Observations of attenuation during the Scripps Pier bubble experiment

An experiment that measured the effects of bubbles just offshore from active surf was performed in the spring of 1997 off the pier at the Scripps Institution of Oceanography. A region from a few tens of meters to 1000 m from the beach was instrumented by several researchers. An emphasis was placed on a region approximately 400 m2 in area and about 300-m offshore where most of the instruments were clustered. Located in this area was a triangular-shaped frame called the Delta Frame, which had sources at two vertices and eight hydrophones along the perimeter (each side was 9.4 m long). Eight frequencies, from 39 to 244 kHz, were propagated sequentially from each source and received on each hydrophone. All 16 transmissions occurred within 12 ms and were repeated at a 1-s interval during each of ten 88-min data runs. Bubbles produced in the surf zone were carried out to the experimental region by rip currents and had dramatic effects on all the instruments. This paper discusses their effects on attenuation as measured at the Delta Frame. [Work supported by ONR.]