High Frequency Backscattering from Sandy Sediments

I did a series of experiments in the lab on broadband backscattering from clean, degassed sand we obtained locally (in San Diego). I used Panametrics broadband transducers to obtain backscattered echoes at a variety of grazing angles. These data were consistent in most regards to field data we obtained off West Destin, FL in 1999 (Greenlaw et al, 2004) but, due to the careful preparation and grooming of the laboratory samples, showed some differences from the field data.

The most interesting result from the field data was a rolloff in Lambert's Parameter above about 700 kHz which, in the paper, we ascribed to some effect due to grain size. This was consistent with some lab work done earlier by Kevin Williams (1988). Our data are the circles in the plot below. Other, historical data are shown down to 10 kHz.


In the lab, we found the expected increase with frequency, this time up to about 1.5 MHz. At this point, the Lambert's Parameter was constant until it began rising again at 3 MHz. This shape was found to bear a striking resemblence to the empirical scattering model obtained by Sheng (1991) for individual sandgrains in water (dashed line in figure below). This is not what would be expected from a compacted body of sand grains but the data are highly suggestive.


With these results, the roll-off found in the field must be due to some other factor. Surface roughness is certainly a candidate but so is the possibility of small numbers of interstitial bubbles in the sediments. At this time, I have no idea.