The 300 km wide north western continental shelf off Bombay with structures of hydrocarbon prospects and number of proven oil and gas fields has been the scene of recent development. These developments required extensive seabed surveys for jackups, production platforms, pipeline routes, relocation of well heads and post lay surveys of pipelines etc. The surveys during 1976-84 covered an area of 72,600 sq. km on the inner and midshelf between 18° N and 21° N latitudes and comprised of echo sounding (12,000 lkms), side scan sonar (7,000 lkms) shallow seismics (11,000 lkms) and seabed samples (252).
The inner shelf in the north (the Gulf of Khambhat) is characterised by uneven topography (upto 13 m) chiefly due to NNE SSW trending sand banks and the intervening channels filled by about 5 m thick clays. The sand banks are superposed by NNE-S5"1 sand waves (3–4 m high), mega ripples and ripples due to the high energy tidal regime of the Gulf. The sand banks taper off at Lat. 20° 50' N (28 m depth) and further the seabed is covered by clays and marked by even topography. The clay thickness increases towards SSW from 10 m to 20–25 m in water depths from 28-30 m to 50–55 m. Between 28 to 38 m water depths in an area of approximately 2500 sq. km the clays are marked by extensive masking due to gases (about 11.5 percent methane).
The seabed between Bombay and the Bassein offshore area (50–55 m) in the west is marked by even topography and is covered by 20 to 30 m thick acoustically transparent clays. The clays attain the maximum thickness (20–30 m) at about 25 m to 30 m water depth and thin gradually to the west where the underlying relict sands of 10,000 yrs BP out-crop at water depth of 60–65 m. The transition is marked by NNW-SSE trending sandy ridges (4–5 meters high, 1–2 km wide). The outer midshelf sandy areas are characterised by uneven topography (up to 15 m) chiefly due to relict forms in oolitic, coral and algal limestones and oolites etc and 1 to 2 m high (100 to 200 m wave length) NNE-SSW trending relict sand waves.
The delineation of a structure by geological and geophysical surveys in offshore areas is usually followed by drilling to test the structure and if oil and gas are proved by the development of the structure. Marine geological and geophysical surveys commonly referred to as seabed surveys form an integral part of the development programme and these range from site surveys prior to drilling to surveys for planning the development~ i.e., location and designing of platforms, pipeline routes and post-constructional surveys i.e., location of well heads and post-lay surveys for pipelines etc.