SUBSURFACE ENVIRONMENTAL AND CONTAMINATION INVESTIGATIONS
The environment and its well being has always been a sensitive issue although of more recent time there are increasing legislation towards its current condition and sustainability. With the increasing population and demand for newer facilities and construction upgrades there is high focus on the current and future state of the surrounding environment. Areas that are currently known to be contaminated require some form of remediation. Whether the area for investigation is currently contaminated or up for investigation, non destructive geophysical techniques offer a safe, efficient and straight forward approach to determine what is happening within the subsurface.
Traditional geophysical technologies for environmental mapping include Ground Penetrating Radar (GPR), electromagnetic (EM) and magnetic surveys. Specific applications for these technologies may include geological/soil mapping, groundwater and salinity investigations, underground storage tank (UST) detection, unexploded ordnance (UXO) detection, and contaminant mapping including landfill and buried object delineation. Repeat surveys have the potential to show site change and progression over time. These changes may include water and contamination flow, ground creep, sediment flow and many other slow scale site changes.
DUALEM instruments measure the conductivity and magnetic susceptibility of the ground, and detect buried metal. The patented DUALEM dual-geometry array simultaneously measures conductivity and susceptibility to two distinct and easily quantified depths. DUALEM instruments operate by electromagnetic induction, so no contact with the ground is needed, which enables surveying at moderate speed over uneven or electrically resistive terrain. DUALEM systems are portable and easily adaptable setups, allowing for time efficient mobilisation and high quality data acquisitioning. The DUALEM-42S sensor has dual-geometry receivers at separations of 2 and 4m from the transmitter, which provide four simultaneous depths of conductivity sounding, four simultaneous depths of susceptibility sounding, and detection of metallic response. The several sounding-depths of the DUALEM-42S enable the analysis of layering in the top several metres of the earth.
CASE STUDY ONE: LANDFILL DEPTH AND EXTENT MAPPING - CARLINGFORD, NSW
A suburban parkland bordering a small creek was believed to be used as a landfill site back in 1970. No records were available except for the words of some residents who have lived in the area for many years. The park is currently used as a football ground for the public. An investigation was undergone to determine potential landfill depth and extent to further understand if any environmental contamination issues are present.
The area was surveyed using both a GPR and DUALEM instruments. GPR was not overly successful, it was believed to have limited signal due to the presence of a conductive clay landfill capping. Strong anomaly metallic objects were identifiable through the GPR. Real success was delivered through the results of the DUALEM survey. Patches of high and low conductivity were visible which ultimately allowed delineation of two distinct landfill areas. Capping layer and depth were noted as being thin although the extent of the landfill appeared to be contained and not leaching into nearby waterways.
CASE STUDY TWO: FRESHWATER WELLS, FIJI
A feasibility study involving coral shelf and saline water mapping was conducted in Fiji. The ABEM Terrameter LS using multi channel gradient protocol was set up over 81 electrodes with 2m spacing. The result after data inversion produces a pseudo-cross section displaying areas of high and low resistivity. The interpretations made available from the dataset included isolation of zones with freshwater pooling, coral shelf extent and saline water/salinity pooling. The entire survey including data collection and imaging was complete in one day. Resistivity data was compared to GPR data which further complimented interpretations of stratigraphic layering.
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