Barbara J. Kraft

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

In-Situ Determination of the Variability of Seafloor Acoustic Properties: An Example from the Onr Geoclutter Area

In support of the US ONR-sponsored Geoclutter program, we have developed, built, and deployed a relatively inexpensive, robust, small-ship-deployable device (ISSAP — In situ Sound Speed and Attenuation Probe) for rapidly measuring sound speed and attenuation in near-surface sediments. We have demonstrated its ability to make reliable and precise measurements (+/- 1–2 m/s for sound speed, < +/- 1 dB/m for attenuation). We have found that in the Geoclutter area the sound speed varies on the order of 200–300 m/s over spatial scales of 10’s of kms and the attenuation (at 65 kHz) varies on the order of 60 dB/m. On scales of less than one kilometer, the sound speed can vary by more than 100 m/s and attenuation by approximately 25 dB/m. On the sub-meter scale, much of the seafloor is relatively homogeneous but some areas show sound speed variation of approximately 50 m/s and attenuation variation on the order of 25 dB/m. These variations are probably related to the presence of large clasts or shells in the measured path.

Detailed Bathymetric Surveys Offshore Santa Rosa Island, FL: Before and After Hurricane Ivan (September 16, 2004)

Two weeks before Hurricane Ivan reworked the shores and nearshore seafloor of the northeastern Gulf of Mexico, swath bathymetry surveys were conducted with high-frequency (300 and 455 kHz) multibeam echosounders in three areas offshore Santa Rosa Island, FL, an 80-km barrier island extending west from Destin to Pensacola Bay, FL. These surveys were repeated in late October 2004, six weeks after the passage of the hurricane, allowing for quantitative pre- and posthurricane seabed comparisons. Bathymetric difference maps (0.2-0.3-m grid cells) show that sediment accretion exceeded 1 m in areas near the 6-7-m isobaths, where a submerged longshore bar was formed below the breaker zone of large storm waves. Accretion of sediment continued seaward tapering off near the 11-12-m isobaths, with evidence of slight seabed erosion (0.1-0.2 m) seaward of this boundary. Between the 6- and 12-m contour lines, the increase in sediment volume is about 279 000 m 3/km 2. Grab samples obtained in the area by Vaughan [IEEE J. Ocean. Eng., vol. 34, no. 4, pp. 495-515, 2009] support the hypothesis that the added material is fine sand eroded from the beach and dunes on Santa Rosa Island by the overwash and inundation associated with Ivans storm surge and eventually deposited offshore by storm-surge ebb currents. Two-dimensional bottom roughness power density spectra computed from colocated east-west (EW) bathymetry swaths near the 12-m isobath show a post-Ivan threefold increase in root mean square (rms) roughness over the [0.104, 0.495] m-1 spatial wave number band. Bottom roughness spectrograms computed along individual north-south (NS) survey track lines perpendicular to the shoreline and extending 10 km offshore indicate that Ivan-induced waves and currents reworked the seabed to water depths of at least 22 m, with a twofold to fourfold increase in rms roughness over the [0.023, 0.156] m-1 spatial wave number band.

Calculation of In Situ Acoustic Wave Properties in Marine Sediments

The importance of estimating compressional wave properties in saturated marine sediments is well known in geophysics and underwater acoustics. As part of the ONR sponsored Geoclutter program, in situ acoustic measurements were obtained using ISSAP (In situ Sound Speed and Attenuation Probe), a device developed and built by the Center for Coastal and Ocean Mapping (CCOM). The location of the Geoclutter field area is the mid-outer continental shelf off New Jersey. Over 30 gigabytes of seawater and surficial sediment data was collected at 99 station locations selected to represent a range of seafloor backscatter types. At each station, the ISSAP device recorded waveform data across five acoustic paths with nominal probe spacing of 20 or 30 cm. The transmit/receive probes were arranged in a square pattern and operated at a nominal frequency of 65 kHz. The recorded waveforms were processed for sound speed using two methods, cross-correlation and envelope detection, and compared. The waveforms were also processed for sediment attenuation using the filter-correlation method. Results show considerable variability in the acoustic properties at the same and nearby seafloor locations.

