GEOPHYSICS, VOL. 71, NO. 5 共SEPTEMBER-OCTOBER 2006兲; P. Q19–Q24, 6 FIGS. 10.1190/1.2335512
A reusable implosive seismic source for midwater or seafloor use
LeRoy M. Dorman1 and Allan W. Sauter2
an oceanic rift and a seamount 共Wiggins et al., 1996; Hammer et al., 1994兲 and in Scholte wave studies of the upper few tens of meters of sediment 关Sauter et al., 1986; Schreiner and Dorman, 1990 共Scholte waves carry seafloor noise兲; Dorman et al., 1991; Schreiner et al., 1991; Bibee and Dorman, 1991; Nolet and Dorman, 1996; Dorman, 1997兴. The explosives that served as the energy sources for these studies have become increasingly difficult to use because of regulatory and safety issues. This situation has motivated us to develop a nonexplosive seafloor source. In addition to these detailed structural studies, there are other areas of current scientific interest that can benefit from using near-bottom sources. Some of the sites of Ocean Seismic Network observatories, at full ocean depths of 4000–5000 m, will be in regions of strong gradients in shear velocities, and these can produce troublesome reverberations. Knowledge of the detailed velocity structure derived independently of recorded teleseismic signals can aid in separating local from distant effects. Hydrothermal systems at spreading centers, a focus of the National Science Foundation RIDGE program, have structural features small enough to make them difficult to image from the surface, and near- or on-bottom techniques are clearly likely to lead to better understanding of the complex processes taking place. These sites are at ridge-crest depths 共⬃2500 m兲. Understanding of conditions at gas hydrate sites could, we think, profit from shear-velocity studies. Seismic work at producing petroleum sites in gassy sediments benefits from the fact that propagation of shear waves is minimally affected by the presence of bubbles, whereas propagation of compressional waves is strongly affected because the bubbles increase compressibility greatly 共Pautet et al., 2001兲. These sites are at depths of 500–1000 m. We have observed correlations among seismic noise, fluid flow, and seismic activity 共Brown et al., 2005兲. Our previous work using on-bottom sources and receivers has shown the importance of Scholte waves and how seafloor noise in the frequency range of 1–10 Hz is strongly influenced by seafloor structure 共papers by Bibee, Dorman, Schreiner, and Sauter cited above兲. At present, the
ABSTRACT We have developed a new implosive seismic or acoustic source for seafloor or midwater use. The fact that this device does not use pyrotechnics simplifies logistic and permitting problems. It produces relatively little high-frequency output, so it is wildlife friendly. This device enables us to place the source nearer to the image target compared to surface sources, which thus increases resolution. The simple 20-l version we have constructed must be reset after each use by bringing it to the sea surface. We present measurements of seafloor shear velocity at a depth of about 1 km in the San Diego Trough. There the surficial shear velocity is 16 m/s, and the gradient is about 10 s−1.
INTRODUCTION Seismic sources used in marine geophysics have evolved significantly during the decades since World War II in response to advances in technology and, to a lesser degree, to the increased awareness of the effects of loud sounds in the sea upon animal life. Today, air guns and water guns are the most commonly used marine seismic sources. To make an equivalent source safer for animals at the surface, to generate shear waves, and to do more detailed refraction experiments, it is helpful for the source to be on or near the seafloor 关Davies, 1965; Whitmarsh and Lilwall, 1982; Sauter et al., 1986; Stoll, 1989; Berge et al., 1991 and Lilwall 共sediment anisotropy兲; Chapman, 1991; Christesen et al., 1992, 1994 共shallow structure of East Pacific Rise兲; Dorman, 1997 共shear velocity of seafloor sediments兲; Hammer et al., 1994 共inhomogeneity of seamount interior兲; Wiggins et al., 1996 共structure of Hess Deep rift zone兲; Collins and Detrick, 1998 共shallow, high velocities on massif兲; Sohn et al., 2004 共shallow structure of the East Pacific Rise兲兴. Accurately timed and navigated seafloor shots have been used for years in refraction experiments to image the subbottom structure of
Manuscript received by the Editor April 18, 2005; revised manuscript received February 3, 2006; published online August 31, 2006. 1 Scripps Institution of Oceanography, Marine Physical Laboratory and Geosciences Research Division, 9500 Gilman Drive, MC 0220, La Jolla, California 92093. E-mail:
[email protected]. 2 Formerly Scripps Institute of Oceanography, La Jolla, California; presently IRIS/PASSCAL Instrument Center, New Mexico Tech, 100 East Road, Socorro, New Mexico 87801. E-mail:
[email protected]. © 2006 Society of Exploration Geophysicists. All rights reserved.
