Marine Resistivity: a Tool for Characterizing Sediment Zones

Motivation Beach Nourishment Beach Nourishment Large Scale Features Large Scale Features Sand vs. clay Sand vs. clay from USACE Philadelphia (2003)

Problem Vibracore Vibracore Large area  lots of samples Large area  lots of samples Heterogeneities  lots of samples Heterogeneities  lots of samples Could a geophysical method guide point measurements? Could a geophysical method guide point measurements? From Waldner et al. (2008)

Geophysical Methods Seismic Seismic Reflections due to different wave velocities Reflections due to different wave velocities Identify geologic structures Identify geologic structures From Waldner et al. (2008)

Geophysical Methods Resistivity Resistivity Responds to electrical conductivity of sediment Responds to electrical conductivity of sediment Map sediment type directly based on results Map sediment type directly based on results Bedrock Clay From Heaney (2007)

Resistivity How materials conduct electricity How materials conduct electricity Current is injected into the ground Current is injected into the ground Measure changes in electric potential Measure changes in electric potential From Peake (2005)

Resistivity Potential Targets Potential Targets Porosity Porosity Pore fluid conductivity Pore fluid conductivity Lithology Lithology From Rhoades et al. (1976).

Terrestrial Resistivity Applications Groundwater exploration Groundwater exploration Pollution monitoring Pollution monitoring Void detection Void detection From agiusa.com Archeological surveys Archeological surveys Borehole logging Borehole logging Dam investigations Dam investigations

Towed Resistivity Surveys Rapid data collection Rapid data collection ~3 km/hr ~3 km/hr Electrodes at water surface Electrodes at water surface ~ 30 m penetration ~ 30 m penetration

Towed Resistivity Results Near Clay Core Near Sand Core From Waldner et al. (2008)

Towed Resistivity Results Resistivity in ohm-m

Ohm-m Towed Resistivity Results

Summary Towed resistivity Towed resistivity Responds directly to lithology/clay content Responds directly to lithology/clay content Rapid data collection supplements vibracore Rapid data collection supplements vibracore Goals Goals Refine/expand existing borrow areas Refine/expand existing borrow areas Map changes between existing vibracore Map changes between existing vibracore Guide placement of future vibracore Guide placement of future vibracore