Aubrey L. Anderson

Acoustics of gas‐bearing sediments I. Background

Acoustical properties of water saturated and gassy sediments are observed to be significantly different. The present state of knowledge of the acoustical properties of saturated sediments, gassy water, and gassy sediments is reviewed. The dynamics of bubbles in water and in various solid materials, including sediments, are experimentally examined in a companion paper. Pulsation resonance is exhibited by the bubbles in all materials examined. Predictions of bubble resonance frequency and damping are shown to agree with the measurements. Equations for sound speed and attenuation, based on the model of resonating gas bubbles, are shown to agree with published measurements in gassy sediments. Parameters required for predicting gassy sediment acoustical properties are identified. Ranges of values of these parameters for various sediments are discussed.

Acoustics of gas‐bearing sediments. II. Measurements and models

Acoustical properties of water saturated and gassy sediments are observed to be significantly different. The present state of knowledge of the acoustical properties of saturated sediments, gassy water, and gassy sediments is reviewed in a companion paper. The dynamics of bubbles in water and in various solid materials, including sediments, are experimentally examined here. Pulsation resonance is exhibited by the bubbles in all materials examined. Predictions of bubble resonance frequency and damping are shown to agree with the measurements. Equations for sound speed and attenuation, based on the model of resonating gas bubbles, are shown to agree with published measurements in gassy sediments. Parameters required for predicting gassy sediment acoustical properties are identified. Ranges of values of these parameters for various sediments are discussed.

ACOUSTIC SCATTERING FROM THE SEAFLOOR : MODELING AND DATA COMPARISON

An existing model of seafloor backscattering [D. R. Jackson, D. P. Winebrenner, and A. Ishimaru, J. Acoust. Soc. Am. 79, 1410–1422 (1986)] was implemented with the additions of volume scattering from a random inhomogeneous continuum and scattering from subbottom interfaces. Results of computer simulations with the extended model were compared with values of scattering strength obtained from processed GLORIA data from the Monterey Fan off the coast of California. Regions of well‐delineated high and low backscatter are seen in the GLORIA imagery. The geoacoustic input parameters for the simulation runs were either taken directly from or estimated from core data obtained by ground truth sampling in the image area. From the model simulation results it appears that the main factor causing the observed dichotomous character of the strength of returns in the Fingers Area is the presence or absence of a topmost, inhomogeneous silt‐clay layer (with a thickness of around 1–1.5 m). In the high backscatter areas, inh...

Predictions of the acoustic scattering response of free‐methane bubbles in muddy sediments

The response of the sediments of Eckernfoerde Bay, Germany to acoustic remote sensing previously has been attributed qualitatively to gas features which were postulated to exist in situ within the sediment. The existence of features as small as 0.5‐mm equivalent spherical radius has now been confirmed by x‐ray computed tomography of cores taken and scanned at in situ pressures. The interaction of an acoustic pulse from the acoustic sediment classification system (ASCS) with this type of gassy sediment was modeled and the results are presented here. The bubble scattering response model includes both the effects of nonspherical bubbles and of sediment resistance to shear deformation (as expressed by the dynamic shear modulus). Model predictions made using the observed gas bubble distribution agree with normal incidence ASCS data, exhibiting extended returns (i.e., greater than the source pulse length) from the seafloor bubble layers as well as high attenuation within the bubble layers. Spectrograms of ASCS ...

Acoustics and Gas in Sediments: Applied Research Laboratories (ARL) Experience

A discussion of the acoustical properties of saturated sediments and of gas bubbles in water is given as background to a discussion of the present state of knowledge of the acoustical properties of gas bearing sediments.

