Robert D. Stoll

Wave Attenuation in Saturated Sediments

This paper considers a mathematical model to describe the propagation of low‐amplitude waves in saturated sediments. Losses due to inelasticity of the skeletal frame and to motion of the pore fluid relative to the frame are both accounted for, and each is found to be significant in a different frequency range. The theory shows favorable agreement with experimental results where available for both sands and finer‐grained sediments over a wide range of frequencies.

Reflection of acoustic waves at a water–sediment interface

Reflection and refraction of plane acoustic waves are studied for the case where the sediment is modeled as a porous viscoelastic medium. The model is based on the classical work of Biot which predicts that three different kinds of attenuating body wave may propagate in the sediment. As a consequence when homogeneous plane waves in water are incident to a water–sediment interface, three nonhomogeneous waves are generated in the sediment. In these waves the direction of phase propagation and the direction of maximum attenuation are not the same and particle motion follows an elliptic path. Moreover the velocity and attenuation of the refracted waves become dependent on the angle of incidence and no ’’critical’’ angle occurs. Numerical examples show that in some cases the reflectivity of a porous viscoelastic model differs significantly from the case where the sediment is modeled as a viscoelastic solid with constant complex modulus. Finally because of the frequency dependence of reflectivity in the porous ...

Acoustic waves in ocean sediments

An acoustic model for unconsolidated sediments is used to study velocity, attenuation, and reflection in ocean sediments. The model predicts attenuation and wave velocity on the basis of physical parameters such as porosity, grain size, permeability, and effective stress. Two mechanisms for energy loss are included in the model; one accounts for intergranular losses in the skeletal frame and the other for viscous losses in the porewater as it moves relative to the frame. As a result, in certain sediments such as sands and silts, attenuation is found to vary in a manner quite different from the usual dependency on the first power of frequency that is almost universally assumed. Furthermore, the amplitudes of reflected and refracted waves at boundaries between water and sediment or between sediment layers become frequency dependent. In the immediate vicinity of such boundaries, a significant amount of energy may be lost owing to the generation of a second kind of dilatational wave with extremely high attenu...

ACOUSTIC WAVES IN SATURATED SEDIMENTS

This paper discusses a phenomenological model that describes the propagation of sound waves in saturated sediments. The compressibility and shearing stiffness of the skeletal frame, the compressibility of the fluid, and two major sources of attenuation are included in the model. Attenuation is attributed to two fundamentally different types of energy loss, one resulting from inelasticity of the skeletal frame and the other due to motion of the pore fluid relative to the frame, with each significant in a different frequency range.

Theoretical aspects of sound transmission in sediments

The structure of a general theory for acoustic wave propagation in ocean sediments is reviewed with emphasis on the determination of meaningful parameters from data in the literature. It is shown that acoustic properties are nonlinear functions of cyclic strain amplitude and static stress level so that only a limited amount of the available data is useful for studies of acoustic propagation. An example of geoacoustic modeling is presented based on the theory and some new data for Stoneley wave velocities measured in situ are evaluated with the aid of the model.

Velocity dispersion in water-saturated granular sediment

Recent experiments in the Gulf of Mexico have yielded a wealth of information on the environmental conditions and geoacoustic response of a uniform sand stratum immediately beneath the seafloor. A comparison of p-wave velocities measured at low (125 Hz) and high (11-50 kHz) frequencies in this layer indicates that there is a significant amount of velocity dispersion that occurs in the interval between these extremes. This narrow-band dispersion, which is not consistent with the often-used assumption of a nearly constant-Q model, is in accordance with the predictions of the Biot theory. It results from viscous damping in the fluid as it moves relative to the skeletal frame. Other recent field data that support this conclusion are presented.

Experimental studies of attenuation in sediments

Recent theoretical work suggests that attenuation of acoustic waves in ocean sediments depends on two distinct types of energy loss; these losses result from inelasticity of the skeletal frame and motion of the pore water relative to the frame. In this paper, experimental evidence is presented which verifies the type of response predicted by the theoretical model. Torsional resonance and logarithmic decrement are measured over a wide frequency range in both water‐saturated and dry sediments. A changeover from dominance by one type of loss to the other is clearly demonstrated by a change of response that is observed as the frequency is varied. Specially designed equipment is used which allows the intergranular stress to be controlled to simulate various depths of embedment in the sea floor. Both the theory and the experiments suggest that for some sediments the attenuation will vary in a manner quite different from the usual dependency on the first power of frequency that is often assumed.

Field experiments to study seafloor seismoacoustic response

Results of a 5-year program to measure the seismoacoustic properties of unlithified sediments in the seafloor are presented. A field technique utilizing an impulsive source and a geophone array located on the seafloor was used to obtain detailed travel-time curves which permit a number of different kinds of analysis. A new signal source powered by electrically detonated, 8-gauge shotgun shells was used to focus energy into the bottom and eliminate much of the high-frequency water-borne noise that accompanies the usual explosive sources. Both interface waves and wide-angle refractions were analyzed. A variety of data inversion techniques including slant stacking and cross multiplication were used to obtain dispersion curves, and constrained, least-squares inversion using partial derivatives was used to develop an iterative inversion method to obtain velocity and attenuation as a function of depth. Examples of data from a wide angle of different kinds of sediment ranging from soft Holocene mud to stiff over...

Shallow seismic experiments using shear waves

During the summer of 1986, a series of seismo‐acoustic experiments was carried out in shallow water off the New Jersey shore. The purpose of these experiments was to measure the geoacoustic properties of the ocean sediments that comprise the upper few hundred meters of the sediment column. Seismic sources and receivers were deployed at or very near the bottom in order to excite shear waves in the sediment and minimize the effects of interference from waterborne propagation. The experiments were performed at several sites where prior field work had established physical properties and a detailed profile of the sediments. By using conventional air guns deployed in an unconventional way, strong interface and diving shear waves were generated; these data were inverted to obtain shear wave velocity as a function of depth. The inversion results were then compared with the predictions of a geoacoustic model that accounts for the effects of voids ratio, overburden pressure, and other physical parameters. The in si...

NEW TOOLS FOR STUDYING SEAFLOOR GEOTECHNICAL AND GEOACOUSTIC PROPERTIES

Three new tools designed to measure certain geotechnical and geoacoustic properties of the sediments just beneath the seafloor are described. One tool is a source–receiver system designed to generate and receive SH waves or Love waves. The source applies a torsional impulse to a small circular area of the seafloor and a linear array of gimballed geophones is used to measure horizontal motion perpendicular to the axis of the array. Energy to generate the torsional pulse is supplied by a rotating flywheel that is suddenly brought to rest. The dispersion of Love waves produced by this method is used as the basis for a least‐squares inversion to obtain shear‐wave velocity as a function of depth. A second source–receiver system generates vertically polarized shear waves or Scholte waves by applying a vertical impulse to the sediment over a small circular area of the bottom. In this case, the receiving array utilizes gimballed geophones that respond to the vertical motion of the seafloor and the Scholte wave di...