GI has conducted many marine resistivity surveys to map subbottom geology, sedimentation, or to find sources for submarine groundwater discharge. Marine resistivity simply refers to a geophysical resistivity survey in an aqueous environment. The acquisition of the data can be conducted as a continuous resistivity profile (also known as streaming resistivity) by towing cables behind a boat, or as a static setup by submersing data acquisition cables on the bottom of most any marine setting.
HGI conducted a large 3D marine resistivity survey of the Panama Canal in support of canal expansion. The data were acquired along continuous and alternating parallel swathes, and 3D models were created to distinguish hard rock from loose sediments.
Typically, individual resistivity profiles across a water body are obtained in conjunction with water temperature, water conductivity, and bathymetry data. These data are entered into the inversion model to more accurately define the subbottom stratigraphy. Additionally, 3D resistivity maps can be produced from a set of parallel profiles to gain a more complete picture of the subsurface. As an example, HGI conducted a large 3D marine resistivity survey to map the geological conditions below the Panama Canal. The data were used to help understand rock strength for the recent expansion project.
An example of marine resistivity profiling is presented below that shows a continuous resistivity profile across Patagonia Lake in southern Arizona. The near surface water conductivity is approximately 30 ohm-m and below the lake bottom are low and high resistivity values representing loose sediments and competent granitic bedrock, respectively. High sediment accumulation can be seen in the valleys between bedrock highs and is thicker towards the inlet (beginning of the line).
In another example, a geological mapping project using marine resistivity was conducted in the Panama Canal to help understand material properties for dredging. Within the Gatun Lake region, loose sandy fill can easily be removed by suction dredging, while harder marine-based sedimentary rock must first be drilled and blasted before using a bucket dredge for removal. Understanding the subsurface properties can limit the use of the more expensive blasting method. The image below shows the results of 3D modeling of streaming resistivity data acquired in the canal, where high resistivity values appeared to correlate with the former route of the Chagres River. In this area, the river bed was primarily coarse sands and gravels that meandered through harder outcrops of rock (shown as lower resistivity). These data were then used by the Panama Canal Authority (ACP) to refine costs of construction.