Stratigraphic Model Predictions of Geoacoustic Properties

Geoacoustic properties of the seabed have a controlling role in the propagation and reverberation of sound in shallow-water environments. Several techniques are available to quantify the important properties but are usually unable to adequately sample the region of interest. In this paper, we explore the potential for obtaining geotechnical properties from a process-based stratigraphic model. Grain-size predictions from the stratigraphic model are combined with two acoustic models to estimate sound speed with distance across the New Jersey continental shelf and with depth below the seabed. Model predictions are compared to two independent sets of data: 1) Surficial sound speeds obtained through direct measurement using in situ compressional wave probes, and 2) sound speed as a function of depth obtained through inversion of seabed reflection measurements. In water depths less than 100 m, the model predictions produce a trend of decreasing grain-size and sound speed with increasing water depth as similarly observed in the measured surficial data. In water depths between 100 and 130 m, the model predictions exhibit an increase in sound speed that was not observed in the measured surficial data. A closer comparison indicates that the grain-sizes predicted for the surficial sediments are generally too small producing sound speeds that are too slow. The predicted sound speeds also tend to be too slow for sediments 0.5-20 m below the seabed in water depths greater than 100 m. However, in water depths less than 100 m, the sound speeds between 0.5-20-m subbottom depth are generally too fast. There are several reasons for the discrepancies including the stratigraphic model was limited to two dimensions, the model was unable to simulate biologic processes responsible for the high sound-speed shell material common in the model area, and incomplete geological records necessary to accurately predict grain-size

In situ measurement of sediment acoustic properties and relationship to multibeam backscatter

In support of the Office of Naval Research’s Geoclutter Program, in situ acoustic and resistivity measurements were obtained using ISSAP, a device developed and built by the Center for Coastal and Ocean Mapping. The primary focus of this research is to understand the relationship between remotely measured backscatter and the acoustic properties of surficial sediments. The field area selected was Portsmouth Harbor (NH) due to the comprehensive sonar data set collected during the Shallow Water Survey 2001 conference. Seawater and surficial sediment measurements of compressional wave sound speed, attenuation, and resistivity were obtained at a large number of stations selected to represent a range of seafloor backscatter types. The ISSAP platform was configured with two orthogonal matched pairs of transducer probes operating at frequencies of 47 and 65 kHz. A Van Veen grab sampler was also used to obtain a sediment sample at each station. Subsampling tubes were used to obtain undisturbed samples; laboratory measurements of density, compressional wave speed and attenuation, resistivity, and grain size were completed. For a small subset of samples, selected to represent a range of sediment types, measurements of permeability, shear wave speed, and attenuation were completed. [Research supported by ONR Grant No. N00014‐00‐1‐0821.]In support of the Office of Naval Research’s Geoclutter Program, in situ acoustic and resistivity measurements were obtained using ISSAP, a device developed and built by the Center for Coastal and Ocean Mapping. The primary focus of this research is to understand the relationship between remotely measured backscatter and the acoustic properties of surficial sediments. The field area selected was Portsmouth Harbor (NH) due to the comprehensive sonar data set collected during the Shallow Water Survey 2001 conference. Seawater and surficial sediment measurements of compressional wave sound speed, attenuation, and resistivity were obtained at a large number of stations selected to represent a range of seafloor backscatter types. The ISSAP platform was configured with two orthogonal matched pairs of transducer probes operating at frequencies of 47 and 65 kHz. A Van Veen grab sampler was also used to obtain a sediment sample at each station. Subsampling tubes were used to obtain undisturbed samples; laboratory ...

