G.B.N. Robb

The frequency dependence of compressional wave velocity and attenuation coefficient of intertidal marine sediments

To advance the present understanding of the frequency dependence of compressional wave velocity and attenuation in marine sediments a series of well-constrained in situ acoustic transmission experiments (16 to 100kHz) were performed on intertidal sediments. The processing techniques incorporated in situ spreading losses, sediment to transducer coupling and thorough error analyses. Significant variations in velocity and attenuation were observed over scales of tens of meters within the same sediment type. Velocity was generally nondispersive in sands, while highly variable silt velocities prevented any meaningful dispersion estimates from being determined. The attenuation coefficient was proportional to frequency for 75% of the experimental sites. The measured compressional wave properties were compared to predictions from the Grain-Shearing model. For the sandy sites, the phase velocities predicted by the Grain Shearing model exceed those measured, while predicted phase velocities agreed with measured gro...

Measurement of the In Situ Compressional Wave Properties of Marine Sediments

Geoacoustic inversion requires a generic knowledge of the frequency dependence of compressional wave properties in marine sediments, the nature of which is still under debate. The use of in situ probes to measure sediment acoustic properties introduces a number of experimental difficulties that must be overcome. To this end, a series of well-constrained in situ acoustic transmission experiments were undertaken on intertidal sediments using a purpose-built in situ device, the Sediment Probing Acoustic Detection Equipment (SPADE). Compressional wave speed and attenuation coefficient were measured from 16 to 100 kHz in medium to fine sands and coarse to medium silts. Spreading losses, which were adjusted for sediment type, were incorporated into the data processing, as were a thorough error analysis and an examination of the repeatability of both the acoustic wave emitted by the source and the coupling between probes and sediment. Over the experimental frequency range and source-to-receiver (S-R) separations of 0.99-8.1 m, resulting speeds are accurate to between 1.1% and 4.5% in sands and less than 1.9% in silts, while attenuation coefficients are accurate to between 1 and 7 dBm in both sands and silts. Preliminary results indicate no speed dispersion and an attenuation coefficient that is proportional to frequency.

Absolute calibration of hydrophones immersed in sandy sediment

An absolute calibration method has been developed based on the method of three-transducer spherical-wave reciprocity for the calibration of hydrophones when immersed in sandy sediment. The method enables the determination of the magnitude of the free-field voltage receive sensitivity of the hydrophone. Adoption of a co-linear configuration allows the acoustic attenuation within the sediment to be eliminated from the sensitivity calculation. Example calibrations have been performed on two hydrophones inserted into sandy sediment over the frequency range from 10 to 200 kHz. In general, a reduction in sensitivity was observed, with average reductions over the frequency range tested of 3.2 and 3.6 dB with respect to the equivalent water-based calibrations for the two hydrophones tested. Repeated measurements were undertaken to assess the robustness of the method to both the influence of the sediment disturbance associated with the hydrophone insertion and the presence of the central hydrophone. A simple finite element model, developed for one of the hydrophone designs, shows good qualitative agreement with the observed differences from water-based calibrations. The method described in this paper will be of interest to all those undertaking acoustic measurements with hydrophones immersed in sediment where the absolute sensitivity is important.

Use of dual methods to infer methane bubble populations in gassy sediment: Inversion of propagation data

The inversion of the acoustic properties of gassy sediments presents the optimum manner of determining the in situ distribution of sediment‐based methane bubbles. An in situ device that measures both compressional wave attenuations and combination‐frequency components in gassy sediment lying within 2 m of the seabed has been developed at the University of Southampton. This device was deployed at an inter‐tidal site along the South coast of England. Compressional wave attenuations were measured from 10 to 100 kHz though the analysis of propagation signals transmitted from a variety of sources to a buried co‐linear hydrophone array, with propagation distances spanning 0.5 to 2 m. Measured attenuations were inverted to infer in situ bubble size distributions using both established and new acoustic models for gassy sediment. The analysis and results of the combination‐frequency component are described in a companion paper.

