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

摘要

A towed electromagnetic survey, mapping theelectrical resistivity of the seafloor, was conducted over an area ofthe Eel shelf off Humboldt Bay, California. Continuous resistivityprofiles to 20 m below the seafloor were measured along 120kmof track line, from water depths of 100m to around 30m. Theshallow structure along the shelf is highly variable and we identifythree distinct environments based on the recorded resistivitiesinferred from them. The first region is a mid-shelf depocenter,characterized by a thin (~2m), moderately high-porosity (45~60%)surface layer, which overlies a less porous (35-45%) andhomogeneous substrate, uniform both laterally and vertically. Thisregion is found to the northwest of the Humboldt Bay entrance,from water depths of about 65m to at least 100m, and is roughlycoincident with recent flood deposits. The second region is locatedcloser to shore and contains extremely high resistivities for ashallow sedimentary environment. It reveals a high degree ofspatial variability on length scales of several hundred meters.Several possibilities exist to explain such high resistivities andthese include: upwelling fresh water channeled to the seafloorthrough local fault and anticline systems; a significant volume ofnatural gas within the sediments; or a continuous process ofcarbonate precipitation through oxidation of methane near theseafloor, which, over time, builds a substantial thickness oflithified material. None of the above explanations are mutuallyexclusive, and all could act in concert to increase resistivities. Thethird region roughly coincides with the Eel River delta andfeatures a buried layer of moderately low porosity (30-35%) at adepth of about 5m and with a thickness of between 5 and 10m.This layer extends from near the entrance to Humboldt Bay in asouthwesterly direction across the shelf. 1999 Elsevier ScienceB.V. All rights reserved.