HR: 1340h
AN: B53A-0930    [Abstracts]
TI: Sonar Determination of Environmental and Geologic Controls of Spatial-Temporal Variability in the Coal Oil Point Seep Field, California
AU: Leifer, I
EM: ira.leifer@bubbleology.com
AF: Marine Sciences Institute, University of California, Santa Barbara, CA 93106, United States
AU: Leifer, I
EM: ira.leifer@bubbleology.com
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106, United States
AU: Luyendyk, B P
EM: luyendyk@geol.ucsb.edu
AF: Department of Earth Science, University of California, Santa Barbara, CA 93106, United States
AU: Luyendyk, B P
EM: luyendyk@geol.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106, United States
AU: * Stubbs, C C
EM: stubbs@umail.ucsb.edu
AF: Marine Sciences Institute, University of California, Santa Barbara, CA 93106, United States
AU: * Stubbs, C C
EM: stubbs@umail.ucsb.edu
AF: Department of Earth Science, University of California, Santa Barbara, CA 93106, United States
AU: Wilson, D
EM: dwilson@geol.ucsb.edu
AF: Marine Sciences Institute, University of California, Santa Barbara, CA 93106, United States
AU: Wilson, D
EM: dwilson@geol.ucsb.edu
AF: Department of Earth Science, University of California, Santa Barbara, CA 93106, United States
AU: Kamerling, M
EM: mkamerling@venocoinc.com
AF: Venoco, Inc., 6267 Carpinteria Ave. Suite 100, Carpinteria, CA 93013, United States
AB: Marine seepage of geologic methane, CH₄, is a significant yet poorly understood contribution to atmospheric greenhouse gases, and is at least 20 times as potent as CO₂. Marine seeps, excluding hydrates, are ~13% of natural emissions although there is significant uncertainty due the lack of published studies. Currently, sonar is the best method for seep emission quantification. Repeat sonar surveys spanning ten years were conducted in the Coal Oil Point (COP) marine hydrocarbon seep field in southern California. The field is one of the largest in the world emitting 10⁵m³day^-1 of CH₄ from ~3 km² of seafloor at depths from 2 to 90 m. Seepage arises from the Monterey Fm reservoir, which is overlain and capped by the Sisquoc Fm. Surveys sought to locate seeps, quantify emissions, relate seepage to geological structures, and identify spatial- temporal variability in the seep field to examine controls of and environmental factors. Sonars used were wide beam 3 kHz and 3-15 kHz. To estimate emissions, RMS sonar backscatter amplitude was calculated for a fixed depth window above the seafloor and normalized to the bottom return (termed J). Then, seepage above noise along track lines was quantified for J and gridded with ~40 m resolution. The noise level was calculated from the probability distribution of J for each track line. Emissions were derived from J using a lab calibration. Results suggest geology has a controlling influence on seep spatial distribution through features including fault planes, outcrops of the contact between the Monterey and Sisquoc Fms, and folded beds. The bed and fault plane interfaces provide pathways for bubbles to migrate from the Monterey Fm through and around the capping units. Emissions show significant variability on tidal to decadal time scales and decameter to km spatial scales, including the activation and deactivation of seepage areas. Environmental factors influence seepage- e.g., emissions inversely relate to tides- with variability occurring along pathways that appear to be geologically controlled. Emission uncertainty arises from the calibration function, which showed very poor dynamic range, and the inherent geometric limitations in wide beam sonar. Further, its low spatial resolution prevents correlation of seepage with smaller geologic structures. Recent studies show high frequency multibeam sonar mapping achieves spatial resolutions < 50 cm. This allows identification of individual bubble plumes, eliminating geometric uncertainty and improving dynamic range.
UR: http://www.bubbleology.com
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 4806 Carbon cycling (0428)
DE: 4820 Gases
DE: 4894 Instruments, sensors, and techniques
SC: Biogeosciences [B]
MN: 2007 Fall Meeting