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High-frequency (200 to 15,000 Hz), high-resolution seismic reflection profiles can be a valuable tool in the exploration for offshore hydrocarbon accumulations. The occurrence of hydrocarbons (particularly natural gas) either in the water column or shallow subbottom can result in strong acoustic impedance (density × velocity) contrasts which may produce a number of anomalous seismic responses including: (1) water-column anomalies, which are strong reflections that rise from the ocean bottom into the overlying water column as a result of natural gas seepage; (2) subbottom amplitude anomalies ("bright spots") that, when found along geologic structures, may indicate shallow hydrocarbon accumulations; (3) seismic smears, which are "turbid," chaotic, high-amplitude events that may indicate shallow, gas-charged sediments; and (4) seismic wipeouts, transparent zones commonly found below seismic smears or water-column anomalies indicating total reflection and/or absorption of seismic energy in an overlying zone of gas.
A high-frequency, high-resolution seismic reflection profile survey of the continental margin off northern Santa Cruz County, California, using both a 300-joule "uniboom" and a 1-kilojoule sparker has resulted in the detection of all four of these seismic anomalies. The shelf here is cut diagonally by the San Gregorio fault zone which marks the southeast boundary of the Outer Santa Cruz basin. Northeast of the San Gregorio fault, the shelf is dissected by the Monterey Bay fault zone. More than 100 water-column anomalies (gas seeps) have been detected, some of which rise over 50 m (160 ft) into the overlying water column. Most water-column anomalies correlate with subbottom geologic structures such as anticlines, faults, and truncated, tilted strata. Several subbottom amplitude anomali s, seismic smears, and seismic wipeouts have also been detected and correlated with structures. All these anomalies have been found in association with the middle Miocene Monterey Formation, the late Miocene Santa Cruz Mudstone, and the Pliocene Purisima Formation. Samples of natural gas have also been collected from a shallow
coastal water well. Geochemical analyses indicate the presence of 74 to 91% methane, 7 to 23% nitrogen, ~ 2% carbon to dioxide, and < 1% ethane. The gas appears to be thermogenic in origin as it has ^dgr13C values of -29.51 to -32.5 ^pmil PDB. Oil-saturated sandstones have also been dredged from a depth of 700 m (2,300 ft) in Ascension submarine canyon, further indicating the natural seepage of hydrocarbons in this area. We interpret these natural hydrocarbon seeps to be the result of migration from depth along active faults within the San Gregorio and Monterey Bay fault zones.
High-frequency, high-resolution seismic reflection surveys in unexplored offshore areas may yield evidence for hydrocarbons that are not detectable with conventional, deep-penetrating, low-frequency (< 100 Hz) seismic reflection systems, and thus may be very valuable in the search for oil and gas accumulations.
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