Q19
Q20
Dorman and Sauter
mechanism of coupling between fluid flow and noise remains a mystery that further studies using seafloor sources can help to answer.
RATIONALE FOR SOURCE DEVELOPMENT It is an appropriate time to consider alternatives to surface sources, such as air guns. In addition to the scientific benefits that a seafloor source can provide, recent news items have cast seismic air-gun exploration in a less-than-perfect light because of its association with beaked whale strandings. Although it is arguable whether air guns have caused whale strandings, there is little doubt that U. S. Navy acoustical sources have, and the public is now aware that anthropogenic sound sources can cause serious damage to cetaceans. The Bahamas stranding event of 2000 共NOAAand U. S. Navy, 2001兲 involved at least 17 cetaceans, including two species of beaked whales. Six beaked whales died. The stranding is attributed to tactical naval sonars in use in the area by several naval vessels. These sonars operate at frequencies centered around 2.6, 3.3, 6.8, and 8.2 kHz. These frequencies are higher than those used in reflection and refraction seismic exploration. In the more recent Gulf of California incident 共Peterson, 2003兲, two Cuvier’s beaked whales were found beached during a seismic survey. Although it is arguable whether the R/V Maurice Ewing caused these whale deaths, the U. S. District Court or Northern California found sufficient probable cause to issue a court order stopping the survey. Also in 2003, another Ewing cruise was cancelled 共the permit was withheld兲 by the Government of Bermuda over potential risk to marine mammals. The scientists involved went to great lengths to protect mammals, and this diligence may have contributed to unease within the citizenry and Government of Bermuda. Discussion of these problems has reached the pages of Nature 共Dalton, 2004兲. Dalton quotes the costs of the Bermuda and Gulf of Mexico cruises as about $2.6 million. Whether air-gun use is a serious threat to ceutacean health or not, that perception has seriously hampered U. S. sponsored marine seismic research. Most scientists would agree that not enough is known about the physiological effects of noise on cetaceans to set safe species-, frequency-, and location-dependent source-level limits. Wartzok and Ketten 共1999兲 assert, “Currently, there are insufficient data to determine accurately the TTS 关temporary threshold shift兴 and PTS 关permanent threshold shift兴 exposure guidelines for any marine mammal.” The Marine Mammal Protection Act established sound levels that are allowable for marine mammals. The limits, however, are neither frequency dependent nor species specific. The species most often affected is the Cuvier’s beaked whale 共Ziphius cavirostris兲. This species is rare and not well studied. Richardson et al. 共1995兲 report nothing about the frequency of its calls, which is presumably one part of the sound spectrum where its hearing is sensitive. These incidents prompt us to look at our sources in the light of environmental considerations. In land seismic surveys, reflection experiments began by using chemical explosives, which are inexpensive, compact, and convenient. Over the years, the need to perform experiments in urban areas 共such as downtown Los Angeles兲, personnel safety, signal-processing advantages, and national security have led to development of controllable electromechanical sources such as Vibroseis, which generates a swept-frequency signal. Marine seismic work has moved from chemical explosives to air guns and water guns. These are nonexplosive, impulsive sources with limited control over the source waveform. They are customarily used near the sea surface. Moving the source away from the sea surface reduces the acoustic exposure of many marine mammals, so it is
advantageous to have a source that can operate at greater depths than can air guns. Towed acoustical chirp sources exist that work at shallow depth; however, in general, their limited power yields less deep insonification compared to air guns, and like air guns, they do not directly excite shear waves. The advantages of implosive sources are that they are able to work at great depths, can be scaled to be as strong as an air-gun array, and can directly excite shear waves if the source is on or very near the seafloor.
