Acoustic masking in sediments due to gases on the western continental shelf of India

Abstract

Surveys carried out on the western continental shelf of India indicate that the inner and middle shelf to a depth of 50–60 m is covered by acoustically transparent clays. On the shelf off Bombay, the clays are thin near the shore where they overlie rocks or sands, and increase to 20–35 m on the middle shelf, beyond which they thin out at a depth of 50–60 m where relict oolitic sands are exposed. The shallow seismic and echosounding records of these clays at places are marked by turbidity which may be categorised as (1) extending over a distance of 50–60 km and covering large areas of over 2500 km 2 and (2) resembling isolated inverted “U” shaped features. The turbidity appears to rise from the underlying reflectors to within 5–10 m of the sea bed. These features are very prominent in two areas, viz. on the middle shelf off Bombay and on the inner shelf. The shallow bore holes drilled and the cores collected in the area show bubbles or fissures in the clays of the turbid zone. The gas predominantly consists of methane with terraces of higher hydrocarbons. The composition of the gas and the occurrence of the gassy sediments on the middle shelf and in the vicinity of the estuaries and rivers favours their biogenic nature. The common depth of occurrence of 5–10 m may represent the boundary between the sulfate-reducing zone and the underlying methane-producing zone. Perhaps the gases were formed by the degradation of large quantities of organic matter produced by increased productivity, or because of higher influx of organic matter in the recent past. However, the location of some of the occurrences in the vicinity of an oil and gas field indicate that the gases in the sediments of the outer shelf off Bombay may be petrogenic and those on the middle and inner shelf off Bombay, Konkan, Karnataka and Kerala may be biogenic.