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Stephen, RA, Collins JA, Peal KR, Hildebrand JA, Orcutt JA, Spiess FN, Vernon FL.  1999.  Seafloor seismic stations perform well in study. EOS Trans. AGU. 89:592. Abstract
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Hussenoeder, SA, Collins JA, Kent GM, Detrick RS, Harding AJ, Orcutt JA, Mutter JC, Buhl P.  1996.  Seismic analysis of the axial magma chamber reflector along the southern East Pacific Rise from conventional reflection profiling. Journal of Geophysical Research-Solid Earth. 101:22087-22105.   10.1029/96jb01907   AbstractWebsite

The thickness and internal properties of the magma sill located at the top of the axial magma chamber (AMC) along the southern East Pacific Rise (EPR) have been investigated through a combination of waveform modeling the near-vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source-receiver offset (or slowness). Our results show that the AMC reflector observed along the southern EPR is best modeled by a thin (< 100 m thick) sill of partial melt (V-s not equal 0 km/s) sandwiched between higher-velocity material, and that the thickest sills are generally associated with the lowest P and S wave velocities. The comparatively high P wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content. This may have important implications for eruption mechanisms and along-axis mixing of magma at the EPR. There is no simple relationship between morphologic indicators of magma supply (e.g., axial depth or volume) and sill thickness, depth, or velocity. Magma sill properties may be closely tied to the eruption and replenishment cycle of the AMC and thus may vary on a much shorter spatial and temporal scale than axial morphology, which reflects longer-term variations in magma supply to the ridge.

Blackman, DK, Orcutt JA, Forsyth DW, Kendall JM.  1993.  Seismic anisotropy in the mantle beneath an oceanic spreading centre. Nature. 366:675-677.   10.1038/366675a0   AbstractWebsite

BENEATH an active mid-ocean ridge, the mantle upwells in response to the divergence of the newly formed plates, leading to high temperatures and pressure-release melting below the ridge axis. The width of the upwelling region and the amount of melting depend on mantle rheology1-5, but all models predict a maximum decrease in seismic velocity at the ridge axis. It has also been suggested, however, that the alignment of anisotropic minerals by shear in the upwelling mantle will increase seismic velocity for rays travelling subvertically through the upwelling zone6,7. Here we report the observation of a consistent pattern of anomalously early P-wave arrival times at an array of ocean-bottom seismographs deployed across the axis of the southern Mid-Atlantic Ridge: P-waves from distant earthquakes arrive earlier at stations near the axis than at those further away. Our results are consistent with a model of anisotropy in which the degree of mineral alignment is greatest directly beneath the ridge axis, and significant anisotropy extends tens of kilometres from the axis.

Singh, SC, Collier JS, Harding AJ, Kent GM, Orcutt JA.  1999.  Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise. Geology. 27:219-222.   10.1130/0091-7613(1999)027<0219:sefahl>2.3.co;2   AbstractWebsite

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50-60-m-thick roof is overlain by a 150-200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

Reid, I, Orcutt JA, Prothero WA.  1977.  Seismic Evidence for a Narrow Zone of Partial Melting Underlying East Pacific Rise at 21°N. Geological Society of America Bulletin. 88:678-682.   10.1130/0016-7606(1977)88<678:sefanz>2.0.co;2   AbstractWebsite

Analysis of a seismic-refraction profile on the crest of the East Pacific Rise at 21 °N reveals considerable lateral inhomogeneity in the velocity structure. A split profile, of a total length of 120 km, was shot to a tripartite array of ocean-bottom seismometer capsules on or close to the spreading axis. The recorded travel times and amplitudes indicate the presence of a low-velocity zone in the immediate vicinity of the spreading axis, at a depth of about 2.5 km below the sea floor. This low-velocity zone is not evident a few (∼10) kilometres from the axis.Additional structural evidence was obtained from a deployment of the capsules at the intersection of the East Pacific Rise crest with the Rivera Fracture Zone, at 20 °N. A group of small earthquakes was recorded by the seismometers, and, for ray paths traversing or close to the rise crest axis, the shear waves appear to be anomalously attenuated, particularly at shorter periods. While the evidence is not conclusive, it suggests the presence of a shear-attenuative region beneath the rise crest, presumably identical with the low-velocity zone and consistent with the interpretation of this zone as a region of partial melting.