IMPLOSIVE SOURCES Researchers have long recognized the usefulness of collapsing water voids at depth to produce seismic or acoustic signals. In the mid-1950s, John Isaacs and Art Maxwell 共Isaacs and Maxwell, 1952兲 attached glass floats 共about 0.2-l volume兲 to piston sediment corers; the floats were broken by a mechanism when the corer hit the bottom. Listeners on board would retrieve the core when they heard the pop. Later, Urick 共1983兲 recorded acoustic signals from the implosion of ⱕ4-l bottles. Orr and Schonberg 共1976兲 examined the potential of spherical, glass, high-pressure floats 共manufactured by Benthos, Inc.兲 for use as acoustic sources. The volume of the floats used was about 34 l. In their tests, Orr and Schoenberg dropped weakened glass spheres through the water column, recorded the implosion, and analyzed the sounds emitted. Heard et al. 共1997兲 found that ordinary light bulbs retain their integrity to great enough depths to perform as useful acoustic sources. In all these studies, the primary interest was in the audible part of the seismo-acoustic spectrum, rather than in the subaudible frequencies used for seismic studies of the crust and sedimentary column. For a nonexplosive source to be useful as a bottom seismic source, it must be able to work on the bottom, and it must be initiated by an accurately timed signal. In the work of Urick, Orr and Schoenberg, and Heard et al., the sources were lowered or dropped into the sea until the increasing water pressure caused failure. This lack of control of either the depth 共making it impossible to initiate an implosion on the seafloor, rather than at some uncontrolled distance above it兲, or the timing of breakage has been difficult to overcome.
SOURCE THEORY None of the treatments of implosive sources we cite made calculations of source behavior, other than comparing total radiated energy with the potential energy available. Nolet and Dorman 共1996兲 calculated the seismic moment for a deep explosion. In that treatment, the significant source parameter at frequencies well below the bubble frequency is shown to be the change in volume between the solid explosive and the equilibrium size of the gas bubble. That treatment is also applicable to an implosion; the primary difference is that the volume change is negative. A modified version of that argument is presented here. In an underwater explosion, the explosive charge is rapidly converted into a bubble of gaseous detonation products 共Cole, 1948兲. The pressure within this bubble is much higher than the ambient water pressure, and the bubble increases rapidly in size. After the bubble expands to its equilibrium size, the momentum of the outwardmoving water mass causes the bubble to expand to a size larger than that necessary to make the gas pressure equal the hydrostatic pressure in the water, and the pressure deficit causes a restoring force that makes the bubble smaller. The size of the bubble thus oscillates for a few cycles. During this oscillation, the bubble radiates acoustic energy, especially when the bubble walls undergo large accelerations
Reusable implosive source during the minima of the bubble size. As energy is radiated, the range of oscillation decreases, and the bubble volume approaches its equilibrium size at the ambient pressure. The low-frequency part of the spectrum of an explosion is described by the moment, which is proportional to the volume change caused by the source. This volume is the difference between the volume initially occupied by the unexploded charge and the volume occupied by the combustion products at the end of the process. We are interested in the spectrum in the frequency range of 0.3–3 Hz, because this is the range in which we observe Scholte waves. The time scale of interest is thus 0.3–3 s — much larger than the detonation 共microseconds兲 and bubble oscillation 共milliseconds兲 time scales, but smaller than the surface-reflection 共seconds in deep water兲 and bubble-dissolution time scales 共minutes兲 — so we can treat the moment time dependence as a step function in time. The theory of volume seismic sources is treated by Müller 共1973兲, Aki and Richards 共2002, at the end of chapter 3兲, and Richards and Kim 共2005兲. By using equation 1 of Richards and Kim, we have for the isotropic moment M I,
