dc.description.abstract | Fossil fuel is still the primary source of energy and can be refined to other side products as raw materials or solvents. Spill, uncontrolled disposal, storage tank/pipeline leakage of fuel oil and solvents are threatening to biological system and impacting our living environment and even for human health risk. Hydrocarbon contaminant whose density is lighter than water would be considered as LNAPLs (Light Non Aqueous Phase Liquids). Common approaches including soil sampling, monitoring well installation, groundwater sampling, in-situ screening, and laboratory analysis are conducted during ordinary site investigation. From field sample collections, specific or multi-compounds analyses to data interpretations, all these complicated and expensive efforts are performed in order to detect and confirm the source of contaminant. One common problem which relies on the above mentioned conventional point measurements usually unable to provide clear range of contaminated area without dense sampling. The important issue is how to maintain a good balance among the investigation purpose, field operation quality control and budget. Near-surface GPR (Ground Penetrating Radar) survey provides an economical geophysical approach that offers large scaled, quick survey, real time interpretation and determination of possible occurrence of contaminated site. With preliminary GPR survey, areas of the site can be categorized according to the probability of being contaminated, and further, the range can be narrowed down.
Attribute analysis, a commonly used data processing technique in seismic exploration, is implemented for GPR data to reveal detailed information in detecting the presents of the hydrocarbon contaminant. Three different types of known hydrocarbon contaminated sites are investigated in order to examine its feasibility. GPR profiles acquired from each sites in conjunction with field sampling and laboratory analyses results proposed in our study would provide a useful approach for site investigation, monitoring and remediation. Detailed GPR attribute analysis demonstrates that the proposed approach is capable of detecting minor attribute changes caused by the contamination phases and thus providing very useful information for re-occupied or even for time-lapsed site investigation and monitoring.
The residual phase of LNAPL in the vadose zone will produce a large area of smeared zone. Instantaneous phase which reflects the polarity change shows relative aggressive change across water table in those seriously contaminated area. High and spiky instantaneous frequency can be observed around reflected signals from water table or capillary fringe which could be related to the detection of thin beds. Studying GPR signal attributes is extremely useful as the proposed methodology could provide as a strong indicator for detecting hydrocarbon contamination events. | en_US |