Mutter, JC, Carbotte SM, Su WS, Xu LQ, Buhl P, Detrick RS, Kent GM, Orcutt JA, Harding AJ.  1995.  Seismic Images of Active Magma Systems Beneath the East Pacific Rise Between 17-Degrees-05' and 17-Degrees-35'S. Science. 268:391-395.   10.1126/science.268.5209.391   AbstractWebsite

Seismic reflection data from the East Pacific Rise between 17 degrees 05' and 17 degrees 35'5 image a magma lens that varies regularly in depth and width as ridge morphology changes, confirming the notion that axial morphology can be used to infer ridge magmatic state. However, at 17 degrees 26'S, where the ridge is locally shallow and broad, the magma lens is markedly shallower and wider than predicted from regional trends. In this area, submersible dives reveal recent volcanic eruptions. These observations indicate that it is where the width and depth of the magma chamber differ from regional trends, indicating an enhanced magmatic budget, that is diagnostic of current magmatism.

Singh, SC, Harding AJ, Kent GM, Sinha MC, Combier V, Bazin S, Tong CH, Pye JW, Barton PJ, Hobbs RW, White RS, Orcutt JA.  2006.  Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise. Nature. 442:287-290.   10.1038/nature04939   AbstractWebsite

The determination of melt distribution in the crust and the nature of the crust - mantle boundary ( the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres(1-4) but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.

Riedesel, MA, Orcutt JA, Adams JA.  1999.  Seismic signals and noise recorded on a seafloor vertical hydrophone array and a colocated OBS during the LFASE experiment. Bulletin of the Seismological Society of America. 89:423-432. AbstractWebsite

During the LFASE experiment conducted in the summer of 1989, a vertical hydrophone array (VHA) was deployed at the site of DSDP Hole 534b in the Blake-Bahama basin. The VHA consisted of 16 hydrophones spaced 30 m apart rising vertically above the seafloor with the bottom of the array anchored to a digital recording package located on the seafloor. The main purpose of the experiment was to record seismic signals and noise above, on, and beneath the seafloor at the same site; the VHA recorded data from above the seafloor, an ocean-bottom seismograph (OBS) was on the seafloor, and a borehole geophone array (BHA) was in a borehole beneath the seafloor. Signal-to-noise measurements of P waves made for airgun shots recorded simultaneously by the VHA and the OBS show that the OBS has a signal-to-noise ratio (SNR) 5 to 10 dB greater than that of a single hydrophone, but P-wave stacks of the VHA data have SNRs for body waves 10 to 15 dB greater than the OBS. This implies that a vertical array is capable of recording distinct body waves out to significantly greater ranges than is a single receiver and so might be a useful tool in seismic refraction experiments. Other stacks of the VHA data revealed S waves not visible on the data from a single hydrophone or geophone. The VHA was also used to construct images of the wavefronts arriving from a distant source and to determine the vertical direction of the source. This array processing capability shows the potential of VHAs for use in 3D seismic reflection surveys, particularly in cases where it is not convenient to use a multi-channel streamer.

Detrick, RS, Harding AJ, Kent GM, Orcutt JA, Mutter JC, Buhl P.  1993.  Seismic Structure of the Southern East Pacific Rise. Science. 259:499-503.   10.1126/science.259.5094.499   AbstractWebsite

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, mid-crustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges.

Kim, II, Smith DK, Menard HW, Orcutt JA, Jordan TH.  1987.  Seismic-Reflection Site Survey - Correlation with Physical-Properties, Leg-91, Deep-Sea Drilling Project. Initial Reports of the Deep Sea Drilling Project. 91:271-305.   10.2973/dsdp.proc.91.104.1987   AbstractWebsite

Seismic-reflection data collected for the site survey of Leg 91 of the Deep Sea Drilling Project (DSDP) are described. Correlations of these reflection data with physical-properties measurements from the recovered cores are discussed. The predicted "reverberant layer" (Houtz and Ludwig, 1979) was not found. Synthetic seismograms indicate the "reverberant layer's" presence in earlier reflection data is due to narrow-band analog recording techniques.

