An electrical resistivity tomography survey (ERT) was carried out to map suspected unknown schedule waste dumped in Warehouse No 2, Greenverse Sdn Bhd, Gebeng, Kuantan, Pahang. decided to employ ERT to obtain preliminary subsurface information.
Two (2) lines were surveyed using the Wenner-Schlumberger protocol with 20-electrodes
configuration and 2m electrode interval. The ERT survey picked-up four (4) significant
blocks which are thought to be man-made features. These features are likely indications of
soil materials being removed (excavated) and replaced with some other materials of
extremely low resistivity values. It is recommended that the Client proceed with further
investigations to confirm the existence of these man-made features and the materials filling
them.
ERT survey
The survey was carried out using the GEOMATIVE GD-20 DC Geoelectrical system. This is
a multi-channel system, capable of measuring of up to a maximum of 12-channels per current
injection.
The survey was carried out using both the Wenner-Schlumberger protocols with 20-electrodes
configuration. The electrode interval was 2m, giving an effective length of 38m for each line.
Two (2) lines were surveyed and their layouts are shown in Figure 1.
Small holes were drilled into the concrete cement floor to plant the electrodes. This is to
ensure good contact between the electrodes and the ground
2D-inversion
All data acquired by the GEOMATIVE GD-20 system were transferred directly to the laptop
via the GEOMATIVE Studio software. This software was also used to filter out bad data points
prior to putting the data through the inversion process. Only good quality data were accepted
for inversion.
The data were then exported to RES2DINV format, the software used to invert the data to
produce subsurface models. RES2DINV have several other options to ensure only good
quality data were passed through for inversion. The robust constraint was selected for the
inversion model as it was suspected that schedule waste would have been buried in a
confined space. By running the robust inversion, the boundary of the confined space would
be mapped more accurately
The results of the ERT survey are shown in Figure 2. The corresponding resistivity block models is shown in Figure 3.
Line GV1
Line GV1 runs along the length of Warehouse No 2 and is located almost in the middle. The
layout of this line was discussed with the Client as they thought there was a strong possibility
that the unknown schedule waste would have been dumped somewhere in the middle of the
warehouse.
The subsurface resistivity structure is shown in Figure 2 (top). The resistivity values range
from as low as < 0.5 Ωm to > 75 Ωm. The extremely low resistivity values of < 0.5 Ωm are
only confined to a depth of a few meters (Blocks A, B, C and D in Figure 2) below the ground
surface. Except for Block C, all the other blocks have a consistent thickness of about 2.5m.
Block C is slightly thicker, extending to about 4m. The almost similar thickness of the blocks
together with the constant gaps of about 2m between the blocks seem to suggest the features
are man-made. It is very likely that the blocks represent soil which has been removed
(excavated) and replaced with some unknown materials with extremely low resistivity values.
If it is the original soil, the resistivity values will not be that low.
Underlying these blocks are typical soil materials with > 25 Ωm. It could likely be mixture of
clay and sand materials. As the depth of investigation is very shallow (about 8m), no bedrock
was encountered. Bedrock would have typically resistivity values of > 1000 Ωm.
Line GV2
Line GV2 runs along the breadth of the warehouse and is perpendicular to Line GV1. It crosses Line GV1 almost at the center of the line. The subsurface resistivity structure (Figure 2, bottom) shows very low resistivity values of < 2 Ωm throughout the whole profiles. The first half of the profile is dominated by low resistivity of < 0.1 Ωm. Beyond this distance, the resistivity values is split into two distinct layers. The upper layer with thickness of about 3m and having extremely low resistivity values of < 0.1 Ωm. Underlying this layer, the resistivity reaches up to about 2 Ωm. The features observed on this line bear no resemblance to Line GV2. This would suggest that Line GV2 is actually running parallel to the features (blocks) observed on Line GV1. It would be very likely that it is running along and parallel to Block D. Thus, it does not pick-up any other features, other than the low resistivity values. However, the range of resistivity values is very similar to that observed in Block D of Line GV1
The ERT survey had identified four significant blocks which are likely man-made features on Line
GV1. These are indications of soil materials which has been removed (excavated) and replaced
with some other unknown materials with extremely low resistivity values. These blocks warrant
further investigation by the Client.
It is recommended that the Client proceed with soil sampling via drilling to confirm the type of
materials filling the man-made features
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