M I = 共 + 2兲␦V,
Q21
analysis was limited to the static case. In Figure 2, arrows indicate the movement of water. At the upper left of the diagram is an EdgeTech 8242 acoustic release cylinder, modified to be triggered electrically. Just to the right of the release cylinder is the release pelican hook, which is attached to a plug that seals a backing chamber. When the release operates, water pressure pushes the piston into the backing chamber, allowing water to flow through the bridge tube 共the pipe at a right angle to the release兲 and flood an actuating chamber shown in the lower-right of the figure. As high-pressure water
共1兲
where is the shear modulus and is the Lamé modulus. Because the shear modulus is zero in a liquid, we are left with M I = ␦V, where is incompressibility. For water, is 2.25 ⫻ 109 N · m−2. Thus the moment is M I = 4.5 ⫻ 107 N · m for a 20-l source. This result corresponds to a moment magnitude of −0.9 according to the definition of Hanks and Kanamori 共1979兲. Because the equilibrium volume of a gas bubble produced by an explosion is pressure dependent, the performance 共as measured by the seismic moment兲 of explosive sources decreases with depth. The moment of an implosive source, however, is not depth dependent because the volume change is determined by the volume of the container. We have calculated gas volumes of small explosions at a range of depths and present these in Figure 1. At depths greater than about 500 m, the 20-l imploder should be more effective than a 200-g 共0.5-lb兲 TNT explosion. A plot of source moment magnitude 共Hanks and Kanamori, 1979兲 is similar in appearance.
Figure 1. Gas volumes produced by explosion of 200 g 共0.5 lb兲 of TNT 共solid line兲 and by 20-l 共0.02 m3兲 imploder 共dashed line兲 at a range of depths.
A SIMPLE REUSABLE IMPLODER We have constructed a prototype 20-l implosive source that can be fired in the water column or on the seafloor to depths of 2000 m. In its present configuration, it is useful for mapping the local sediment shear-velocity gradients, but for more general seismic surveys, the source volume needs to be increased, and a system for in situ source reloading must be developed. Our present design shares some features with an air gun. Both are energetically driven by a large differential pressure gradient, and both use hydraulically controlled valves to open a chamber. In air guns, opening the chamber allows the interior, high-pressure air to flow out to the sea; in the imploder, high-pressure seawater flows into the low-pressure chamber. Figure 2 shows a schematic of the device, and Figure 3 is a photograph of it on the deck of the R/V R. G. Sproul. In designing the imploder, a concern was that the high stresses would cause the device to fail at depth, either by exceeding material strengths or by causing strains exceeding piston-movement tolerances. To prevent these problems, we iterated to the final design assembly by using CAD Solidworks and the accompanying stressstrain analysis package 共COSMOS兲 before manufacture. Our stress
Figure 2. Schematic of single-shot imploder. The 20-l energy chamber is at the lower left. The pear-shaped orifices are where water enters the energy chamber upon firing. At the lower right, the piston that keeps the water out before firing can be seen in light shading, which simulates a partially transparent valve housing. That piston is connected to a pressure-balancing piston by a spacer rod, much as in an air-gun shuttle. The dashed arrow shows the entry of the triggering fluid pulse that causes the shuttle to begin to move.