Blackman, DK, Nishimura CE, Orcutt JA.  2000.  Seismoacoustic recordings of a spreading episode an the Mohns Ridge. Journal of Geophysical Research-Solid Earth. 105:10961-10973.   10.1029/2000jb900011   AbstractWebsite

A period of very active seismicity near 72.7 degrees N, 4 degrees E marks an episode of seafloor spreading on the Mohns Ridge. The earthquakes were recorded from November 1995 to January 1996 by onshore seismic stations and by U.S. Navy hydrophone arrays in the North Atlantic. Both the temporal and spatial histories of the activity suggest that volcanism accompanied the tectonic events. The hydrophone arrays recorded 2-3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day observed during the most intense phase of activity. A level of 50-200 events per day was sustained throughout the episode. Initial locations of the events were obtained from the seismic bulletin. Further refinement of the epicenters was possible using P, S (converted to an acoustic phase at the seafloor), and T waves in the hydrophone data, Analysis of arrival time differences between these phases indicates that one main area and two subsidiary areas along the rift were active during the swarm. A few events occurred at a more distant location. The activity tends to concentrate in one area or another for short periods (a few days), but at times it is clear that events occur simultaneously at more than one location. We have not found evidence of steady migration of activity, such as might accompany propagation of a magma-filled dike. We thus infer that despite the 50-70 km length of ridge involved in the spreading episode, rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas.

Tolstoy, M, Constable S, Orcutt J, Staudigel H, Wyatt FK, Anderson G.  1998.  Short and long baseline tiltmeter measurements on axial seamount, Juan de Fuca Ridge. Physics of the Earth and Planetary Interiors. 108:129-141.   10.1016/s0031-9201(98)00091-0   AbstractWebsite

Long-term observations of seismic activity and ground deformation at mid-ocean ridges and submarine volcanoes are required for an understanding of the spatial and temporal characteristics of magma transport and intrusion. To make precise records of tilt on the seafloor we have installed short baseline tiltmeters in six ocean bottom seismometers (TILT-OBS) and developed a long baseline (100-500 m) two-fluid tiltmeter (LBT). In the TILT-OBS, the seismometer platform is levelled to better than 1 degrees after deployment. The tiltmeter consists of a pair of electrolytic bubble sensors mounted on a secondary levelling stage on the seismometer platform. The levelling stage uses two motor-driven micrometers on a triangular mounting plate to bring the sensors to null. The sensitivity of these tiltmeters is 0.05 mu rad, at a dynamic range of 0.2 mrad. A long baseline instrument was developed to achieve a better spatial average of deformation. Most approaches used on land to measure stable long baseline tilt cannot be applied to a submarine instrument, but tiltmeters in which the pressure of a fluid in tubes is measured are amenable to installation on the seafloor. The development resulted in a device that is essentially a center-pressure instrument folded back on itself, with fluids of different densities in the two tubes. During July to September 1994, these instruments were deployed on Axial Seamount, on the Juan de Fuca Ridge off Washington state, for a test of their relative performance on volcanic terrain, yielding 9 weeks of continuous data (seismic, tilt, and temperature) from five TILT-OBS and one long baseline instrument. Drift on all instruments was of the order of 1 mu rad/day, with higher frequency variations of order 5-10 mu rad. Initial drift on the TILT-OBS is shown to be associated with platform settling rather than with the sensor or its mounting. High frequency noise is coherent across instruments and tidal in character, and we conclude that tidal currents moving the sensors are responsible. (C) 1998 Elsevier Science B.V. All rights reserved.

Shearer, PM, Adair RG, Orcutt JA, Jordan TH.  1987.  Simultaneous Borehole and Ocean Bottom Seismometer Recordings of Earthquakes and Explosions - Results from the 1983 Ngendei Experiment at Deep-Sea Drilling Project Hole 595-B. Initial Reports of the Deep Sea Drilling Project. 91:377-384.   10.2973/dsdp.proc.91.109.1987   AbstractWebsite

We use data from the 1983 Ngendei Seismic Experiment in the southwest Pacific to compare vertical component seismograms recorded by a borehole seismometer with those from ocean bottom seismometers. The borehole seismometer, the Marine Seismic System (MSS), was emplaced 54 m into the oceanic crust at the Ngendei site. Ocean bottom seismometers (OBSs) from the Scripps Institution of Oceanography were located atop 70 m of sediment and all were within 0.5 km of the borehole. Numerous seismic refraction shots were recorded simultaneously by both the borehole and ocean bottom instruments, as well as a small number of regional earthquakes. The waveforms and instrument-corrected spectra for compressional waves from these events are nearly identical for the two different instruments, while the absolute amplitudes of the arrivals differ by no more than 4 dB. Since borehole noise levels are 10 to 15 dB lower than theseafloor levels, there is a general increase in the signal-to-noise ratio for the buried instrument. The similarity in absolute amplitudes for these observations agrees with results obtained from simple synthetic seismogram calculations.

