High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

3-D Volume Inhomogeneity Measurements

Kevin Briggs (NRL), Peter Jackson (BGS) and Tony Lyons (SACLANTCEN)
Naval Research Laboratory, Code 7431
Stennis Space Center MS 39529-5004
kevin.briggs@nrlssc.navy.mil
(228) 688-5310, -5752 (fax)


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Goals

The overall task is to measure sediment volume inhomogeneities in three dimensions. From these measurements, correlation lengths in x, y, and z dimensions can be calculated in order to model the acoustic scattering from the seafloor. Inductive measurement of electrical resistivity is an in-situ, non-intrusive technique that will image porosity and density of sand sediments without sediment disturbance. The sediment sound speed and porosity will be measured by standard techniques (diver cores) in the vertical (z) direction. Collocation of the two techniques should allow estimation of Archie’s m parameter and tortuosity. The Sediment Resistivity Array Imaging System is new and is currently being fabricated to operate with the aid of divers; it is derived from experience with a previous version used to measure two-dimensional electrical resistivity in sediment slabs [1].


Sediment Resistivity Array Imaging System

This system has 768 independent electrodes arranged in a rectangular array (32 x 24) with 1-cm spacing. By varying the geometry of the electrical field generated between transmitting and receiving electrodes, different sediment depths are examined for electrical resistivity. The system is diver-deployed and, through communication with the surface where the data are logged, can be moved to various locations on the seafloor, including contiguous segments for the purpose of creating a mosaic of a larger area. For SAX99, we plan to take measurements on multiple slabs of sediment to 8-cm depth. In order to convert the resistivity data to sediment porosity, density, Archie’s m parameter, and tortuosity, we will collect diver cores from the areas measured for resistivity. The cores will be assayed almost immediately to obtain reliable porosity with the gravimetric method in order to provide a bulk porosity/density baseline. From these data, 3-D images are created from which correlation lengths are resolved and density spectra estimated. Such measurements are used as inputs to the composite roughness model.


High-Resolution Bottom Roughness Measurements (Kevin Briggs and Tony Lyons)

Goals: The overall task is to measure sediment interface roughness at 0.5-mm spacing over a two-dimensional area of the seafloor. Interface roughness is an essential parameter required to model acoustic scattering from the seafloor. Stereo photogrammetry will be used to make the measurements of relative sediment height at the seafloor. Both conventional film emulsion and digital cameras will be utilized to perform image calibration. The manually operated stereo comparator will be used on the film images of the identical seafloor from which the digital images are made.


Seafloor Stereo Photogrammetry

A Photosea 2000M stereo camera will be employed to collect periodic stereo photographs of the seafloor. During the same time frame, a digital stereo camera system developed at SACLANTCEN, La Spezia, Italy, will be used to take almost identical stereo images. The collocation of the images will enable us to compare the digital system with the calibrated, manually operated stereo comparator in order to calibrate the stereo-correlation calculations created by the Desktop Mapping System (DMS) by R-Wel Inc. The DMS is a low cost software package that facilitates image processing for photogrammetric, remote sensing, and GIS applications using off-the-shelf personal computers. The DMS uses area-based matching to locate the exact points that are the same on each image and is the first personal computer software package that permits the generation of digital elevation models by automated stereo-correlation. The approach to be taken in the field experiment is to depend on the digital images for creating the two-dimensional power spectra from which model parameters of spectral slope and strength are calculated for predicting backscattering strength.


Measurement Time Considerations

The electrical resistivity measurements will occupy an entire 35-min dive at a water depth of 60 ft. During that dive many contiguous slabs of sediment can be measured and a couple of cores can be collected. Several (5–9) dives devoted to this measurement are planned. Processing of the resistivity data to obtain porosity, density, and other parameters will occur in the laboratory after the field experiment is concluded. The stereo camera imaging will occupy an entire dive time at 60 ft. We expect to make 3–5 dives in which we collect seafloor roughness imagery. The roughness data is processed aboard ship, but will require comparison with manually operated processing after the field experiment is concluded.


References

[1] Briggs, K.B., P.D. Jackson, R.J. Holyer, R.C. Flint, J.C. Sandidge, and D.K. Young. 1998. Two-dimensional variability in porosity, density, and electrical resistivity in Eckernförde Bay sediment. Contin. Shelf Res., 18: 1939–1964.

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