Q22
Dorman and Sauter
fills this actuating chamber, it pushes a piston to the right. This piston forms the right-hand end of a shuttle, whose left-hand end is the end cap closing the 20-l energy cylinder at the lower left of the figure. The movement of the shuttle to the right opens the main volume and allows seawater to flood inside. The time elapsed between electrical initiation and opening of the main volume is about 70 ms. The fill time 共moment rise time兲 at 1135-m depth is about 25 ms. For recharging, the device must be returned to the surface, where water is poured out of the working volume and the control valves are reset. The energy source is ultimately gravity, which creates water pressure. Our existing device was designed for a maximum depth of 2400 m, and our testing has been at depths of about 1000 m. Use at greater depths would require scaling of the large main valve, making it smaller, but stronger. The volume change of our current design is 20 l 共0.02 m3兲, which is as effective as 200 g 共0.5 lb兲 of TNT detonated at 500-m depth. The next step in our imploder’s development is to make it capable
of being refired without having to return it to the surface, as is currently required. The working volume could be emptied by using an electrically driven high-pressure pump or by electrochemical means. We plan to design a larger version whose source waveform is more controllable, so that the output energy is concentrated in the seismic frequency band of interest 共1–200 Hz or 1–500 Hz, depending on the problem兲. Our goal is to make a source that is effective for refraction studies while minimizing the harm to the marine ecosystem. The effectiveness of the source is illustrated in Figure 4. This record section shows Scholte waves generated by the imploder out to a distance of about 240 m. The pulses at the ends of the records are the fundamental-modeAiry phases whose velocity approximates the shear velocity at the top of the sediments. In this record, this velocity is about 16 m/s, among the slowest we have observed. Figure 5 shows the time-frequency decomposition of one seismo-
Figure 3. Photograph of single-shot imploder. The mass is about 275 kg 共600 lb兲. The cookie-tin tool container is about 300 mm 共12 in.兲 in diameter.
Figure 4. Record section of three imploder shots recorded on a vertical-component ocean-bottom seismograph at a water depth of 1135 m. The location of this experiment was 32°40⬘N, 117°35⬘W.
Figure 5. Time-frequency decomposition of 共a兲 one of the seismograms 共vertical component; range, 0.111 km兲. 共b兲 The upper abscissa is group slowness, which is linear in time. Group velocity, the reciprocal, is shown on the same plot’s lower abscissa. The prominent feature that slopes up gently to the Airy phase is the dispersion curve of the fundamental mode. There is some structure to the Airy phases, so we are probably seeing a little multipathing. The energy faster than 70 m/s between 3 and 6 Hz represents higher modes. The smooth lines are the computed dispersion from a model. The lower 共black兲 smooth line is the group velocity of the fundamental mode. The next-lowest 共red兲 line is the first overtone, and the green and blue lines are the second and third overtones, respectively. If this model were a perfect fit, the lines 共which are the mode frequencies for a given slowness兲 would lie atop the contoured ridges that represent the time-frequency decomposition of the observed data. This representation is sometimes called a sonogram.
Reusable implosive source gram. The group velocity-dispersion curves from our interim model are also plotted. The first step in modeling was to adjust a model by hand so that the fundamental-mode dispersion curve fit the time-frequency curves in Figure 5. We are now working on waveform modeling of these dispersed wavetrains by using Guust Nolet’s nonlinear modeling program that has been adapted for Scholte waves 共Nolet and Dorman, 1996兲. The model from group velocity matching produced a signal envelope that was sharply unimodal, with a peak at the Airy phase. The first iteration of the modeling code produced the models in Figure 5. The gross timing and amplitude ratios are not too far off, but detailed phase matching is yet to be achieved. The interim velocity model from the San Diego Trough is shown 共the bold line兲 in Figure 6, along with a suite of other models from Dorman 共1997兲. As is evident, the San Diego Trough surficial shear velocity, 16.3 m/s, is among the slowest yet observed, and the shearvelocity gradient with depth is the highest, about 10 s−1. The other very low shear velocity is from the Mississippi Fan 共Bibee and Dorman, 1995兲. Both of these are lower than any reported by Hamilton 共1980兲 and the extensive references cited therein. The great majority of those measurements were made on cores. These sampling methods are known to have difficulty in capturing the very soft surficial sediments, which are nearly liquid. Implosive sources, towed slightly above the seafloor, will require slower towing than air guns. Experience of the Scripps Marine Physical Laboratory deep-tow devices indicates 2.8–3.7 km/h 共1.5–2 knots兲 will be achievable. Imploder survey mileage per day will be less than for an air gun, but the tradeoff will be the greater resolution and added shear imaging provided by the implosive source.