Stark, PB, Parker RL, Masters G, Orcutt JA.  1986.  Strict bounds on seismic velocity in the spherical earth. Journal of Geophysical Research-Solid Earth and Planets. 91:13892-13902.   10.1029/JB091iB14p13892   AbstractWebsite

We address the inverse problem of finding the smallest envelope containing all velocity profiles consistent with a finite set of imprecise τ(p) data from a spherical earth. Traditionally, the problem has been attacked after mapping the data relations into relations on an equivalent flat earth. Of the two contemporary direct methods for finding bounds on velocities in the flat earth consistent with uncertain τ(p) data, a nonlinear (NL) approach descended from the Herglotz-Wiechert inversion and a linear programming (LP) approach, only NL has been used to solve the spherical earth problem. On the basis of the finite collection of τ(p) measurements alone, NL produces an envelope that is too narrow: there are numerous physically acceptable models that satisfy the data and violate the NL bounds, primarily because the NL method requires continuous functions as bounds on τ(p) and thus data must be fabricated between measured values by some sort of interpolation. We use the alternative LP approach, which does not require interpolation, to place optimal bounds on the velocity in the core. The resulting velocity corridor is disappointingly wide, and we therefore seek reasonable physical assumptions about the earth to reduce the range of permissible models. We argue from thermodynamic relations that P wave velocity decreases with distance from the earth's center within the outer core and quite probably within the inner core and lower mantle. We also show that the second derivative of velocity with respect to radius is probably not positive in the core. The first radial derivative constraint is readily incorporated into LP. The second derivative constraint is nonlinear and cannot be implemented exactly with LP; however, geometrical arguments enable us to apply a weak form of the constraint without any additional computation. LP inversions of core τ(p) data using the first radial derivative constraint give new, extremely tight bounds on the P wave velocity in the core. The weak second derivative constraint improves them slightly.

Vera, EE, Mutter JC, Buhl P, Orcutt JA, Harding AJ, Kappus ME, Detrick RS, Brocher TM.  1990.  The Structure of 0.2-MY Old Oceanic Crust at 9-Degrees-N on the East Pacific Rise from Expanded Spread Profiles. Journal of Geophysical Research-Solid Earth and Planets. 95:15529-15556.   10.1029/JB095iB10p15529   AbstractWebsite

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0–0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity-depth models for these profiles have been obtained by travel time inversion in the τ-p domain, and by x-t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100–200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ∼500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ∼500 m of the crust. The layer 2–3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on-axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ∼6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1-km-thick Moho transition zone.

Orcutt, JA.  1987.  Structure of the Earth - Oceanic-Crust and Uppermost Mantle. Reviews of Geophysics. 25:1177-1196.   10.1029/RG025i006p01177   AbstractWebsite

The past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere. The application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere. Borehole seismic instrumentation has become more widely employed; ocean bottom seismographs, while generally decreasing in number, have become significantly more reliable and useful; and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes. The techniques available for the analysis of seismic data have become more sophisticated. Waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models, and travel time data are regularly inverted directly for structure. Trial and error modeling has become increasingly unimportant. Marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere. This has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves.

Harding, AJ, Orcutt JA, Kappus ME, Vera EE, Mutter JC, Buhl P, Detrick RS, Brocher TM.  1989.  Structure of Young Oceanic-Crust at 13-Degrees-N on the East Pacific Rise from Expanding Spread Profiles. Journal of Geophysical Research-Solid Earth and Planets. 94:12163-12196.   10.1029/JB094iB09p12163   AbstractWebsite

We present the results of the analysis of expanding spread profiles (ESPs) collected on and near the axis of the East Pacific Rise at 13°N. These profiles were collected at 0, 1.1, 2.1, 3.6, and 9.5 km from the rise axis, and all but the most distant profile show a distinct low-velocity zone (LVZ) located within layer 3 of the oceanic crust. At the ridge crest, the top of the magma chamber is at the base of layer 2, while 3.6 km off axis, the roof of the LVZ is 1.1 km below the top of layer 3. The profile farthest from the ridge could possibly have a residual LVZ confined to the lower 1.5 km of the crust. The total width of the LVZ, as determined from the ESP data, is at least 6 km, and possibly much greater. This wide LVZ apparently contradicts multichannel seismic data which show cross-axis reflections from the magma chamber with a width of <5 km. We suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt. The ESP data are sensitive to this larger region, while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast. We identify the presence of a layer at the top of the oceanic crust with initial P wave velocities between 2.35 and 2.6 km/s, while the S wave velocity is estimated as being ≤0.8 km/s. The layer thickness lies between 100 and 200 m. These velocities are consistent with previous estimates for the Pacific and recent results for the Atlantic. The thickness of this layer is consistent with that of layer 2A determined from geophysical measurements at Deep Sea Drilling Project hole 504B.

