A near-surface geophysical investigation of the effects of measured and repeated removal of overlying soil on instrument response
Long, Zachary Ryan
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A geophysical survey presents many challenges. A scientist must be able to not only understand the theory and nature of the geophysics being applied but must also be able to identify features of interest in a dataset. It is also of extreme importance to be able to determine where, in the subsurface, the features identified in the data occur. This research is designed in an attempt to identify the locations of subsurface heterogeneities that affect geophysical instrument response. An experiment was conducted in which topography, magnetics, ground-penetrating radar (GPR), and electromagnetic induction (EM) data were collected over a defined survey line. An excavator with a modified flat-bladed bucket was used to remove, or skim, a 5 to 10 cm thick layer of material from the survey line. Upon removal of the material, datasets from the above mentioned instruments were again collected along the same survey line. This process was repeated for 10 skims, resulting in a total of 11 sets of data for each instrument. Having collected data with various instruments in the same location as material was progressively removed allowed for an empirical study with the goal of noting how the response of each instrument changed with respect to the removal of material. By observing how the anomalies changed in the data from one skim to the next, a better understanding of the location of the causative heterogeneities could be had. Data for each instrument was compared to the equivalent data collected from each subsequent skim to determine how similar or different the data appeared as the depth of the trench increased. The experiment also sought to determine if the topographic variations, or roughness, along the survey line had any impact of the geophysical signals. The data collected from each instrument were compared to the topographic roughness of the survey line for the corresponding skim.