CHARACTERISTICS OF HIGH-RESOLUTION MARINE REFLECTION PROFILING SOURCES

Midwater signatures of eight common types of marine seismic sources ha ve been recorded and characteristics of importance for high-resolution reflection profiling have been analyzed. Analyzed characteristics inc lude wavelet shape, peak frequency, bandwidth, repeatability and direc tivity. Digitization rates required to describe the signatures before any processing and after whitening deconvolution were determined. It w as intended that most of the broad range of available source types be represented. Toward that end, both resonant and impulsive types were s tudied; the impulsive types being both electric-discharge and pneumati c. They included a conventional sonar transducer, a chirped sonar tran sducer, a boomer, a plasmagun, a multitip sparker, a watergun, an airg un and three sizes of sleevegun. It was observed that the peak frequen cy of each conventional sonar is similar to its nominal value and that of the chirped sonar is about the midpoint of its nominal sweep bandw idth. The dominant frequencies of the electric-discharge impulsive sou rces are about an order of magnitude higher than those of the pneumati c impulsive sources. Among impulsive sources, the boomer has the highe st peak frequency and the watergun has the broadest bandwidth. The sle evegun and airgun were found to be comparable, both having low frequen cies, poor repeatability and weak directivity. The conventional sonar and the boomer are the most repeatable sources. The boomer shows the s trongest directivity with the chirp sonar and the watergun being the m ost non-directional. Sample rates required to describe the unprocessed signatures vary from 2 to 88 times the peak frequency and from 1 to 4 times the bandwidth 40dB below peak power. The required rate depends largely on source type but can also be affected by energy level. Bandw idth seems to be a more stable indicator than peak frequency. Rates re quired to describe the results of applying whitening deconvolution to signatures were about 1.5-3 times higher than rates calculated for the unprocessed signatures. This leads to the conclusion that the digitiz ation rate required in a specific situation can depend as much on the intended processing scheme as it does on source type.