condition, then it will be recovered and restored. Contemporary photographs suggested a section of the filled-in canal may prove fruitful for investigations (Fig. 2). A 1999 investigation, undertaken by Alan Lovelock (technique unknown) had determined three likely locations at the southern end of the canal, where it joined the still-functioning Trent & Mersey Canal (Fig. 3). Available background information (from EDINA) showed the site bedrock geology to be the Etruria Formation mudstones, sandstone and conglomerates with alluvium top-soil, albeit all made-ground. Once an initial reconnaissance site visit had been undertaken (27th May 2010), it was found that there was significant difficulties onsite for a geophysical survey; most of the canal itself had been infilled with a variety of artificial materials, including metal slag from the now-demolished Shelton Bar steelworks, a significant proportion of the canal had been subsequently removed by a ~20 m deep, storm drain, metal fences bordering the survey area with metal and domestic appliance recycling industries also adjacent to the survey area (Fig. 4). There was also a ‘lump’ of material present in the middle of the old boatyard (marked in Fig. 4). Keele are, however, very experienced in undertaking near-surface geophysical detection surveys (see Pringle et al. 2002; 2007; 2008; 2009) and thus felt they were still worthwhile to undertake. 3. Methods 3.1 Ground Penetrating Radar (GPR) surveys It was decided to acquire GPR 2D profiles initially, as the PulseEKKO™ 1000 equipment used had shielded antennae and thus would be less affected by above-ground structures. GPR typically provides the highest resolution of geophysical surveys and can also determine the depth to target(s) (see Milsom, 2001). Three 2D fixed-offset (1 m) profiles, orientated approximately north -> south were therefore acquired on the 9th June 2010 using 110 MHz frequency antennae (see Figs. 4/5a). This frequency was decided as small objects would not be resolved and it should penetrate the furthest below the present ground surface. A step size of 0.25 m was used, with a time window of 100 ns and 32 repeat ‘stacks’. Once the data was collected, it was imported into Reflex-W™ software. First break arrivals were picked and then adjusted, before a manual gain filter function was applied to boost deeper reflection events whilst retaining relative amplitudes before a time cut was lastly applied to delete blank data at the base of the profiles. 3.2 Magnetic surveys It was decided to use a GSMP-40™ potassium-vapour gradient magnetometer as this would be oriented vertically and would hopefully be less affected by the above-ground metals present onsite. The same three survey lines and direction of data collection, as used for the GPR data collection, were used to collect lower/upper sensor and gradient data on the 9th June 2010 (Fig. 5b). There were significant problems onsite attempting to get the equipment calibrated onsite, presumably


4

due to the plentiful above-ground metallic objects. Data was collected every 0.2 seconds and GPS enabling allowed a GPS measurement of each data point to be collected. Once downloaded, profiles having non-calibrated data were then removed, before being ‘de-spiked’ to remove anomalous readings and contoured with ArcGIS™ ArcMap v.9.3.1 software. 3.3 Electrical Resistivity Imaging (ERI) surveys ERI surveys are commonly undertaken to detect anomalous areas near the surface and importantly create vertical 2D profiles, unlike fixed-offset methods (see Reynolds, 1997 for background). After viewing initial data, a Campus TiGRE™ system was used to collect one 2D ERI profile on Line 2 on the 9th June 2010 using ImagerPro™ 2006 software (Fig. 5c). 32 stainless steel electrodes were used with an electrode spacing of 0.7 m determined onsite to maximise the line length. It was found to be very difficult to acquire equivalent contact resistances at each electrode, presumably due to the very heterogeneous near-surface materials being present onsite. Raw ERI data were then processed and inverted using Geotomo Res2Dinv v.355 software in accordance with Loke & Barker (1996) resistivity surveying recommendations. 3.4 Bulk ground conductivity surveys Due to the mixed results of the previous surveys, a bulk ground conductivity survey was lastly decided to be utilised. These should penetrate to ~10 m below ground level given ideal conditions and may show where the barge is located in comparison to background values (see Reynolds, 1997 for further details). A Geonics™ EM-31-Mk2 system (Fig. 5d) was therefore used on the 5th July 2010 to collect both vertical and horizontally-orientated data (although the latter will almost certainly be affected by above-ground structures) along five profiles that broadly followed the previous survey line locations (Fig. 9 for location). This was GPS enabled to allow a GPS measurement of each data point to be collected. Once downloaded, data were ‘de-spiked’ to remove anomalous readings and contoured with ArcGIS™ ArcMap v.9.3.1 software. 4. Results & interpretation GPR data was generally poor, only two anomalies were shown, one ~80 m on L1 and the other ~10 m on L2 (Fig. 6). These positions are plotted on Figure 4 and are consistent with anomaly 2 on Lovelock’s map – Fig. 3). No anomalies were present at the junction of the present canal (anomaly 3 on Lovelock’s map – Fig.3). These poor results were probably due to the variable materials that were used to fill in the canal. Magnetic data was also very variable, presumably due to the ground conditions and above-ground structures present. However, the area around the old boatyard did have high magnetic values which may be suggestive of a barge location (Fig. 7). This was also in a similar area to the GPR anomalies (see Fig. 4). Resistivity data was unusable due to the too high electrode variations when data collecting; even when trying to mitigate against this model errors were over 40%. Conductivity data was also very variable, with the horizontal (HMD) data too variable to be of use as suspected (Fig. 8). The vertical (VMD) data was also variable although a low conductivity region was present in the boatyard area. This may have been due to data being collected over the ‘lump’ of dumped material however.