Figure 6. Shear velocities in surficial sediment from shallow and deep environments.
Q23
CONCLUSION We have developed an explosive-free seismic source that can be used on the seafloor. This device has the advantage of concentrating seismic energy at lower frequencies than explosives and eliminating most of the safety and regulatory concerns. It can replace small explosions in generating Scholte 共interface兲 waves. It is electrically initiated so that source timing is well known. In comparison with a source at the sea surface, the advantages of having the source directly on the bottom include shrinking the footprint area of ray entry and enhanced excitation of shear waves. Because the energy source is gravitational potential energy and there is no automatic way to empty the working volume, the device must be returned to the sea surface between firings.
ACKNOWLEDGMENTS This work was supported by the National Science Foundation Ocean Technology Program under grant OCE 97-12605. Phil Harbin of Lawrence Livermore National Laboratories participated on some test cruises with a glass-sphere imploder. Eric Canuteson worked on an earlier design.
REFERENCES Aki, K., and P. G. Richards, 2002, Quantitative seismology, 2nd ed.: University Science Books. Berge, P. A., S. Mallick, G. J. Fryer, N. Barstow, J. A. Carter, G. H. Sutton, and J. I. Ewing, 1991, In situ measurement of transverse isotropy in shallow-water marine sediments: Geophysical Journal International, 104, 241–254. Bibee, L. D., and L. M. Dorman, 1991, Implications of deep water seismometer array measurements for Scholte wave propagation, in J. Hovem, R. Stoll, and M. Richardson, eds., Shear waves in marine sediments: Kluwer, 221–230. ——–, 1995, Full waveform inversion of seismic interface wave data, in O. Diachok, A. Caiti, P. Gerstoft, and H. Schmidt, eds., Full field inversion methods in ocean and seismo-acoustics: Kluwer 377–382. Brown, K. M., M. Tryon, H. R. DeShon, L. M. Dorman, and S. Y. Schwartz, 2005, Correlated transient fluid pulsing and seismic tremor in the Costa Rican subduction zone: Earth and Planetary Science Letters, 238, doi: l0.1016/j.epsl.2005.06.0055. Chapman, N. R., 1991, Estimation of geoacoustic properties by inversion of acoustic field data, in J. Hovem, R. Stoll, and M. Richardson, eds., Shear waves in marine sediments: Kluwer, 511–520. Christeson, G. L., G. M. Purdy, and G. J. Fryer, 1992, Structure of young upper crust at the East Pacific Rise near 9°30⬘N: Geophysical Research Letters, 19, 1045–1048. ——–, 1994, Seismic constraints on shallow crustal emplacement processes at the fast spreading East Pacific Rise: Journal of Geophysical Research, 99, 17957–17973. Cole, R. H., 1948, Underwater explosions. Princeton University Press. Collins, J. A., and R. S. Detrick, 1998, Seismic structure of the Atlantis Fracture Zone megamullion, a serpentinized ultramafic massif: Eos, Transactions, American Geophysical Union, 79, 800. Dalton, R., 2004, Push to protect whales leaves seafloor research high and dry: Nature, 428, 681. Davies, D., 1965, Dispersed Stoneley waves on the ocean bottom: Bulletin of the Seismological Society of America, 55, 903–918. Dorman, L. M., 1997, Propagation in marine sediments, in Crocker, M. J., ed., Encyclopedia of acoustics: John Wiley, 409–416. Dorman, L. M., A. E. Schreiner, and L. D. Bibee, 1991, The effects of sediment structure on sea floor noise, in J. M. Hovem, M. D. Richardson, and R. D. Stoll, eds., Shear waves in marine sediments: Kluwer, 239–245. Hamilton, E. L., 