Monday, November 13, 2006

Acoustic Profiling of Impounded Sediments Using an Acoustic Probe in Shallow Waters.

craig j. Hickey1, del leary1, and Daniel Wren2. (1) National center for physical acoustics, 1 coliseum drive, university, MS 38677, (2) National Sedimentation Laboratory, oxford, MS 38655

An increasing number of dams and their reservoirs throughout the country are approaching their predicted life expectancy.  Acoustics will play a key role by remotely determining the volume of impounded sediment and possibly characterizing these sediments in-situ. Sub-bottom profilers are acoustic based systems that measure the time it takes for an acoustic pulse to travel through the sediment, reflect at an interface, and return to the receiver. To extract the thickness of the layers between interfaces from the recorded time between echoes, an estimate of the velocity for each layer must be known. Therefore, an inherent weakness of acoustic sub-bottom profiling techniques is the need to assume an acoustic wave speed before estimates of the thickness of impounded sediment can be made. In order to address this issue an acoustic probe was built with the aim of directly measuring the sound speed profile in impounded sediments. The acoustic probe is comprised of eight piezoelectric 50 kHz transducers at 10 cm spacing mounted on a rigid rod. The source is placed in the water column above the sediment floor and is moved to various offset distances from the probe. The source signal is an acoustic chirp waveform, having a typical bandwidth of 35 to 75 kHz and a pulse length of 0.3 ms. Classical geophysical signal processing uses the measured travel time and the source-receiver geometry to determine the acoustic speed of the sediment as a function of depth. Acoustic measurements and physical core samples were obtained at collocated sites. Initial results indicate that the interval speed profile correlates well to the bulk density measured at the same location. Future plans include additional field data measurements using the current system and more elaborate signal processing techniques in an attempt at in-situ characterization.