Fifteen high-resolution multi-node electrical resistivity river bottom surveys (marine surveys) were completed at a proposed pipeline crossing along the Colorado River in San Saba and Burnet Counties, Texas. The objective of the surveys were to provide preliminary subsurface information along a proposed pipeline right-of-way and to identify geologic hazards such as faults, fractures, and voids in the subsurface that may impact horizontal directional drilling activities.

The resistivity data collected from each survey line was processed and modeled using a two-dimensional inversion modeling package. A penetration depth of approximately 90 feet beneath the river bed was obtained in each of the modeled profiles. The modeled resistivity values ranged from less than approximately 5 ohm-meters in the low resistivity (high conductivity) silty and clayey river bed sediment to approximately 100 ohm-meters and higher in the competent higher resistivity carbonate bedrock along all the profiles. The existence of unstable material in the path of the proposed pipeline would be a potential concern that needed to be evaluated prior to advancing the horizontal directional drilling bore through the right-of-way.

An overview of the data collection methods as well as a brief summary of the results will be provided during the presentation.