1980, Geoacoustic modeling of the sea floor: Journal of the Acoustical Society of America, 68, 1313–1340. Hammer, P. T. C., L. M. Dorman, J. A. Hildebrand, and B. Cornuelle, 1994, Jasper seamount structure: Seafloor seismic refraction tomography: Journal of Geophysical Research, 99, 6731–6752. Hanks, T. H., and H. Kanamori, 1979, A moment magnitude scale: Journal of Geophysical Research, 84, 2348–2350. Heard, G. J., M. McDonald, N. R. Chapman, and L. Jaschke, 1997, Underwater light bulb implosions, a useful acoustic source: IEEE Oceans ’97, Pro-
Q24
Dorman and Sauter
ceedings of IEEE Oceans ’97, 755–762. Isaacs, J. D., and A. E. Maxwell, 1952, The ball-breaker, a deep water bottom signalling device: Journal of Marine Research, 11, 63–68. Müller, G., 1973, Seismic moment and long-period radiation of underground nuclear explosions: Bulletin of the Seismological Society of America, 63, 847–857. NOAA and U. S. Navy, 2001, Joint interim report, Bahamas marine mammal stranding event of 15–l6 March 2000: Technical Report, National Oceanic and Atmospheric Administration and Department of the Navy. Nolet, G., and L. M. Dorman, 1996, Waveform analysis of Scholte modes in ocean sediment layers: Geophysical Journal International, 385–396. Orr, M., and M. Schoenberg, 1976, Acoustic signatures from deep water implosions of spherical cavities: Journal of the Acoustical Society of America, 59, 1155–1159. Pautet, L., W. A. Kuperman, and L. Dorman, 2001, Using refracted shear waves for velocity estimation: Geophysical Prospecting, 49, 281–286. Peterson, G., 2003, Whales beach seismic research: Geotimes, January. Richards, P. G., and W.-Y. Kim, 2005, Equivalent volume sources for explosions at depth: Theory and observations: Bulletin of the Seismol Society of America, 95, 401–407, doi: 10.1785/0120040034. Richardson, W. J., C. R. Greene, C. I. Malme, and D. H. Thomson, 1995, Marine mammals and noise: Academic Press. Sauter, A. W., L. M. Dorman, and A. E. Schreiner, 1986, A study of sea floor structure using ocean bottom shots and receivers, in T. Akal and J. Berkson, eds., Ocean seismo-acoustics: Plenum Press NATO Conference Se-
ries IV, 16, 673–682. Schreiner, A. E., and L. M. Dorman, 1990, Coherence lengths of seafloor noise: Effect of ocean bottom structure: Journal of the Acoustical Society of America, 88, 1503–l514. Schreiner, A. E., L. M. Dorman, and L. D. Bibee, 1991, Shear wave velocity structure from interface waves at two deep sites in the Pacific Ocean, in J. M. Hovem, M. D. Richardson, and R. D. Stoll, eds., Shear waves in marine sediments: Kluwer, 231–238. Sohn, R. A., S. C. Webb, and J. A. Hildebrand, 2004, Fine-scale seismic structure of the shallow volcanic crust on the East Pacific Rise at 9°50⬘N: Journal of Geophysical Research, 109, doi: 10.1029/2004JB003152. Stoll, R. D., 1989, Sediment acoustics: Springer-Verlag. Urick, R. J., 1983, Principles of underwater sound, 3rd ed.: McGraw-Hill Book Company. Wartzok, D., and D. R. Ketten, 1999, Marine mammal sensory systems, in J. E. Reynolds, III, and S. A. Rommel, eds., Biology of marine mammals: Smithsonian Institution Press, 117–175. Whitmarsh, R. B., and R. C. Lilwall, 1982, A new method for the determination of in-situ shear-wave velocity in deep-sea sediments: Oceanology International 1982, Proceedings of Oceanology International 1982, OI82 4.2. Wiggins, S., L. M. Dorman, B. D. Cornuelle, and J. A. Hildebrand, 1996, Hess Deep rift valley structure from seismic tomography: Journal of Geophysical Research, 22335–22353.