5

5. Conclusions Keele University carried out GPR, magnetics, ERI and conductivity surveys to look for a buried coal-barge within the filled-in Burslem Branch canal. Two locations looked promising and are therefore recommend for intrusive investigations. 6. Acknowledgements We wish to thank the Burslem Port Volunteers for allowing us to undertake this project. The Wisniewski brothers and two French summer placement students are also thanked for assistance in data collection. 7. References Loke, M.H. & Barker, R.D. 1996. Rapid least-squares inversion of apparent resistivity pseudosections by a quasi-Newton method. Geophysical Prospecting, 44, 131–152. Milsom J. 2001. Field geophysics. Geological Society of London Handbook. Milton Keynes, U.K.: Open University Press. Pringle, J.K., Lenham, J.W. & Reynolds, J.M. 2009. GPR investigations to characterise Medieval and Roman foundations under existing shop premises: a case study from Chester, Cheshire, UK. Near Surface Geophysics, 7(2), 93-100. Pringle, J.K., Jervis, J., Cassella, J.P. & Cassidy, N.J. 2008. Time-lapse geophysical investigations over a simulated urban clandestine grave. Journal of Forensic Sciences, 53, 1405-1416. Pringle, J.K., Stimpson, I.G., Toon, S.M., Caunt, S., Lane, V.S., Husband, C.R., Jones, G.M., Cassidy, N.J. & Styles, P. 2008. Geophysical characterisation of derelict coalmine workings and mineshaft detection: a case study from Shrewsbury, UK. Near Surface Geophysics, 6(3), 185-194. Pringle, J.K., Doyle, P. & Babits, L.E. 2007. Multi-disciplinary investigations at Stalag Luft III Allied Prisoner of War camp, the site of the 1944 ‘Great Escape’, Zagan, Western Poland. Geoarchaeology, 22(7). 729-746. Pringle, J.K., Westerman, A.R., Schmidt, A., Harrison, J., Shandley, D., Beck, J., Donahue, R.E. & Gardiner, A.R. 2002. Investigating Peak Cavern, Castleton, Derbyshire, UK: integrating cave survey, geophysics, geology and archaeology to create a 3-D digital CAD model. Cave & Karst Science, 29, 2, 67-74. Reynolds, J.M. 1997. Introduction to Applied and Environmental Geophysics, Wiley & Sons Ltd., Chichester, UK, 806pp.


6

Figures

Figure 1. Location map of the survey area (red) with co-ordinates in OSGB. © Crown Copyright Database 2007 (an EDINA supplied service).


7

Figure 2. 1961 breach photographs (unless otherwise stated). © Courtesy of Burslem Port volunteers.


8

Figure 3. Survey map of Alan Lovelock (not to scale) undertaken in 1999. The three likely positions (location technique unknown) are marked. © Courtesy of Burslem Port volunteers.


9

Figure 4. Site-map showing modern location-map (green lines) with historic (1937) map underlain (blue). Survey lines where GPR, magnetic and ERI geophysical data were collected are also marked (see key). The start of survey lines are where the relevant line number is positioned. GPR and magnetic anomaly positions are marked (see key). © Crown Copyright Database 2007 (an EDINA supplied service).

OSGB


10

Figure 5. Photographs taken of geophysical equipment onsite of: (A) PulseEKKO™ 1000 equipment using 110 MHz frequency (1 m fixed off-set) antennae; (B) GSMP-40™ potassium-vapour (1-m separated vertical) gradient magnetometer with GPS positioning; (C) Campus™ TIGRE Electrical Resistivity Imaging (ERI) system and; (D) Geonics™ EM-31-Mk2 conductivity meter. © Jamie Pringle, 2010.


11

(A)

(B) (C)

Figure 6. GPR profiles 1 (A) ,2 (B) and 3 (C) acquired (see Figure 3 for location). Anomaly locations are marked (red arrows) and also marked on Fig. 3. © Jamie Pringle, 2010.


12

Figure 7. Potassium-vapour magnetic gradiometer data acquired over the survey area showing (A) upper sensor; (B) lower sensor and; (C) gradient data (see respective keys). Black lines indicate survey locations. Dotted question mark rectangle indicates potential anomaly location. © Jamie Pringle, 2010.


13

Figure 8. EM-31-Mk2 (A) vertical (VMD) and (B) horizontal (HMD) fields see respective keys). Black lines indicate survey locations. © Jamie Pringle, 2010.

Disclaimer Analysis of the geophysical data obtained has followed standard practices and methodologies. However, this is no guarantee that the interpretations given are correct. Keele University do not guarantee the accuracy of the interpretations shown.

environmental forensic geophysical survey for a buried canal … › sites › default › files ›...

Documents