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Report on the

Ground Penetrating Radar Survey

At Inverness (Scotland)

By Scantech Geoscience Ltd.

For Brian Macgregor

February 2006

Contents

Introduction..................................................................................................................3Survey and site description..........................................................................................3Processing and data analysis......................................................................................4Amplitude Time/Depth Slices.......................................................................................4Interpretation and results.............................................................................................5Recommendations.......................................................................................................5

Figures

Figure 1 : Site location on the A9 with the yellow mark from metal detector surey......3Figure 2 : Sketch map of the site................................................................................6Figure 3 : GPR profile locations...................................................................................7Figure 4 : GPR profiles from MC1 to MC11 after processing.......................................8Figure 5 : 3D display of the GPR profiles after processing..........................................9Figure 6 : GPR profiles MC1 after processing..........................................................10Figure 7 : Horizontal amplitude depth slices..............................................................11Figure 8 : 3D volume created from interpolated depth slices.....................................12Figure 9 : GPR anomalies identified and metal detector anomaly.............................13

Scantech Geoscience. Heather House, Heather Road, Sandyford, Dublin 18, IRELAND

Tel: +353 1 506 0793 Fax: +353 1 213 5343 Email: [email protected] Web: www.scantech.ie

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http://www.scantech.ie/

mailto:[email protected]

Introduction

Scantech Geoscience carried out a GPR survey around Inverness on the 26/02/2006 under the recommendations of Brian McGregor in order to identify a possible location where the bodies of a missing wife and her son could have been buried with a pushchair.

Survey and site description

The GPR survey was carried out with a RAMAC X3M GPR using a 500MHz shielded antenna as central frequency and a RAMAC Monitor XV11 professional firmware.

A preliminary reconnaissance survey with a metal detector made by Brian McGregor gave a significant response in the middle of the road on the north side of the bridge deck. The GPR survey was designed around that location. The target area is located in the middle of a national road just at the northern side of a bridge that allows the road (A9) to pass over the railways line (see Figure 1 and 2).



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Figure 1 : Site location on the A9 with the yellow mark from metal detector surey

A total of 11 GPR profiles were acquired in zigzag mode (up and down) with a line spacing of 45 cm (see Figure 3). The profiles started 35 cm from the curb and ended just before the other curb of the road because of the GPR cart. All the profiles acquired in the West to East direction have been reversed for processing purposes and all the profiles are displayed in the East to West direction in the report.

Processing and data analysis

The GPR data are displayed in cross sections with the distance along the profile for the X axis and with the two-way travel time of the reflected GPR waves for the Y axis. To convert time sections into depth sections, an average velocity of 10 cm/ns was used assuming a possible variation in depth of +/-20%.

After being reversed (the odd ones) and resampled from 16 to 8 bits, a DC filter and an amplitude correction were applied to remove constant offset and compensate for loss spread and attenuation respectively. The enhanced profiles are displayed on the Figure 4. Another 3D display of the profiles is shown in Figure 5.

As we can see in Figure 6, the profile MC1 shows a good penetration of the signal up to 2.5 meters below ground level (bgl). A strong anomaly is visible at the start of the profile (within the first 2 m). This anomaly is related to the response of the northern side of the bridge deck and its metal reinforcement joint. We can also see that this anomaly is getting shorter and shorter on each profile as they are being run mainly over the north side of the bridge (see Figure 4).

Amplitude Time/Depth Slices

In order to interpret better any disturbed ground or weaker anomalies that could point out the location of the target, we created amplitudes time slices or depth slices if we calibrated the data with an average velocity. This way of looking horizontally at the data instead of vertically is complementary to traditionnal data processing. The GPR profiles have been interpolated to create a set of amplitudes depth slices. Theses slices are then interpolated between each other and used to create a 3D vloume. This way of looking at the data allows identifying any change in the geology or any disturbed ground quickly and efficiently depending on the contrast of GPR energy reflected.

The amplitude time slices were converted into depth slices using an average velocity of 10 cm/ns. The Figure 7 displays a set of 20 amplitude depth slices with an overlap



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of 30 cm to avoid any gap in the 3D volume. The thickness of the depth slices is around 50 cm. All the radar reflected energy is averaged within a predefined time interval or depth interval if the velocity is used.

Interpretation and results

The strong anomaly related to the bridge deck and the metal joint is well defined on the depth slices at the bottom right corner.

The depth slices n6, n7 and n8 present a strong anomaly visible from 88 cm to 1.88 cm bgl (see Figure 7) that could be a possible location of the suspected target. The dimensions of 1x2 m square and the depth range of the anomaly are in accordance with the size of human bodies. The presence of a deeper anomaly at the same location could be related to the pushchair.However some reserves should be made concerning the major linear anomaly visible on Figure 8 at around 132 cm bgl could suggest the presence of a buried utility or reinforcement structure within the bridge foundation.

Recommendations

We have identified two anomalies that could be of a major interest for the case. The first is located at X= 4.1 m and Y= 5.8 m between 90 and 140 cm bgl. The second anomaly worth of interest is located at X=2.8 m and 5.7m between 137 and 185 cm bgl (see Figure 9).

Further investigations concerning the presence of any buried utility line or reinforcement structure should be made prior any digging.

Because the data were depth calibrated assuming an average velocity, all the depth should be taken within a range of error of +/- 20% until on site calibration can be made.

Last recommendation further GPR survey should be made prior excavations to improve the results and confirm the possibility of a bodies remain at the site.

Respectfully submitted on the 13th of March 2006,

_____________________Brice Le Comte



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BSc, MSc GeophysicistScantech Geoscience



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Figure 2 : Sketch map of the site



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Figure 3 : GPR profile locations



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Figure 4 : GPR profiles from MC1 to MC11 after processing



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Figure 5 : 3D display of the GPR profiles after processing

Figure 6 : GPR profiles MC1 after processing



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NORTHGPR ANOMALIES

Figure 7 : Horizontal amplitude depth slices



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NORTHGPR ANOMALIES

Figure 8 : 3D volume created from interpolated depth slices



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Figure 9 : GPR anomalies identified and metal detector anomaly



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