The estimation of geoacoustic properties from broadband acoustic data, focusing on instantaneous frequency techniques

The compressional wave velocity and attenuation of marine sediments are fundamental to marine science. In order to obtain reliable estimates of these parameters it is necessary to examine in situ acoustic data, which is generally broadband. A variety of techniques for estimating the compressional wave velocity and attenuation from broadband acoustic data are reviewed. The application of Instantaneous Frequency (IF) techniques to data collected from a normal-incidence chirp profiler is examined. For the datasets examined the best estimates of IF are obtained by dividing the chirp profile into a series of sections, estimating the IF of each trace in the section using the first moments of the Wigner Ville distribution, and stacking the resulting IF to obtain a composite IF for the section. As the datasets examined cover both gassy and saturated sediments, this is likely to be the optimum technique for chirp datasets collected from all sediment environments

A multitechnical analysis of the shallow gas blanket in Southampton Water

The presence of bubbles of biogenic gas in marine sediments has a dramatic effect on the acoustical properties of the bulk sediment, and can act as an acoustic blanket through which acoustic pulses cannot penetrate. A combination of bore hole data, chirp profiling and forward modelling was used to examine the intertidal sediments in Dibden Bay, Southampton Water. The sediment consisted of stratified Holocene clays, Pleistocene sands and Eocene Bedrock and satisfied necessary conditions to allow the production of biogenic gas. A gas blanket, which dipped in an offshore direction, lay between 1 and 2 m below the seabed. This covered the majority of the bay, with some small gas free windows present. Results suggested that factors controlling the lateral variations in the depth of the gas blanket could be split into two categories: those which control variations on large scales, including pressure, salinity, temperature and depth of the sulphate reducing zone; and those which control variations on smaller scales, i.e. sedimentary boundaries. In Dibden Bay lateral variations in sedimentary boundaries appeared to control the depth of the gas blanket

The 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 inter-tidal sediments using a purpose-built in situ device, the Sediment Probing Acoustic Detection Equipment. Compressional wave velocity 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 separations of 0.99 – 8.1 m, resulting velocities are accurate to between + 1.1 to + 4.5 % in sands and less than + 1.9 % in silts, while attenuation coefficients are accurate to between + 1 to + 7 dB•m-1 in both sands and silts. Preliminary results indicate no velocity dispersion and an attenuation coefficient which is proportional to frequency

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 100 kHz) 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 group velocities at specific locations for the silty sites. For both silts and sands predicted dispersions are comparable to the intrinsic errors in group velocity and hence undetectable. The attenuation coefficients predicted by the Grain Shearing model adequately describe the measured attenuation coefficients, within the observed variability

The compressional wave and physical properties of inter-tidal marine sediments

New quadratic regression equations for inter-tidal sediments are presented, which relate compressional wave properties (velocity ratio and proportionality constant) to porosity, bulk density and mean grain size. The compressional wave properties were derived from compressional wave velocities and attenuation coefficients measured on inter-tidal sediments from 16 - 100 kHz using common experimental and processing techniques. The regression equations are more robust for velocity ratio than proportionality constant. Discrepancies between the new regression equations and those derived for submerged sediments support the conclusion that the sediment structure of inter-tidal sediments differs from that of submerged sediments. This is attributed to the different physical processes which govern the supply, deposition and dynamics of the sediment in each environment

Hydrophone performance in sediment

In situ measurements of acoustic signals in marine sediment are oftenperformed using hydrophones which have been designed for use in water. Typically, thesehydrophones are characterised for transmit or receive sensitivity in water. When thehydrophone is submerged in a medium other than water, the sensitivity (both amplitudeand phase response) of the hydrophone, and its resonant characteristics, can bedramatically affected as a result of the differences in acoustic impedance of the mediumand the different coupling to the medium. To investigate these changes, a series ofmeasurements of electrical impedance and receive sensitivity were performed on ahydrophone in fine sand sediment using a novel method which does not require a prioriknowledge of the absorption in the medium. The initial results of this investigation arepresented in this paper demonstrating the change in the hydrophone characteristics whenused in sediment at frequencies above 40 kHz, and the factors affecting hydrophoneperformance in sediment are discussed

Use of dual methods to infer methane bubble populations in gassy sediments: 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. ©2008 Acoustical Society of Americ