The shallow porosity structure of the Eel shelf, northern California: results of a towed electromagnetic survey

Abstract

A towed electromagnetic survey, mapping the electrical resistivity of the seafloor, was conducted over an area of the Eel shelf off Humboldt Bay, California. Continuous resistivity profiles to 20 m below the seafloor were measured along 120 km of track line, from water depths of 100 m to around 30 m. The shallow structure along the shelf is highly variable and we identify three distinct environments based on the recorded resistivities and the porosities inferred from them. The first region is a mid-shelf depocenter, characterized by a thin (∼2 m), moderately high-porosity (45–60%) surface layer, which overlies a less porous (35–45%) and homogeneous substrate, uniform both laterally and vertically. This region is found to the northwest of the Humboldt Bay entrance, from water depths of about 65 m to at least 100 m, and is roughly coincident with recent flood deposits. The second region is located closer to shore and contains extremely high resistivities for a shallow sedimentary environment. It reveals a high degree of spatial variability on length scales of several hundred meters. Several possibilities exist to explain such high resistivities and these include: upwelling fresh water channeled to the seafloor through local fault and anticline systems; a significant volume of natural gas within the sediments; or a continuous process of carbonate precipitation through oxidation of methane near the seafloor, which, over time, builds a substantial thickness of lithified material. None of the above explanations are mutually exclusive, and all could act in concert to increase resistivities. The third region roughly coincides with the Eel River delta and features a buried layer of moderately low porosity (30–35%) at a depth of about 5 m and with a thickness of between 5 and 10 m. This layer extends from near the entrance to Humboldt Bay in a southwesterly direction across the shelf.