The major APL-UW goals in the SAX99 field experiment and associated modeling are to answer the following four questions. What are the dominant penetration mechanisms in sand sediments for grazing angles below the critical angle? What are the dominant backscattering mechanisms in sand sediments as a function of frequency and incident angle? Which acoustic propagation models are consistent with the attenuation measured in sand sediments as a function of frequency? Can the degradation of the spatial and temporal coherence of the acoustic field propagating into and within the sediment be understood in terms of measured sediment properties? An additional goal is to examine the spatial and temporal variability of acoustic scattering from the bottom and from objects buried in the bottom. The experimental approaches used to obtain these goals are described below.
Numerical simulations have indicated that an array of 18 buried hydrophones is sufficient to distinguish between penetration mechanisms for the experimental procedure we plan to use. The experiment will include a mobile tower, an array of buried hydrophones, a 5 m x 10 m frame deployed on the bottom, and a tracking system mounted on this frame. The frame will be deployed first and serves several purposes. It is a stable platform from which to guide placement of the buried array, it protects the area of the bottom that must not be disturbed, and it can be used to mount the environmental-characterization instrumentation of other investigators if the need arises.
The buried array will be deployed via a cofferdam such that the hydrophones are inserted horizontally into the sediment and do not disturb the top 10 cm. After insertion, the hydrophone locations will be surveyed using an in-water tracking array mounted to the frame. After this, the tracking array will be reconfigured to allow tracking of the sources on the mobile tower. The sources on the tower operate from 10–50 kHz, and transmission data from the tower to the buried array will be taken for a large number of tower positions at tower-to-array ranges of 5 to 25 m. This allows for measurements of penetration both above and below the critical angle and for multiple measurements that can be averaged for comparison with the predictions of different penetration hypotheses.
Environmental characterization of the bottom in the region of the acoustic measurements is essential to our hypothesis testing. In addition to our own environmental characterization using conductivity and acoustic tomography instrumentation, we anticipate the need for Richardson et al. (NRL-SSC), Wheatcroft (OSU), Orsi (PSI), Bennett (Seaprobe Inc.), and Jaffe (UCSD-MPL) to perform characterization measurements near the experimental area. We plan to alter the surface topography after our initial data acquisition obtained under the natural conditions.
The mobile tower used to examine penetration mechanisms will also be used to deploy transducers to measure monostatic and near-monostatic backscattering. These transducers allow acquisition of data from 10–150 kHz. Each time the tower is moved during the penetration experiment, backscattering data will be taken for at least 15 center frequencies. This tower movement will give independent measurements that can be averaged. Acquiring data on both monostatic and near-monostatic backscattering at the same frequency will allow us to search for enhanced backscattering, an indication that multiple scattering is important.
Two additional towers will be used to obtain backscattering data. These towers are autonomous and will be deployed approximately 300 m from the penetration experiment. One tower operates at both 40 and 300 kHz, and the other operates only at 300 kHz. In addition to examining the spatial and temporal variability (discussed below), they extend the frequency range over which backscattering data are acquired and allow independent checks of scattering strengths obtained using different instrumentation. The data acquired by Chotiros et al. (ARL/UT), Lopes (CSS), and Holliday et al. (Marconi Aerospace) on backscattering should allow further opportunities for comparing backscattering strengths.
The environmental characterization carried out for the penetration experiment will also be important for examining dominant backscattering mechanisms; however, additional characterization measurements are needed in the vicinity of the two autonomous towers. The spatial resolution needed for backscatter modeling is higher than that required for the penetration experiment. The resolution required for modeling runs from 7.5 cm at 10 kHz to 2 mm at 300 kHz. This resolution is expected to exceed the capabilities of at least some of the characterization instrumentation for the higher frequencies, forcing a reliance on more qualitative data/model comparisons.
A diver-deployable set of two receivers and two transmitters will be used. They will be mounted on a small jig that allows the divers to deploy them easily within 50 m of the penetration experiment. The transmitters and receivers will allow measurement of attenuation at 80–300 kHz. This frequency range overlaps the frequency ranges of the NRL-SSC attenuation measurements (10–100 kHz) and the APL-UW acoustic tomography measurements (130–180 kHz). As with the backscattering measurements, these overlaps allow useful comparisons of results. Measurements by all three systems in a localized region will allow attenuation to be examined from 10 kHz to 300 kHz.
The loss of spatial coherence due to forward scattering from interface roughness or volume inhomogeneities in the sediment is important for imaging buried targets. Two other measures of the acoustic field are also relevant to such imaging. First, forward scattering can change the structure of the pulse waveform as it propagates through the sediment, leading to degradation of the correlation between the transmitted and received waveforms. Second, biological reworking of the sediment can change the scattering properties over time, leading to changes in the temporal coherence for measurements separated in time. The spatial structure of the acoustic fields propagating in the sediment will be measured with the buried array to quantify the degradation of spatial coherence. The other two measures of the acoustic field will also be readily available from the buried array data. Environmental characterization again will be essential for quantifying the interface roughness and sediment inhomogeneities leading to forward scattering (as in the items above) and for monitoring the temporal change in the sediment due to biological activity. The biological monitoring will be carried out by Jumars and Schmidt (UW) and Holliday et al. (Marconi Aerospace).
The mobile tower and the autonomous towers used in the penetration and backscattering experiments will also be used to examine the spatial and temporal variability of scattering from the bottom and objects buried in the bottom. The objects will consist of two 24-inch hollow spheres (provided by CSS). One will be buried within the field of view of one of the autonomous towers and one within the field of view of the mobile tower. Collection of data on biological activity in the region of the autonomous towers is an essential part of this effort. Data from the autonomous towers will be uploaded so that images can be formed of the scattering, bathymetry, and acoustic decorrelation in their vicinity. This work will be carried out in collaboration with Jumars and Schmidt (UW) and Holliday et al. (Marconi Aerospace).
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