Modern Assessment of Natural Hydrocarbon Gas Flux at the Coal Oil Point Seep Field, Santa Barbara, California

The Coal Oil Point seep field is among the most active and studied hydrocarbon seep fields in the world. The water column of the Coal Oil Point seep field was acoustically surveyed from 31 August to 14 September 2016 with a 200-kHz split-beam echo sounder to map the distribution of natural hydrocarbons in the region. An in situ direct capture device was used to measure the volumetric gas flux of natural hydrocarbons for three localized seep sites while simultaneously collecting acoustic volume backscatter measurements of the hydrocarbons within the water column. The acoustic volume backscatter was calibrated with the measured volumetric gas flux, and the resulting relationship was used to determine flux over the entire seep field. The estimate of integrated volumetric gas flow rate over a survey area of approximately 4.1km(2) was 23,800m(3)/day. The estimates of integrated volumetric gas flow rate and volumetric gas flux were compared to measurements reported in previous studies and were 2 to 7 times smaller than results obtained by Hornafius et al. (1999, ), which had a total survey area of 18km(2). However, differences between methodologies limit the ability to assess natural variability in the Coal Oil Point seep field. Plain Language Summary Coal Oil Point is one of the largest and most studied natural underwater hydrocarbon seep sites in the world. Coal Oil Point is located within the Santa Barbara Channel off California's coast, and researchers have been studying this natural hydrocarbon site for five decades to understand how the release of petroleum from the seafloor affects the ocean, atmosphere and living organisms. This study combines acoustic measurements from a broad-scale survey, with direct observations of gas flow rates, in order to map the contemporary distribution of seeps and obtain estimates of total gas flow rate for the study site. The total gas flow rate for the surveyed area, a total area of 4.1km(2), was approximately 23,800m(3)/day. The gas flow rates from this study were compared to estimates reported in previous studies and showed that current gas flow rates range from 2 to 7 times lower than those reported in 1999 (total survey area of 18km(2)). However, due to differences in the approaches used to estimate the gas flow rate in the region, it is difficult to address if the change in gas flow rates is caused by natural variability or due to differences in methodology between studies.