Tong, CH, Lana C, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2005.  Subsurface tectonic structure between overlapping mid-ocean ridge segments. Geology. 33:409-412.   10.1130/g21245.1   AbstractWebsite

Our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation. The significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level. The overlap region between mid-ocean ridge segments at 9 degrees N on the East Pacific Rise is characterized by aligned cracks that are structurally more closely related to the propagating-ridge segment. These aligned cracks, which are approximately parallel to the ridge segments, provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers, especially those with overlap basins covered by volcanic edifices. The aligned cracks of the 9 degrees 03'N overlapping spreading center are more similar to the ridge-parallel lineaments observed between overlapping axial-summit collapse troughs than those found at larger overlapping spreading centers. Our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related.

Kent, GM, Kim II, Harding AJ, Detrick RS, Orcutt JA.  1996.  Suppression of sea-floor scattered energy using a dip moveout approach - Application to the mid-ocean ridge environment. Geophysics. 61:821-834.   10.1190/1.1444007   AbstractWebsite

Multichannel seismic (MCS) images are often contaminated with in- and out-of-plane scattering from the sea floor. This problem is especially acute in the midocean ridge environment where sea-floor roughness is pronounced. Energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as Moho, and scattering off of elongated sea-floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections. Moreover, stacking at normal subsurface velocities may enhance these water-borne events, whose stacking velocity depends on azimuth and generally increases with time, making them indistinguishable from subsurface arrivals. To suppress scattered energy in deep water settings, we propose a processing scheme that invokes the application of dip moveout (DMO) to deliberately increase the differential moveout between sea-floor-scattered and subsurface events, thereby facilitating the removal of unwanted energy in the stacked section. After application of DMO, all sea-floor scatterers stack at the water velocity, while subsurface reflections like Moho still stack at their original velocity. The application of DMO in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used. Unlike previous approaches to suppress scattered energy, dip filtering is applied in the common-midpoint (CMP) domain after DMO. Moreover, our DMO-based approach suppresses out-of-plane scattering, and therefore is not limited to removal of in-plane scattering as is the case with shot and receiver dip filtering techniques. The success of our DMO-based suppression scheme is limited to deep water (a few kilometers of water depth for conventional offsets), where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle. The application of DMO-based suppression to synthetics and MCS data collected along the southern East Pacific Rise demonstrates the effectiveness of our approach. Cleaner images of primary reflectors such as Moho are produced, even though present shot coverage along the East Pacific Rise is unduly sparse, resulting in a limited effective spatial bandwidth.

Shearer, PM, Orcutt JA.  1987.  Surface and Near-Surface Effects on Seismic-Waves - Theory and Borehole Seismometer Results. Bulletin of the Seismological Society of America. 77:1168-1196. AbstractWebsite

A simple plane wave model is adequate to explain many surface versus borehole seismometer data sets. Using such a model, we present a series of examples which demonstrate the effects of the free-surface, near-surface velocity gradients, and low impedance surface layers on the amplitudes of upcoming body waves. In some cases, these amplitudes are predictable from simple free-surface and impedance contrast expressions. However, in other cases these expressions are an unreliable guide to the complete response, and the full plane wave calculation must be performed. Large surface amplifications are possible, even without focusing due to lateral heterogeneities or nonlinear effects. Both surface and borehole seismometer site responses are almost always frequency-dependent.Ocean bottom versus borehole seismic data from the 1983 Ngendei Seismic Experiment in the southwest Pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation. The borehole seismic response to upcoming P waves is reduced at high frequencies because of interference between the upgoing P wave and downgoing P and SV waves reflected from the sediment-basement interface. However, because of much lower borehole noise levels, the borehole seismometer enjoys a P-wave signal-to-noise advantage of 3 to 7 dB over nearby ocean bottom instruments.

