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

A towed electromagnetic survey, mapping the electrical resistivity of the s eafloor, was conducted over an area of the Eel shelf off Humboldt Bay, Cali fornia. Continuous resistivity profiles to 20 m below the seafloor were mea sured 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 por osities inferred from them. The first region is a mid-shelf depocenter, cha racterized by a thin (similar to 2 m), moderately high-porosity (45-60%) su rface layer, which overlies a less porous (35-45%) and homogeneous substrat e, uniform both laterally and vertically. This region is found to the north west of the Humboldt Bay entrance, from water depths of about 65 m to at le ast 100 m, and is roughly coincident with recent flood deposits. The second region is located closer to shore and contains extremely high resistivitie s for a shallow sedimentary environment, It reveals a high degree of spatia l variability on length scales of several hundred meters. Several possibili ties exist to explain such high resistivities and these include: upwelling fresh water channeled to the seafloor through local fault and anticline sys tems; a significant volume of natural gas within the sediments; or a contin uous process of carbonate precipitation through oxidation of methane near t he seafloor, which, over time, builds a substantial thickness of lithified material. None of the above explanations are mutually exclusive, and all co uld act in concert to increase resistivities. The third region roughly coin cides with the Eel River delta and features a buried layer of moderately lo w 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. (C) 1999 Elsevier Science B.V. A ll rights reserved.