Variable bandwidth filter for multibeam echo-sounding bottom detection

The accuracy of a seafloor map derived from multibeam swath bathymetry depends first and foremost on the quality of the bottom detection process that yields estimates of the arrival time and angle of bottom echoes received in each beam. Filtering of each beam with a fixed bandwidth filter, with the bandwidth based on the length of the transmitted pulse, reduces the error associated with the time-angle estimates. However, filters of this type can not be optimal over the wide range of operational environments encountered. Better results are obtained with a processing scheme that varies the filter bandwidth across the swath width using detected time and angle information from the previous ping. This method is evaluated using sonar data obtained with a Reson SeaBat 8111ER and the results compared with those obtained using a fixed bandwidth filter.

In situ measurement of geoacoustic sediment properties: An example from the ONR Mine Burial Program, Martha’s Vineyard Coastal Observatory

In support of the Office of Naval Research’s Mine Burial Program (MBP), in situ acoustic and resistivity measurements were obtained using ISSAP, a device developed and built by the Center for Coastal and Ocean Mapping. One of the field areas selected for the MBP experiments is the WHOI coastal observatory based off Martha’s Vineyard. This area is an active natural laboratory that will provide an ideal environment for testing and observing mine migration and burial patterns due to temporal seabed processes. Seawater and surficial sediment measurements of compressional wave sound speed, attenuation, and resistivity were obtained at 87 stations. The ISSAP instrument used four transducer probes arranged in a square pattern giving acoustic path lengths of 30 and 20 cm with a maximum insertion depth of 15 cm. Transducers operated at a frequency of 65 kHz. The received acoustic signal was sampled at a frequency of 5 MHz. A measurement cycle was completed by transmitting 10 pulses on each of the five paths and repeating three times for a total 150 measurements. Resistivity measurements were obtained from two probes mounted on ISSAP following completion of the acoustic measurements. [Research supported by ONR Grant Nos. N00014‐00‐1‐0821 and N00014‐02‐1‐0138.]

Comparison of in situ compressional wave speed and attenuation measurements to Biot–Stoll model predictions

The importance of estimating acoustic wave properties in saturated marine sediments is well known in geophysics and underwater acoustics. As part of the ONR sponsored Geoclutter program, in situ acoustic measurements were obtained using in situ sound speed and attenuation probe (ISSAP), a device developed and built by the Center for Coastal and Ocean Mapping (CCOM). The location of the Geoclutter field area was the mid–outer continental shelf off New Jersey. Over 30 gigabytes of seawater and surficial sediment data was collected at 99 station locations selected to represent a range of seafloor backscatter types. At each station, the ISSAP device recorded 65 kHz waveform data across five acoustic paths with nominal probe spacing of 20 or 30 cm. The recorded waveforms were processed for compressional wave speed and attenuation. Experimental results are compared to predicted values obtained using the Biot–Stoll theory of acoustic wave propagation. Several methods are examined to estimate the required model p...

Angular dependence of high-frequency seafloor acoustic backscatter (200–400 kHz)

Acoustic backscatter measurements were made with a RESON 7125-V2 multifrequency, multibeam sonar at 50 kHz increments between 150 and 450 kHz. The sonar was hull-mounted on a vessel held in a four-point moor in 17 m of water depth. The resulting constrained ship motion provided thousands of independent samples of the angular dependence of seafloor acoustic backscatter for grazing angles ranging from 90 deg to about 25 deg, over a well defined seafloor patch. Focused beamforming at all frequencies yielded fore-aft beam footprints with nearly constant width across the swath. Calibrated results are presented at 200 and 400 kHz, and relative results at all other frequencies. [Work funded by ONR-Ocean Acoustics (Code 32).]Acoustic backscatter measurements were made with a RESON 7125-V2 multifrequency, multibeam sonar at 50 kHz increments between 150 and 450 kHz. The sonar was hull-mounted on a vessel held in a four-point moor in 17 m of water depth. The resulting constrained ship motion provided thousands of independent samples of the angular dependence of seafloor acoustic backscatter for grazing angles ranging from 90 deg to about 25 deg, over a well defined seafloor patch. Focused beamforming at all frequencies yielded fore-aft beam footprints with nearly constant width across the swath. Calibrated results are presented at 200 and 400 kHz, and relative results at all other frequencies. [Work funded by ONR-Ocean Acoustics (Code 32).]