Mapping Shallow Geological Structure With Towed Marine CSEM Receivers

Marine controlled-source electromagnetic (CSEM) surveying using nodal ocean-bottom recorders has become a standard tool for offshore hydrocarbon exploration. A combination of low-noise receivers and transmitters emitting 1,000 amps or more allows data collection at source–receiver offsets of up to 10 km, with depths of investigation reaching several kilometers. However, characterization of shallow geological structure (less than several hundred meters below mudline) is limited by the typical node spacing of 500 m or more. A 3-axis electric field receiver has been developed that is towed behind the EM transmitter in order to collect continuous constant-offset data, either as a stand-alone surveying technique or as a supplement to a node-based survey. Low frequency noise on the towed receiver is significantly higher than that for sea-floor nodes, but at 10–100 Hz approaches that of sea-floor instruments when the shorter (1–2 m) antenna length is considered. Early applications of this new technology were limited to source–receiver offsets of a few hundred meters, for fear of the array fouling on the sea-floor. To address this we have developed a telemetry protocol that can be used on twisted-pair copper cables to distances of up to 4 km, allowing real-time monitoring of the array depth during towing. By careful trimming of the buoyancy we are able to "fly" an array of four receivers with offsets of up to 1,000 m at an altitude of 100 m above the seafloor, with only a few meters variation in depth across the array during level flight. Tests were carried out in the San Nicolas Basin, offshore southern California, over an area where a seismic bottom-simulating reflector (BSR) had been identified in heritage seismic data. Increases of 30% in amplitude and 20° in phase were observed when the array was over the BSR, suggesting minor amounts of hydrate above the BSR or free gas accumulation below the BSR.