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Methane Seeps, Faults, and a Giant Cricket

posted Jul 3, 2012, 6:27 PM by Shannon Casey
Chirp! Chirp! Chirp! I fell asleep early this morning to the sound of our sub-bottom echosounder, after the SeepTeam’s second overnight survey. The echosounder emits a sound pulse every few seconds that sounds kind of like a giant cricket. We’ve now completed over 40 hours of survey time, operating the Melville’s multibeam sonar and sub-bottom echosounder, in search of methane seeps at the seafloor. Both of these geophysical tools send sound waves through the water to the seafloor. The sound is reflected off of objects in the water column, the seafloor, and sedimentary layers beneath the seafloor. We can then image these reflections on our computers, and look for evidence of gas in the water column, or in the sediments below. During each survey, we give the captain locations to drive the ship while we eagerly watch the data appear on computer screens, hopeful that we’ll observe features indicative of methane seeps. So far, we’ve been able to identify some areas that warrant further exploration with both the geophysical tools and the remotely operated vehicle (ROV).

Photo: Jillian Maloney monitors live survey data on board R/V Melville for
signs of gas in the water column that would indicate a possible seep site.

As a geologist on the Seeps Team, my primary goal is to help my biologist teammates locate methane seep communities. In addition, I’m interested in studying faults located offshore San Diego. It just so happens that faults often provide pathways for methane and other hydrocarbons to migrate upwards from deep below the seafloor, making this an exciting collaboration between geologists and biologists. All of our survey sites are within the California Continental Borderlands, which is an area of strike-slip faulting offshore southern California. These faults move laterally, similar to local onshore faults like the San Andreas and San Jacinto Faults. Because the faults in the Borderlands are under hundreds of meters of water, they are harder to study, and are not as well understood as their onshore counterparts. During our surveys, we observed several strands of the Coronado Bank Fault marked by offset and deformed sedimentary layers beneath the seafloor. The Coronado Bank Fault lies ~13 miles west of the coast of San Diego, and so understanding how often, and in what way it moves, is important for assessing any potential geohazard posed to San Diego. I’m excited for more surveys, and to analyze the data more closely. These data will not only help us to locate seep sites, but will also help us to understand better the tectonics of the California Continental Borderlands.

--Jillian Maloney, Scripps Graduate Student