Variation in the Spatial Distribution of
Natural Marine Hydrocarbon Seeps in the Northern Santa Barbara Channel,
D Quigley, (Inst. for Crustal Studies, and Dept. of Geological Sciences, Univ. of Calif.Santa Barbara, CA 93106-1100; J S Hornafius, (Inst. for Crustal Studies, Univ. of Calif., Santa Barbara, CA; 93106-1100; B P Luyendyk, (Inst. for Crustal Studies, and Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-1100 R.D. Francis, (Dept. of Geology, Calif. State Univ., Long Beach, CA; 90840-3902); E Bartsch, (Inst. for Crustal Studies, and Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-1100)
Natural gaseous marine hydrocarbon seepage offshore of Coal Oil Point, Santa Barbara can readily be imaged by underwater sonars because the rising seep gas bubbles act as acoustic scattering targets. Utilizing a 3.5 kHz system, seeps were mapped during cruises in 1995 and 1996 with differential GPS navigation. Historical 3.5 kHz data acquired by P. Fischer in 1973 in the same area allows an analysis of the distribution of seepage at that time but with lower navigational accuracy. These data provide a glimpse of changes in the natural seep distribution over 1-20 year time scales. For purposes of the historical comparison, analog records were digitally scanned and quantitatively analyzed in terms of the image darkness. For the 1995-1996 surveys the 3.5 kHz data were also digitally acquired and analyzed. From both methods a spatial distribution of relative seep intensity is obtained. Some variation in the distribution of seep intensity could be an artifact of differing survey coverage. Other real short term variations in seep distribution are probably due to differences in ocean currents since this method identifies the seep gas in the water column rather than at the seep source. Long term changes in the spatial distribution of seepage suggest offshore oil production may have reduced some seepage due to the reduction in reservoir pressure in the Monterey formation. The total seep gas flux within the survey area during the 1995 surveys suggests an emission rate on order of 1.4x10^5 m3/day. This can be used as the input source function for problems of oceanographic modeling of seep product dispersal.