Conductivity probe and stereo camera measurements of roughness during SAX04

Abstract

The detection and imaging of buried object in ocean sediments has been the subject of a series of recent high-frequency sediment acoustic experiments. These experiments have focused primarily on quantifying the acoustic backscatter-ing, penetration, and propagation within the sediment. Integral to these experiments as well as to the application of sonar systems to the detection of buried objects is knowledge of the roughness of the sediment interface as environmental input. The primary tool used to make direct measurements of seafloor roughness has been analog stereo photography. In the last ten years, a number of new technologies have been developed to measure roughness including digital stereo photography, laser-line scanning, structured lighting, and conductivity measurement. During the Sediment Acoustics Experiment 2004 (SAX04), systems incorporating these different measurement techniques were deployed to measure seafloor roughness. This provided a unique opportunity to compare the performance and results of these technologies under a common set of conditions and at a single location. In particular, this provided an opportunity to compare the performance of optically based systems to non-optical systems. This paper compares two of the systems deployed by APL-UW during the experiment: a digital stereo camera system and the In-Situ Measurement of Porosity (IMP2). The distinction between measurement techniques became especially important because of the effects of Hurricane Ivan which made landfall 100 km to the west of the site, several days prior to the beginning of the experiment. The storm deposited significant amounts of lagoonal mud onto the sediment interface and reduced visibility to as little as a foot at the beginning of the experiment. However, because IMP2 uses a conductivity probe to measure roughness, it was not hampered by the lack of visibility. This system consists of a single constant current probe which is mounted on an x-z positioning system such that measurements of the bottom height can be made along a 4m track at 1 cm increments. In addition to being able to measure the roughness of the sand sediment interface, IMP2 was also able to measure the mud layers and detect the presence of sand suspended in those layers. Analysis of the acoustic backscattering and penetration measurements at the site indicate that these mud layers and sand inclusions can play significant roles in the imaging of buried objects. As the experiment progressed, visibility gradually improved and reached the point where the APL-UW stereo camera system could be deployed. This camera system consists of two Easier A102 digital cameras, each with a 1300 times 900 pixel resolution, mounted on a diver portable frame and connected to a PC104 stack, also attached to the frame, which triggered the cameras and flash. The stereo camera system can capture simultaneous stereo image pairs at a rate of 20 pairs per minute. This is a distinct advantage of optical measurement systems in that they can capture larger areas of the sea floor than non-optical systems and can provide 2-D roughness spectra whereas IMP2 cannot. The results of this experiment indicate the need for multiple measurement techniques when quantifying bottom roughness to improve our understanding of acoustic techniques related to mine-countermeasures. In order to address this need in a single system, efforts are currently underway to extend the measurement capabilities of IMP2 through the addition of a laser profiler. This new system will be capable of imaging the sea floor up to a 0.5 m on either side of the conductivity probe and along the 4 m length of the IMP2 trolley