The oblique seismic experiment in oceanic crust

Dissertation submitted for the degree of Doctor of Philosophy, Darwin College, Cambridge, UK, May 1978The Oblique Seismic Experiment (OSE} is proposed to increase the usefulness of the IPOD crustal borehole as a means of investigating layer 2 of oceanic crust. Specific objectives are: to determine the lateral. extent of the structure intersected by the borehole, to analyse the role of cracks in the velocity. structure of layer 2, to look for anisotropy which may be caused by large cracks with a preferred orientation, and to measure attenuation in oceanic crust. Both travel time and amplitude techniques are used to plan the experiment and to interpret the data. The reflectivity method for computing synthetic seisDDgrams is · developed for the case of· the receiver within the reflectivity zone and ray method results 'are shown for comparison. A three-component borehole geophone w1th discrete variable gain pre-amplifiers was developed for the experiment. The first successful Oblique Seismic Experiment in oceanic crust was carried out in March 1977 in a hole 400 miles north of Puerto Rico. An adequate study of lateral velocity variations was impossible because the hole was not deep enough, the hole was inadequately logged, and the small scale basement topography was not known. Wyllie's relation, self-consistent theory, and non- interactive theory are applied to the observed velocity profiles in an attempt to quantitatively determine the crack structure. In general both P-wave and S-wave profiles suggest that the crack density decreases with depth in layer 2. Velocities at the bottom of layer 2 are the same as matrix velocities for basalt implying that crack density may be negligible at this depth. No convincLng evidence for anisotropy in either layer 2 or 3 is found. Since the large fissures observed in the FAMJUS area should produce anisotropy it appears that large fissures are not present in the studied crust (110 My) . The results agree with the theory that large fissures are less prevalent in older crust, perhaps sealing with age, and that the density of small cracks decreases with depth. The hole was not deep enough to measure attenuation from normal incidence shots. Auplitudes were not consistent enough to obtain a measure of attenuation from long range shots. The Oblique Seismic Experiment in March 1977 was a tenuous operation and a higher priority should be given to the experiment before it is attempted again.The Natural Environment Research Council funded the other field work and development; Shell Canada Ltd. for supporting personal expenses for three and a half years

Modeling seafloor geoacoustic interaction with a numerical scattering chamber

Author Posting. © Acoustical Society of America, 1994. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 96 (1994): 973-990, doi:10.1121/1.410271.A numerical scattering chamber (NSC) has been developed to compute backscatter functions for geologically realistic seafloor models. In the NSC, solutions are computed to the elastic (or anelastic) wave equation by the finite-difference method. This has the following advantages: (a) It includes all rigidity effects in the bottom including body and interface waves. (b) It can be applied to pulse beams at low grazing angles. (c) Both forward scatter and backscatter are included. (d) Multiple interactions between scatterers are included. (e) Arbitrary, range-dependent topography and volume heterogeneity can be treated simultaneously. (f) Problems are scaled to wavelengths and periods so that the results are applicable to a wide range of frequencies. (g) The method considers scattering from structures with length scales on the order of acoustic wavelengths. The process is discussed for two examples: a single facet on a flat, homogeneous seafloor and a canonically rough, homogeneous seafloor. Representing the backscattered field by a single, angle-dependent coefficient is an oversimplification. In a strong scattering environment, time spread of the field is a significant issue and an angle-dependent separation of the wave field may not be valid.This work was carried out under support from the Office of Naval Research Acoustic Reverberation Special Research (Grant Number N00014-90-J-149

