The SandScan sensor is a derivative of the WHAPS sensor, using the same electronics and case but a different transducer. It was built rather quickly to provide a field sensor to test the hypothesis that algae in the sediments could and would produce oxygen that, under proper circumstances, could come out of solution and form bubbles -- thus enhancing the scattering strength of the bottom by a potentially large amount. The original version of SandScan was self-contained (as shown above). It was dropped to the bottom and began taking backscattering measurements of the bottom using the transducer mounted on an arm at a 45 degree angle to the center post. Backscattering was measured at several frequencies from 600 to 900 kHz and the intensities simply summed into a single value that was saved, along with the time, in the (small) internal memory of the CPU card. Data were taken at programmed intervals. The device could be left in place for up to several days and the data downloaded after retrieval.

The figure below is a contour plot of relative backscattered intensity vrs range and time for a period in June of 2006 off Scripps Pier.


Recently, I have modified SandScan to take backscattering data at seven frequencies around 250 kHz and output the echo amplitudes after every ping. We will be able to observe trends in bottom backscattering and also inspect echo statistics with this data set. I have built a datalogger to fit inside a case identical to the electronics case, using a TAPS-6 NG CPU/IO card with a CF-RAM card to hold the data. It should be good for at least a week of data at fairly rapid intervals. We purchased a birdcam that we enclosed in a plexiglas box to take timed pictures of the bottom so we can observe wave and biological perturbations as well.

We have data from several deployments. The first several were done in a local bay where the water was quite turbid. Although the light levels were reasonably high at the bottom, we found no evidence that the interstitial phytoplankton were producing oxygen at a rate sufficient to produce bubbles. This July, however, we (Dave Thistle and grad students @ FSU) deployed the SandScan in very shallow water in Port St. Joe Bay, FL. The plot below shows the diel evolution of bottom backscattered echo level (in dB) as a function of time and of frequency. There is clear evidence of bubble evolution and dramatic increases in echo level after sunrise each day. The slower trend towards increased night-time scattering levels is harder to explain. The only clue so far is that this week-long deployment corresponded to the change from neap to spring tides.


This figure is very similar to the result obtained in aquarium experiments we (Van Holliday and I) conducted in the lab in San Diego in 2003. We obtained sand from Panama Beach, FL and innoculated it with local algae from Mission Bay. We set the aquarium by a window covered with window film to reduce ambient light levels to those expected at a few meters depth in the ocean. Ensonifying the surface with a broadband transducer, we recorded 32-ping coherent average waveforms. Data processing consisted of computing echo spectra and normalizing these by a similar spectrum taken at the beginning of the experiment. Those data covered a frequency range of about 150 - 1000 kHz. The results showed that the bulk of the enhanced scattering occured at the lower frequencies -- up to ca. 350-400 kHz, characteristic of gas bubbles. This result drove our modification of SandScan to use frequencies in the 250 kHz range.