Marine Acoustical Hydrocarbon Detection

Abstract

ABSTRACT Hydrocarbon seepage in the marine environment has long been recognized. The presence of dissolved, light gaseous hydrocarbons can be detected by the various sniffing devices currently in use. However, because of oceanic currents and the six to eight minute sampling lag time it is very difficult to pinpoint the source of the seepage. Escaping hydrocarbon bubbles can be detected rising in the water column by means of high resolution subbottom profiling equipment. Bubbles have been detected upon 3.5 kHz acoustical recorders and also on a 30 kHz recorder. The precise point of seepage can be located by these instruments. The velocity contrast between gas and sea water is approximately 1100 meters/second. This reflectivity contrast plus a resonant energy source from the bubbles give rise to a very strong return signal. This shows up as an apparent cloud in the water. The presence of bubbles as the causitive reflectivity factor has been proven by visual sighting of the bubble streams from the ship and the bubbles have been recorded upon video tape both emanating from the bottom and also at various midwater depths. The relationship between seepage, subsurface structure and bottom topography is demonstrated. It is suggested that there may be a direct relationship between near surface, Pleistocene structures and secondary accumulation, and that the seepage comes from thesesecondary traps. There is ample historic and contemporary evidence of hydrocarbon seepage in the marine environment, particularly in the Gulf of Mexico. Spanish explorers reported tar and bitumen floating in the Gulf in the early 1500's. There is also archaeological evidence for the use of natural tar by the Pre-Columbian Karankawa Indians. There were reports of tar on the Galveston beaches in 1841. In the early 1900's large patches of viscous oil were reported in the north central Gulf. In 1909 the ship COMMEDIAN reported 3 jets of the brownish liquid spurting into the air in a water depth of 600 fathoms (Soley, 1910.) In 1970 a research project designed to locate and study hydrocarbon seepage in the Gulf of Mexico was initiated. Until the present time it has been very difficult to pinpoint a precise seepage site within the Gulf itself. Various companies have developed instruments for measuring dissolved hydrocarbons in the ocean. Using two different instruments of this type scientists at Texas A&M detected anomalously high concentrations of dissolved hydrocarbons at various locations in the Gulf of Mexico. However, as presently used the sniffer, as the instrument is called, is a reconnaissance type exploration tool and is not an effective instrument for pinpointing seepage locations. Concentrations of dissolved gasses may also be swept away from their sources by tidal and other currents. Figure 1.