Instrumentation of SEASWAB experiment

Abstract

From September 1975 to April 1976 offshore production Platform V in South Pass, Block 28 (East Bay, Louisiana), was instrumented to measure the effect of storm waves on the soft sediments typical of the Mississippi delta (in a project given the acronym SEASWAB). A portion of this project consisted of four identifiable units of instrumentation (see note): (1) an accelerometer package buried 1 m in the sediment to measure three‐dimensional sediment accelerations and an associated pressure transducer, which measured wave‐induced pressures; (2) an array of instruments that included a wave staff, electromagnetic current meter, and a pressure transducer to examine various relationships between wave properties; (3) a wave‐, current‐, and wind‐measuring station 3.35 km inshore of Platform V to determine the transformation of the waves as they moved over the sediments; and (4) a transponder buried in the mud, the position monitored so that long‐term mudflow could be measured. The direct measurement of seafloor oscillations required the unique instrumentation of the accelerometer system. Three Bruel and Kjaer 8306 accelerometers mounted at right angles to each other made possible the measurement of small oscillations (～0.01 m) at low frequencies (0.1–0.3 Hz). The acoustic method of measuring long‐term mudflow was subject to problems associated with sound propagation in shallow water. The range of the system was found to be 2.74 km, apparently independent of depth. Multiple returns received after single interrogations of the transponder decreased the accuracy of the system.