The Shallow Porosity Structure Of The Continental Shelf Off Humboldt Bay, California: Results Of A Towed Electromagnetic Survey

We have completed a towed electromagnetic survey, mapping the electrical resistivity of the seafloor across<br>an area of the Californian continental shelf. The area has been designated as a natural laboratory by the<br>Office of Naval Research as part of the STRATAFORM initiative. More than 120km of tow-lines were<br>completed, making measurements of resistivity over the top 20m of sediment every 20m or so along track<br>and from water depths of 1OOm to around 30m. We identify three distinct depositional environments based<br>on the resistivities recorded and the porosities inferred from them: one, a mid-shelf depo-center also<br>associated with recent flood deposits; another an uplifted, buried anticline system; and the third associated<br>with the Eel river delta.<br>We present the electromagnetic method used as a technique that can provide valuable information about<br>facies conditions across the littoral zone. Such information can be used for a variety of purposes, but from<br>an environmental perspective these include: identifying sand and gravel deposits; monitoring the effects of<br>storms on continental shelf erosion; finding suitable locations for the siting of offshore cables and pipelines;<br>quantifying the transport of material from river estuaries across and along the shelf; and mapping areas of<br>fine grained sediment that might preferentially scavenge pollutants from the water column.