National Academies of Sciences, Engineering, Medicine.  2017.  Sustaining ocean observations to understand future changes in earth’s climate. :150., Washington, DC: The National Academies Press   10.17226/24919   Abstract

The ocean is an integral component of the Earth’s climate system. It covers about 70% of the Earth’s surface and acts as its primary reservoir of heat and carbon, absorbing over 90% of the surplus heat and about 30% of the carbon dioxide associated with human activities, and receiving close to 100% of fresh water lost from land ice. With the accumulation of greenhouse gases in the atmosphere, notably carbon dioxide from fossil fuel combustion, the Earth’s climate is now changing more rapidly than at any time since the advent of human societies. Society will increasingly face complex decisions about how to mitigate the adverse impacts of climate change such as droughts, sea-level rise, ocean acidification, species loss, changes to growing seasons, and stronger and possibly more frequent storms. Observations play a foundational role in documenting the state and variability of components of the climate system and facilitating climate prediction and scenario development. Regular and consistent collection of ocean observations over decades to centuries would monitor the Earth’s main reservoirs of heat, carbon dioxide, and water and provides a critical record of long-term change and variability over multiple time scales. Sustained high-quality observations are also needed to test and improve climate models, which provide insights into the future climate system. Sustaining Ocean Observations to Understand Future Changes in Earth’s Climate considers processes for identifying priority ocean observations that will improve understanding of the Earth’s climate processes, and the challenges associated with sustaining these observations over long timeframes.

De Groot-Hedlin, CD, Orcutt JA.  1999.  Synthesis of earthquake-generated T-waves. Geophysical Research Letters. 26:1227-1230.   10.1029/1999gl900205   AbstractWebsite

T-phases excited by earthquakes propagate with low transmission loss along the ocean sound channel, and are characterized by long duration wavetrains, often having multiple peaks associated with bathymetric highs in the source region. Although T-waves were first identified almost fifty years ago, their activation has not been adequately explained. In this paper we show that, for continental margin earthquakes, realistic T-phase coda can be computed by expressing the acoustic energy in terms of several low order modes excited by point sources distributed uniformly over the seafloor. The conversion of seismic energy to acoustic energy is thus consistent with seafloor scattering.

Sereno, T, Orcutt J.  1985.  Synthesis of Realistic Oceanic Pn Wave-Trains. Journal of Geophysical Research-Solid Earth and Planets. 90:2755-2776. AbstractWebsite
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Sereno, TJ, Orcutt JA.  1987.  Synthetic Pn and Sn Phases and the Frequency-Dependence of Q of Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 92:3541-3566.   10.1029/JB092iB05p03541   AbstractWebsite

The oceanic lithosphere is an extremely efficient waveguide for high-frequency seismic energy. In particular, the propagation of the regional to teleseismic oceanic Pn and Sn phases is largely controlled by properties of the oceanic plates. The shallow velocity gradient in the sub-Moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle. The confinement of Pn/Sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed. The rapid disappearance of Sn and the discontinuous drop in Pn/Sn group velocity beyond a critical distance, dependent upon the local thickness of the lithosphere, are interpreted as a shadowing effect of the low Q asthenosphere. Wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere. The results were compared to data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific. The Pn/Sn coda is successfully modeled as a sum of leaky organ-pipe modes in the sediment layer and oceanic water column. While scattering is present to some degree, it is not required to explain the long duration and complicated nature of the Pn/Sn wave trains. The presence of extremely high frequencies in Pn/Sn phases and the greater efficiency of Sn than Pn propagation are interpreted in terms of an absorption band rheology. A shorter high-frequency relaxation time for P waves than for S waves results in a rheology with the property that Qα > Qβ at low frequency while Qβ > Qα at high frequency, consistent with the teleseismic Pn/Sn observations. The absorption band model is to viewed as only an approximation to the true frequency dependence of Q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed.

Sereno, TJ, Orcutt JA.  1985.  Synthetic Seismogram Modeling of the Oceanic-Pn Phase. Nature. 316:246-248.   10.1038/316246a0   AbstractWebsite

The seismic oceanic Pn phase is characterized by low spatial attenuation, a high-frequency wavetrain of extremely long duration and an onset velocity near 8.0 km s−1. In an effort to explain these properties, models have been proposed for the mode of propagation of this phase which include energy entrapment within a low-velocity channel1,2, propagation within an extremely low-attenuation lithosphere waveguide3,4 and forward-scattering from local heterogeneity within the oceanic crust5,6. Through synthetic seismogram modelling, we have generated realistic oceanic Pn phases for an oceanic lithosphere model that did not contain any of these intrinsic properties. The synthetic Pn phase can be explained rather simply as a set of refractions from the lower lithosphre with subsequent reverberations within the oceanic water column and sediment layer. Many of the spectral properties associated with layer reverberation observed in the synthetic Pn phase are revealed in data collected in the south-west Pacific. Complicated oceanic lithosphere models are not required to explain the gross characteristics of Pn propagation.