Broadband elastic scattering by fiberglass spherical shells and plates measured in a water tank: Acoustic inversion and wave analysis

Spherical shells and plane plates made of different types of fiberglass (either random or textured) were measured in the backscatter direction, suspended in a water tank in a broadband frequency range between 30 and 350 kHz. The range of ka for the spheres was approximately 8 to 90, with the fd range for the plates approximately 0.15 to 1.75 MHz*m. The aim of the study was to investigate the effects of the fiber type on the object signature, as the frequency and the type of fiber layers vary. Inversion of the material parameters was conducted on the basis of the objects temporal echo. In particular, the estimate of material loss is crucial to determine at what frequency elasticity becomes irrelevant to the objects global response. The spherical shells were measured either void or filled with different materials (liquid and solid) in order to evaluate the contribution of the shell‐borne elastic waves with respect to sound scattered from the interior of the object. Elastic wave analysis and analytical mod...

A method for calibrating hydrophones immersed in sandy sediment

Hydrophones are frequently used as receivers for in situ sediment acoustic experiments. At present, processing techniques use receiver sensitivities measured from water-based calibrations. It is, however, accepted that the receive sensitivity will depend on the medium surrounding the hydrophone, particularly at frequencies close to the transducers resonance frequency. To assess this affect, a series of calibrations were performed over the frequency range of 10 to 200 kHz on two types of hydrophones (with cylindrical and spherical elements) inserted into degassed sandy sediment. Sensitivities were measured using a modified three-transducer reciprocity technique, which uses a co-linear arrangement to allow the sediment attenuation to be omitted from the sensitivity calculation. The insertion of the hydrophones into the sediment reduced the measured receive sensitivities by a maximum value of 3.8 dB with respect to the equivalent water-based calibrations. The co-linear arrangement adopted allowed the transmission between the outer devices to be recorded with and without the central hydrophone present. Repeat measurements indicated that the sediment disturbance associated with the removal of the central hydrophone caused sensitivity differences of less than 1.2 dB, while the inclusion of the central hydrophone caused a shadowing effect which increased sensitivities by between 1.3 to 4.0 dB. ©2008 Acoustical Society of America

Use of dual methods to infer methane bubble populations in gassy sediments: Inversion of combination‐frequency data

Bubbles can dramatically change the acoustic properties of their host medium even if they are present in very small amounts. This paper describes the combination‐frequency component of tank and field measurements taken using a device which measures bubbles in marine sediments using multiple acoustic techniques (allowing the results of the various techniques to be compared). The combination‐frequency method uses the nonlinear scattering property of bubbles when insonified by two primary frequencies. For low void fractions, there is a monotonic relationship between the scattered field and the population of bubbles resonant at either of the primary frequencies or combination of these and/or their subharmonics. This principle is used to infer the bubble size distribution. In contrast to the case of gas bubbles in water, in marine sediments the shear properties of the host medium must be incorporated into the model for the bubble dynamics and a new model for this is presented. This model is then inverted to ob...

Acoustic backscatter at very high frequencies from rough seabeds

Acoustic backscatter data were gathered from a variety of seabed types, both in the laboratory tank and in coastal waters. Data were gathered from sandy sediments in the tank, with a variety of characterized rough surfaces, using narrow‐band pulses at frequencies between 100 and 950 kHz. Wideband data gathered at sea were obtained at frequencies between 80 and 650 kHz. Data gathered at sea from the Acoustic Range at QinetiQ Bincleaves included backscatter from the natural sandy seabed, but also scatter from several artificial sediments (sand, gravel, and pebbles) placed in a rotator. The latter equipment allowed acoustic interrogation of the same patch of seabed from multiple angles. Experimental data are compared with fluid, poroelastic, and discrete scatterer models, with a view to recommendations for the modeling of seabed backscatter in the frequency band 100 kHz to 1 MHz.