Airgun03

During the SSCP03 (Seismic Signature Characterization Project), two Environmental Acoustic Recording Systems (EARS) buoys were deployed in the northern Gulf of Mexico (GoM) by the Littoral Acoustic Demonstration Center (LADC).

The Littoral Acoustic Demonstration Center (LADC) was funded and tasked to deploy, recover and analyze the data from a high speed Environmental Acoustic Recording System (EARS) buoy during the 2003 Sperm Whale Seismic Study (SWSS) experiment. The EARS buoy (leased from NAVOCEANO by LADC) had an increase in useable bandwidth in excess of 25kHz and represents the next generation of buoy. The main objective of the LADC task was to obtain sufficient data from an industry standard seismic airgun to permit an unbiased, calibrated acoustic signature determination. The airgun and ship time were made available as part of the larger SWSS 2003. The International Association of Geophysical Contractors (IAGC) in cooperation with the Minerals Management Service (MMS) sponsored the LADC effort.

The EARS buoys were second-generation buoys with an increase in useable bandwidth in excess of 25kHz. In order to compensate for the expected high level received signals from the airgun and to simultaneously record the ambient noise between airgun shots; two separate EARS data loggers with different gains were required. Technical improvements in electronics and power consumption allowed for an overall reduction in physical size requirements. These improvements would allow the two EARS data loggers to be mounted in the same mooring frame, which in turn would allow both buoys to be deployed quite simply on a single mooring.

The two buoys were co-located on the same mooring (see Fig. 2) near Green’s Canyon in the Northern GoM and recorded ambient noise and seismic airgun array shots up to approximately 25 kHz. The hydrophone of each buoy was approximately 250m from the bottom in a water depth of about 983m. The M/V Kondor towed an airgun array on linear tracks with horizontal closest points of approach to the buoy of 0, 500, 1000, 2000, and 5000m. Experimental data are available for a wide range of horizontal distances (up to 7 km) and arrival angles. The raw data were calibrated in post processing using the EARS system parameters to produce calibrated pressure time series for each shot. These data were analyzed in both the time and frequency domains. Maximum pressures for each shot were obtained as well as spectrogram analysis for a fixed window length. Analysis is made in one-third octave bands.

Arrival times for direct path, reflected path and critically refracted events from the series of five parallel survey lines were analyzed. Near uniform sound speed in the survey area allowed for classification of first-break and secondary arrivals as direct arrival and either reflected or refracted path arrival, respectively, using simple propagation geometries. Beyond a certain horizontal range (approximately 4.5 km), the critically refracted arrival overtook the reflected arrival. In the survey line farthest from the recording buoy (approximately 5 km closest point of approach), the refracted arrival arrives prior to the seafloor reflection in every shot record, and actually begins to precede the direct arrival. These data allow for estimates of the critical angle and sound propagation speed in the seafloor sediment layer.

Number of buoys:
2
Location:

27°40.0996'N and 90 °21.966'W

Dates:

9 June 2003- 23 June 2003

Days of recorded data:
14.5 days
Amount of data:

186 Gb

Depth:
983m

Temperature and conductivity time series were collected over 14.5 days at a depth of 733 m. 17 XBTs were cast.

Additional enviornmental data was collected by Texas A&M and is available through the SWSS program manager.