Douglas N. Lambert
Donald J. Walter
Naval Res. Lab., Code 7431, Stennis Space Center, MS 39529
William R. Bryant
Niall C. Slowey
Texas A&M Univ., College Station, TX 77843
John C. Cranford
Neptune Sci., Inc., Slidell, LA 70458
The Naval Research Laboratory has been developing a normal-incidence, high-frequency (15--30 kHz), narrow beamwidth (6(degrees)--12(degrees)), high-resolution (>92-dB dynamic range) seismic system with the capability to predict, in near real time, acoustic impedance of the upper several meters of the seafloor using inversion techniques. Acoustic impedance, predicted in a series of ten selectable time windows, is then used to estimate other sediment properties through empirical relationships. A series of ground truth sediment cores have been collected along a seismic trackline in the southwestern Baltic Sea with sediment types varying from glacial till to soft, methane gas-charged clayey silts. Comparison of the high-resolution seismic data to sediment structure determined from the cores shows excellent correlation for both 15- and 30-kHz data. The comparison of laboratory-measured sediment geotechnical properties and acoustically estimated properties shows good correlation in the surficial sediments and somewhat less correlation with depth in the sediments. Since gas bubbles within the sediment are strong acoustic reflectors at these frequencies, the use of standard algorithms for nongassy sediments can lead to overestimation of predicted impedance values.