Modeling sea surface scattering by the time-domain finite-difference method

Author Posting. © Acoustical Society of America, 1996. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 100 (1996): 2070-2078, doi:10.1121/1.417917.A numerical scattering chamber based on the time-domain finite-difference solution of the two-way elastic wave equation is applied to a sea surface scattering problem, and excellent agreement is obtained in amplitude and phase with a reference solution obtained by an integral equation method. The sea surface roughness is one representation of a Pierson–Moskowitz spectrum for a wind speed of 15 m/s. The incident field is a 400-Hz continuous wave generated by a Gaussian tapered vertical array. This problem demonstrates a number of issues in numerical modeling of wave scattering. The spreading of Gaussian beams, even in homogeneous media, creates an asymmetry in the insonification of the surface footprint or scattering area. Because of beamspreading, Gaussian tapered vertical arrays do not generate Gaussian beams. Scattering from a rough, free, fluid surface can be accurately solved with careful treatment of the numerical boundary representing the free surface. Continuous wave (cw) scattering problems can be solved in the time domain. For the second-order, explicit, staggered finite-difference formulation used in this study, a spatial sampling of 20 points per acoustic wavelength was necessary for acceptable grid dispersion. However, to correctly compute the scattered field for the test model, it was sufficient to specify the free surface at a spatial sampling of only ten points per acoustic wavelength.This work was carried out under Office of Naval Research Grant Nos. N00014-90-J-1493, N00014-95-1-0506, and N00014- 96-1-0460

Solutions to range-dependent benchmark problems by the finite-difference method

Author Posting. © Acoustical Society of America, 1990. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 87 (1990): 1527-1534, doi:10.1121/1.399452.An explicit second-order finite-difference scheme has been used to solve the elastic-wave equation in the time domain. Solutions are presented for the perfect wedge, the lossless penetrable wedge, and the plane parallel waveguide that have been proposed as benchmarks by the Acoustical Society of America. Good agreement with reference solutions is obtained if the media is discretized at 20 gridpoints per wavelength. There is a major discrepancy (up to 20 dB) in reference-source level because the reference solutions are normalized to the source strength at 1 m in the model, but the finite-difference solutions are normalized to the source strength at 1 m in a homogeneous medium. The finite-difference method requires computational times between 10 and 20 h on a super minicomputer without an array processor. The method has the advantage of providing phase information and, when run for a pulse source, of providing insight into the evolution of the wave field and energy partitioning. More complex models, including velocity gradients and strong lateral heterogeneities, can be solved with no additional computational effort. The method has also been formulated to include shear wave effects.This work was supported by the Office of Naval Research under Contract No. N00014-87-K-0007

User's guide for FINDIF at SACLANTCEN

FINDIF solves the elastic wave equation for a line source in two dimensions by the finite difference method (Virieux,1986; Stephen,1988b). The solution is carried out in the time domain for either pulse or CW sources. Arbitrary distributions of compressional velocity, shear velocity and density (including fluids and solids) can be defined on the finite difference grid. All compressional waves, shear waves, interface waves and evanescent waves are included as are all conversions between wave types. Multiple forward and backward scattering is automatically treatedThis work was cared out under ONR Contract #N00014-89-J-1012 and under ONR Grant #NOOOI490J154

Finite difference modeling of geoacoustic interaction at anelastic seafloors

Author Posting. © Acoustical Society of America, 1994. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 95 (1994): 60-70, doi:10.1121/1.408298.A major problem in understanding seismic wave propagation in the seafloor is to distinguish between the loss of energy due to intrinsic attenuation and the loss of energy due to scattering from fine scale heterogeneities and bottom roughness. Energy lost to intrinsic attenuation (heat) disappears entirely from the system. Energy lost to scattering is conserved in the system and can appear in observations as incoherent noise (reverberation, time spread, angle spread) and/or mode converted waves. It has been shown by a number of investigators that the seafloor scattering problem can be addressed by finite difference solutions to the elastic wave equation in the time domain. However previous studies have not considered the role of intrinsic attenuation in the scattering process. In this paper, a formulation is presented which includes the effects of intrinsic attenuation in a two-dimensional finite difference formulation of the elastodynamic equations. The code is stable and yields valid attenuation results.This work was carried out under Office of Naval Research Grant no. N00014-89-J-1012

