Climate change has a pronounced effect on water resources in many semiarid climates, causing populated areas such as San Diego County (USA), to become more vulnerable to water shortages in the coming decades. To prepare for decreased water supply, San Diego County is adopting policies to decrease water use and to develop additional local sources of water. One new local source of freshwater is produced by a desalination facility that purifies brackish groundwater from the coastal San Diego Formation. This formation has been studied extensively onshore, but little is known about the geology or groundwater quality offshore in the adjacent continental shelf. Because most groundwater systems are interconnected and complex, further analysis is needed to identify offshore geology, possible sequestration of freshwater in the shelf, and potential pathways for saltwater intrusion. This comprehensive understanding is important because seawater intrusion may limit use of the San Diego Formation and longevity of desalination facilities. Controlled-source electromagnetic methods are uniquely suited to detecting offshore groundwater as they are sensitive to changes in pore fluids such as the transition from fresh to brackish groundwater. This paper describes results from surface-towed electromagnetic surveys that mapped the pore-fluid salinity and possible fluid pathways in the continental shelf off the coast of San Diego. The results indicate a considerable volume of fresh-to-brackish groundwater sequestered in the shelf, both in continuous lenses and isolated pockets, that appear influenced by fault systems and shallow stratigraphy.

Understanding the depth of investigation for electromagnetic and electrical methods is important in experimental design and for interpreting inversions. Many studies have defined the depth of investigation for electromagnetic sounding methods, but none have included continuously towed controlled-source electromagnetic methods.Nodal controlled-source electromagnetic surveys using ocean-bottom electromagnetic receivers have generally been found to have a depth of investigation limited to about half the maximum source-receiver spacing, but experience using continuously towed arrays suggests sensitivity to targets at depths approaching the source-receiver spacing. We test this on two-dimensional synthetic data using two methods. A rigorous approach is to re-invert data as a highly conductive or resistive basement is included at successively shallower depths. When the data misfit becomes unacceptably high we can conclude that the maximum depth of inference has been passed. Rather than rely on overall misfit, we note that it is more realistic to preferentially examine those data most sensitive to largest depths (longest offsets and lowest frequencies). A more practical approach is to determine the depth at which a conductivity contrast can be imaged by inversion, noting that knowledge of a contrast is geologically useful even if the actual conductivities cannot be recovered. Both approaches confirm that the increased data density of towed electromagnetic systems at longest offset increases the depth of investigation to about the maximum offset distance.

As applications for offshore renewable energy projects increase, state and federal land managers have become concerned over potential impacts to cultural heritage resources along submerged landscapes. Identification, documentation, and management of historical shipwrecks have been relatively common, but methods for identifying submerged pre-contact archaeological deposits are developing in many coastal regions of the continental United States. Permitting agencies in certain regions along the Gulf of Mexico and the Atlantic Ocean typically require management plans that include mitigation measures for submerged archaeological sites. Over the last decade, resource managers along Pacific Coast regions have become increasingly aware of the need for submerged archaeological site protection. This is especially important since the eastern Pacific continental shelf has become a focal point in the search for late Pleistocene migrations into the Americas and other evidence of pre-contact habitation in coastal regions since the Last Glacial Maximum (LGM). Integral to this search is the identification of submerged Pleistocene landforms that may favor preservation of pre-contact archaeological sites. Stemming from this, our multidisciplinary and multi-institutional effort includes marine geologists, marine biologists, and archaeologists synthesizing existing data using GIS models, and collecting new side scan sonar, CHIRP, and multibeam bathymetry data ground truthed with marine sediment cores. This methodology for the identification of submerged archaeological deposits is not new; however, the landscape approach that defines our research, and our focus on understanding paleolandscapes using modeling, sonar survey, and marine coring is the first of its kind on the eastern Pacific continental shelf. The goal of our project is to develop an archaeological sensitivity model, which the Bureau of Ocean Energy Management can consult in the offshore energy permitting process. As such, we are building our model using data from California’s Northern Channel Islands and testing the model along Oregon’s central coast. Results suggest that with the right technologies, sensitive landscape features such as paleochannels, paleoestuaries, and offshore tar seeps – all features used by Native American communities during the late Pleistocene and Holocene along the Pacific Coast – can be identified and used to model sensitive archaeological landscapes. We also are testing the efficacy of controlled-source marine electromagnetic methods in conjunction with sonar survey data for the identification of tar seeps, paleochannels, and buried archaeological shell midden deposits. This combined methodological approach is unique to North America’s Pacific Coast and represents a pioneering effort in the search for submerged archaeological deposits, which will help identify, document, and preserve underwater cultural heritage resources.