In Situ Measurement of Marine Sediment Acoustical Properties during Coring in Deep Water

In order to obtain in situ measurements, to several meters depth in a marine sediment, of compressional wave speed and attenuation, a device has been built for attachment to ocean bottom sediment corers. This profilometer consists of transducers on the lower end of the coring tool which are connected by armored cables to an electronics package on the upper end of the tool, thus allowing acquisition of acoustical data with a minimum of modification to a standard corer. Early measurements with a prototype profilometer, which required electrical cables to the surface, established the accuracy and usefulness of the technique. The new deep water version of the profilometer is self contained including provisions for recording the data in the electronics package on the corer. Design operating water depth for the new system is 5 km. Test results for the new profilometer are presented including several high resolution in situ sound speed profiles to 12 m depth in deep water (4.5 km) sediments. Detailed layering is described including a high strength, high water content layer, several thin high speed sand layers, and several low speed layers. The data indicate that the near surface low speed zone in these sediments is composed of several thin low speed layers imbedded in a higher speed matrix.

Predictions of the acoustic response of free‐methane bubbles in muddy sediments

The response of the sediments of Eckernfoerde Bay, Germany to acoustic remote sensing has been attributed to gas features found within the sediment. The existence of features as small as 0.5 mm equivalent spherical radius has been confirmed by x‐ray computed tomography of cores taken and scanned under in situ pressures. The interaction of an acoustic pulse from the Acoustic Sediment Classification System (ASCS) with this type of gassy sediment was modeled. The bubble scattering response included the effects of shear modulus and nonspherical bubbles. Model predictions made using the observed gas feature distribution and normal incidence ASCS data agree and show extended returns (greater than a pulse length) from the seafloor bubble layers as well as high attenuation within the bubble layers. These results show that the acoustic returns from the gas bubble layers are probably due to scattering and that differences in level of return can be attributed to random variations in the distributions of scatterers.

Predicted geoacoustic properties of gas hydrate saturated marine sediments

Natural gas hydrates are clathrates, a class of compounds in which an expanded crystalline structure of water forms about a host gas molecule. Gas hydrates found in the marine environment are generally composed of methane combined with varying amounts of other hydrocarbon gases such as ethane and propane. The occurrence of gas hydrates within the seafloor may significantly alter elastic properties of the bulk sediment resulting in the possibility of identifying hydrated sediments by acoustic remote sensing. The Biot poroelastic theory [R. D. Stoll, Geophysics 42, 715–725 (1977)] is used to gain insight into the acoustic response of an unconsolidated marine sediment with included gas hydrates. Available geoacoustic parameter measurements for hydrated sediments, and their constituents, are of limited extent. Therefore, physical sediment model parameters are estimated both theoretically and empirically for varying concentrations of gas hydrate saturation within the sediment. These geoacoustic modeling result...

A Method for Measuring in Situ Acoustic Properties During Sediment Coring

A system developed for attachment to sediment corers in order to obtain an in situ sound speed profile during a coring operation is described. The system uses two electroacoustic transducers mounted in the cutting head of the corer and associated electronic circuitry to measure the travel time of an acoustic pulse traversing the diameter of the sediment core. Results of laboratory and field tests are presented. There is an ongoing study of the feasibility of expanding the sound speed measurement system capabilities to include a measure of sediment acoustic attenuation and internal volume scattering.

The effects of free‐methane bubbles on the propagation and scattering of compressional and shear wave energy in muddy sediments

Free‐methane bubbles cause significant scattering of acoustic energy in the soft sediments of Eckernfoerde Bay, Baltic Sea. In situ and laboratory measurement of sediment geoacoustic and physical properties were made in an attempt to understand the physical mechanisms responsible for this scattering. In situ shear wave velocities (at 100–500 Hz) increased from 5–7 near the surface to 15–20 m s−1 at 2 m into the seafloor, whereas in situ compressional wave velocities (at 38 and 58 kHz) varied little (∼1425 m s−1) with depth. Methane bubbles apparently caused significant attenuation of compressional waves at depths below 1 m, whereas shear wave attenuation was unaffected by gas and decreased with depth. Compressional waves (at 400 kHz) in cores (1%–5% free gas) maintained at in situ pressures were highly attenuated but show little evidence of velocity dispersion. Comparison of geoacoustic data with theory suggest that primary control of the interaction of acoustic profilers with this gassy seafloor is by bu...