Site synthesis report of DSPP sites 417 and 418

This document summarizes information relevent to planning, execution, and interpretation of results from a study of the interaction of sound in the 2-30Hz band with deep ocean seafloor using sea-surface sources, seafloor receivers, and borehole seismometers emplaced by wireline re-entry at Deep Sea Drilling Project sites 417 and 418 in the western North Atlantic. We summarize published scientific results from borehole sampling of water, sediment, and rock, from wire line logging, and from borehole seismic experiments. We present new results from analysis of total power recorded by receivers clamped in basement during the borehole seismic experiment on DSDP Leg 102. We document non-drilling investigations of the site and the nature and location of re-entry cones and transponders. We describe the physical oceanography of the region and the speed of sound in water. We provide an extensive bibliography on published results from scientific investigations at 417/418. This document was completed prior to 1989 surveys of sites 417 and 418.Funding was provided by the Johns Hopkins University, Applied Physics Laboratory under contract Number 602809-0

Bathymetry and sediment thickness survey of the Hawaii-2 cable : cruise report for Kiwi expedition leg 2 on the R/V Roger Revelle

The primary purpose of this cruise was to identify at least two potential observatory sites along the Hawaii-2 cable that would be suitable for drilling a hole to basement. There is a funded program, the Hawaii-2 Observatory (H20), to install a junction box on the cable about mid-way between California and Hawaii (Figures 1 and 2). We want to identify sites in advance so that drilling will be possible near the observatory. This will permit a large range of borehole experiments to be cared out continuously and in real time. Based on available data we chose a section of cable between 140° and 143°W. This cable lies on a ribbon of 'normal' oceanic crust with well defined magnetic anomalies and relatively smooth bathymetry. The goals were to acquire SEABEAM bathymetry data and single channel seismic reflection data along this section of cable, to identify at least two potential sites along the cable and to car out SCS surveys within about 10km radius of the sites. Since the H20 cable has been given to the scientific community it is a valuable resource for research. While transiting to and from the site we felt that it would be wortwhile to acquire SEABEAM data along as much of the cable track as possible. This 'spec' data, Seabeam and magnetometer data between 130°W and 155°W may be useful to other investigators in the future.This work was carried out under the JOI Prime Contract OCE-93020477, JOI Budget Code 44505-J130l0

Frequency division multiplexing for interferometric planar Doppler velocimetry

A new method of acquiring simultaneously the signal and reference channels used for interferometric planar Doppler velocimetry is proposed and demonstrated. The technique uses frequency division multiplexing (FDM) to facilitate the capture of the requisite images on a single camera, and is suitable for time-averaged flow measurements. Furthermore, the approach has the potential to be expanded to allow the multiplexing of additional measurement channels for multicomponent velocity measurement. The use of FDM for interferometric referencing is demonstrated experimentally with measurements of a single velocity component of a seeded axisymmetric air jet. The expansion of the technique to include multiple velocity components was then investigated theoretically and experimentally to account for bandwidth, crosstalk, and dynamic range limitations. The technique offers reduced camera noise, automatic background light suppression, and crosstalk levels of typically <10%. Furthermore, as this crosstalk is dependent upon the channel modulations applied, it can be corrected for in postprocessing

A user’s manual for finite difference synthetic seismogram codes on the CYBER 205 and CRAY XMP-12

Over the past eight years, a software package has been developed to solve the elastic wave equation by the method of finite differences (Hunt et al., 1983; Stephen, 1983; Stephen, 1984a; Stephen, 1984b; Nicoletis, 1981). The elastic wave equation can be solved in two dimensions for point sources in cylindrical coordinates or line sources in rectangular coordinates. Compressional and shear velocity and density are allowed to vary both vertically and radially. Since the code is very computationally intensive for realistic size models, it has been implemented on two Class VI super computers: the Cyber 205 at Purdue University and the Cray XMP-l2 at the Naval Research Laboratory. This technical report is a user's manual for running the code on these machines. It is assumed that the reader is already familiar with running the code on the VAX ll-780 (Hunt et al., 1983).Funding was provided by the National Science Foundation under grant Nos. OCE-811 7571 and OCE-8409155; and by the Office of Naval Research under contract No. N00014-85-C-001NR