It will be shown in the next sections that the deviations in these acoustic parameters for sediments containing gas and contaminates is so large, that though desirable, an accurate model is not necessary when considering using the resultant data for clean up dredging purposes. Typical subbottom data acquisition schematic diagram. Figure 3 . Figure 2 illustrates the acquisition of typical subbottom data near a core site. The sound source can be pingers, chirp, or boomers.
Final selection is dependent on local geology and desired resolution.
For the Trenton Channel site discussed in this paper, two pingers with frequencies of 3.5 KHz and 7.0 KHz were used along with a boomer to determine depth to the compact river bottom. Various receiver types are available and are normally selected to optimize the beam pattern and to match the sound source utilized. The data from the receiving arrays are amplified with calibrated amplifiers and digitized for future processing. An important addition to the acquisition procedure is the use of a calibration hydrophone of known bandwidth and sensitivity. This calibration phone allows absolute calibration of the source power and the receiver sensitivities, allowing absolute computation of the bottom reflection coefficients.