Conductivity and Seismic Investigation to Delineate Explosive Burial Pits

The objective of this investigation was to locate and delineate suspected burial pits of explosives, within specified areas of the investigation site. The survey work was carried out in three days.

The areas investigated were either known to be sites of disposal of explosives, or were suspected of containing buried explosive materials. The location of the burial pits is required to facilitate site remediation, involving removal or destruction of this explosive material as may be considered necessary.

Methodology

The EM31 investigation was undertaken over the areas presumed to contain burial pits. Two portions were identified by the owner’s representatives to be possible sites of burial pits. One site was an area of 0.65ha, defined as Area A, the second region surveyed was an area of 0.23ha, described as Area B. Data profiles were collected at 5m intervals along lines 5m apart.

A grid was pegged for both areas. On the larger area (Area A) In addition to these grided areas, two lines of conductivity data were collected across the contaminant catchment dam below the gully (see Photo below), and one line was collected in an East – West direction from the western base line of Area A over undisturbed ground to assess the background conductivity. Repeat readings were conducted on the first point of the 10m line throughout the survey of Area A to evaluate drift in the equipment. Further to this, the last line of conductivity readings collected on the first day was repeated on the second day to evaluate possible changes caused by the rain on the afternoon and during the night.

A number of refraction seismic lines were collected using either 2m or 3m geophone spacing over areas considered to be of interest from both visual and EM data assessment.

Five refraction seismic spreads were collected; two across the area of the largest EM31 survey . These utilised a geophone spacing of 3m. Nine shots were acquired with four offsets at each end of the spreads and one central shot. For the remainder of the seismic spreads 2m geophone spacings were employed with seven shot points used on each spread. Each shot was stacked between 3 and 6 times, as necessary to give a suitable signal.

Results of the Investigation.

Repeat readings on the first data point on the 10m survey line returned the same value; equipment drift was concluded to be minimal to nonexistent over the time of this survey.

With the rain on the afternoon and evening of the first day it was felt, that the data collected subsequently may not have been comparable with that acquired on the first day. To evaluate the change in results the 50m line was recollected on the morning of the second day. The resulting data was plotted and is shown below in Figure 1. Comparison of these two lines of data and the constant reading obtained at the repeat reading station, established that the variations in the conductivity along the 50m line were less than the “background” conductivity, and that the effect of the rain on the survey would be minimal.

Figure 1. Plotted conductivity on Days 1 and 2

The ground conductivity data collected during this investigation has been used to map two areas within the property suspected as possible sites burial. Six locations have been identified on the site that require further investigation to test for possible explosive residue. Most of these areas display elevated conductivity readings over the background.

The hill on the site is of higher conductivity than the surrounding portions of the survey. Within this area, three distinct regions of high conductivity were mapped. The general high area could be the result of leaching of the buried material, with the distinct high areas being possible disposal sites. In addition to these three targets, there is an area of low conductivity which may be indicative of a buried linear object; this is consistent with the findings of a metal detector survey undertaken prior to this investigation.

Another area within the site contains two regions requiring further investigation. One is on the edge of the hardstand, and the second is in the floor of the dam. Both of these regions display a marked increase of conductivity over the local ground conductivity.

Figure 2.Location of Seismic Spread and Conductivity Profiles

The use of seismic techniques has allowed the mapping of local strata. The collected seismic spreads, have delineated the depth of three layers within the area of the ground conductivity survey. One of these spreads was also modelled as four layers. The data from the remaining spreads collected were two layer models. The top layer is; top soil, fill or clay capping, the second layer is weathered bedrock and the third layer is the local bedrock. These are found to be consistent with local exposures. The depth of the silt in the dam was determined to be approximately 3.5m and the thickness of fill (waste) material in the “Burn Gully” was determined to be 2 to 4.5m.