GEOPHYSICAL DETERMINE OF DEPTH TO BEDROCK IN AGRICULTURAL FIELDS

Surface geophysical methods combined with drone technology are enhancing our ability to develop more accurate depth-to-bedrock maps in eastern Wisconsin. Depth-to-bedrock data is essential for land-use planning in Wisconsin, from recent rules governing land-spreading of manure based on soil thicknesses to construction of infrastructure such as windfarms, roads and pipelines. However, existing depth to bedrock maps are commonly out of date and were not created to meet present day needs. For example, the rules governing manure land-spreading require the field be delineated for two, three, five, and twenty foot depths to bedrock. Traditional bedrock mapping does not provide an adequate level of detail.

The geology of the area of interest is well suited to application of geophysical methods. In general, a till overlies dolomite bedrock. These lithologies provide excellent contrasts in many physical properties. As a result, multiple geophysical methods such as ground conductivity electromagnetics (EM), electrical resistivity imaging (ERI), ground penetrating radar (GPR), and HVSR seismic offer potential mapping solutions. We also applied a novel method using an infrared camera mounted on a small unmanned aerial vehicle to map soil temperature, making use of the contrast in thermal properties between the soil and rock. We tested the different methods by comparing geophysical techniques with hand push probe data at depths less than 4 feet.

We found that the ERI provided reliable data but required greater time and effort than the other methods. The EM methods were able to quickly collect data over large areas of the field but single measurements were poorly correlated to push probe depth (r²=0.4). Repeated measurments at multiple coil frequencies, orientations, and separations greatly improved accuracy (r²=0.67). The GPR signal was highly attenuated by the clayey till soils and worked reliably only to depths of around 2 feet. The HVSR seismic method is better suited to depths greater than 4 feet and so was not compared. The thermal imagery showed correlation to push probe data similar to a single EM measurement. However, this method was the fastest and had the greatest coverage and resolution. We intend to couple this thermal data with other observations to improve its applicability. We conclude that coupling the ease and higher resolution of geophysics with the greater certainty and availability of push probe, drilling, and excavation methods should lead to better field mapping of depth to bedrock than using either alone.