Monrovia

Collaborating Authors

Atlantic Crossing Film

OnePetroMay, 1969

This film is a 30-minute, illustrated, seismic profile cross...section of the deep ocean floor and sub-bottom sedimentary layering from the island of Trinidad, B.W.I. to the port of Monrovia, Liberia. The survey program was initiated in June, 1967. The traverse to Liberia was completed in 14 days at an average vessel speed of 10 knots. The seismic profiling system was Teledyne Exploration's 120,000 Joule SSP System. Seismic data was recorded on a facsimile recorder, the detector streamer cable was Teledyne's 100 sensor, single-channel Hydrostreamer. The actual traverse length was about 3,500 miles.. Our film will show perhaps two thirds of the actual field sections. The data has not been processed in any manner. The data shown consist of essentially continuous seismic profile records, with the vertical scale of the record a 4-second [from top to bottom] time scale, and a horizontal distance scale [across the screen] of approximately 15 miles. This produces a vertical-horizontal scale ratio of approximately 1:8. While this ratio could be altered by changing either the firing interval of the underwater seismic source or the vessel speed, the parameters selected were considered optimum for this traverse. Thus, we will see an unfolding of the .subsea bottom geology with the water surface at the top of the record. The first major event is the sea floor, followed by numerous reflector events which detail the sedimentary layering below the sea floor. One may estimate the depth of the sea floor by remembering that each second [one fourth of the vertical record length] from the top of the record down toward the sea floor, covers a distance of 800 fathoms; i.e., if the sea floor is seen at about 2 seconds, the water depth is approximately 1,600 fathoms. Throughout the film, inserts have been added illustrating the equipment and the manner and methods of usage aboard ship. Geological interpretation is based on data in current publications. Positioning was by dead reckoning and celestial navigation except at the origination and termination of the seismic profile. As mentioned earlier, portions of the profile were edited out because of monotonous geology, and the need to limit the film to 30 minutes. One breakdown occurred near the halfway point of the traverse [mid-Atlantic rise] and about 300 miles of data were lost. The vessel used on the program was a typical work boat, 136 ft long and 26 ft wide [M/V Stranger]. The first film cut shows a similar survey vessel rigged with the same seismic profiler system. The vessel is shown leaving Port Arthur through the Sabine Channel. Illustrations of the deck rigging detail the 20-ft outrigger booms, from which the arcer electrodes are towed. This scene shows a wire dispenser for the Teledyne wire explorer [WASSP] attachments. Each boom is lowered into position and two electrical cables are released into the water. The cables are approximately 18 ft apart and extend about 150 ft astern of the vessel. The Teledyne crew is shown lowering one of the cables over the side. Cable pullers are used to simplify retrieving the cables while underway.

OnePetro

doi: 10.4043/1127-MS

OTC

OTC-1127-MS

Offshore Technology Conference