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Newman, HB, Ellisman MH, Orcutt JA.  2003.  Data-intensive e-science - Frontier research. Communications of the ACM. 46:67-75. AbstractWebsite
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Tolstoy, M, Harding AJ, Orcutt JA, Detrick RS, Kent GM, Mutter JC, Buhl P.  1997.  Deepening of the axial magma chamber on the southern East Pacific Rise toward the Garrett Fracture Zone. Journal of Geophysical Research-Solid Earth. 102:3097-3108.   10.1029/96jb03226   AbstractWebsite

A wide-aperture profile along the ridge axis from 14 degrees 29'S to 13 degrees 39'S, 120 km to 30 km south of the Garrett Fracture Zone, is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector. Five areas along the 90 km line are examined in detail, with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area. A genetic algorithm code is used to find a best fit model from a comparison of the data and WKBJ synthetic seismograms. One hundred starting models are generated using a predefined set of velocity nodes, with a fixed window of allowable depth variations between nodes. An evolutionary process favors the better fitting models in each generation, and a satisfactory misfit is usually obtained within 40 generations. Within individual areas the models were in good agreement with the depth of a given velocity node, generally varying by not more than 20 m, the depth discretization interval for the models. A consistent deepening trend of the axial magma chamber (AMC) is observed across the five areas as the Garrett Fracture Zone is approached. The depth varies from 0.99 km at area 1, which is approximately 100 km south of the Garrett, to 1.23 km at area 5, which is approximately 40 km south of the Garrett. The depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age. For the areas examined here, layer 2a is observed to be relatively constant in thickness along the axis, although it is about 40 m thicker over area 5, where the axial magma chamber is deepest. This variation is within the scatter of previously detailed layer 2a measurements at 13 degrees N on the East Pacific Rise, where an effectively constant thickness is observed. This implies that layer 2a thickening is not a significant factor along this profile and that the AMC deepening is rear rather than apparent. Theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location. Thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the Garrett Fracture Zone, which marks a major interruption of hundreds of kilometers of continuous ridge axis. The deepening may also be interpreted as: a downward limb from a central injection point; however, there is no indication of a similar downward trend in the other (southern) direction. Furthermore, there is no accompanying systematic variation in axial depth or axial volume, both of which are proposed to be indicators of central injection and along-axis flow.

Adair, RG, Harris MM, Orcutt JA, Jordan TH.  1987.  Description and Performance of the Marine Seismic System During the Ngendei Experiment. Initial Reports of the Deep Sea Drilling Project. 91:335-345.   10.2973/dsdp.proc.91.106.1987   AbstractWebsite

The Marine Seismic System (MSS) and its performance during the Ngendei Experiment are described. The MSS is a digital, triaxial submarine borehole seismograph that comprises a borehole sensor package connected by coaxial cable to an ocean-bottom recording unit. Two shipboard recording systems provide redundant data logging and instrumentation monitoring capability. One of the shipboard consoles, the Teledyne system, monitored the borehole package. The other, the Gould system, emulated the functions of the ocean-bottom recorder. A submerged mooring facilitates the installation and recovery of the ocean-bottom recorder. The borehole package is emplaced by means of a special carriage tool employed at the bottom end of the drill string. The package rested undamped at the bottom of DSDP Hole 595B, 54 m within basement rock overlain by 70 m of sediments. The hole-lock and cable-isolating mechanisms of the borehole package were disconnected after they were found to damage the coaxial cable. One of the horizontal seismometers was apparently damaged and consequently yielded no usable data. Totals of 120, 83, and 43 hours of data were collected on the Teledyne and Gould shipboard recorders and on the ocean-bottom recorder. The ocean-bottom recorder failed after approximately 2 days of operation because of a water leak in one of its two battery spheres.

Orcutt, JA, Moore RD, Jordan TH.  1987.  Description and Performance of the Scripps Ocean Bottom Seismographs During the NGENDEI Experiment. Initial Reports of the Deep Sea Drilling Project. 91:347-356.   10.2973/dsdp.proc.91.107.1987   AbstractWebsite

Six ocean bottom seismographs from the Scripps Institution of Oceanography were used in the Ngendei experiment in conjunction with the Marine Seismic System. The systems were modified to meet the requirements of this experiment. Significant buffer memory was added to several of the instruments to eliminate the effects of a rotating tape recorder housed in the same capsule as the inertial sensors. In addition, the response of the instruments was modified for the triggered portion of the experiment to improve the likelihood of detecting low frequency teleseismic signals. Finally, the software used by the seismographs central processing unit was modified to allow simple, but important, changes in the operating characteristics immediately before launch. Each instrument was deployed several times and all capsules were recovered. Several deployments were found to be unsuccessful through acoustic diagnostics and several capsules returned to the surface shortly after launch because of small leakages revealed by the leak detectors. Nevertheless, all deployments following the correction of these faults were successful in returning data. This chapter outlines the operating characteristics of each instrument, describes the instrument response functions, lists the locations of the seismographson the seafloor, and enumerates the events recorded by each of the capsules.

Kent, GM, Harding AJ, Orcutt JA.  1993.  Distribution of magma beneath the East Pacific rise between the Clipperton transform and the 9-degrees-17' n deval from forward modeling of common depth point data. Journal of Geophysical Research-Solid Earth. 98:13945-13969.   10.1029/93jb00705   AbstractWebsite

We have reprocessed seven cross-axis common depth point (CDP) profiles between the Clipperton transform and the 9-degrees-17'N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross-axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9-degrees-53'N-9-degrees-35'N Devals, the 9-degrees-35'N-9-degrees-17'N Devals, and south of the 9-degrees-17'N Deval were < 0.7 km, 1.0-1.2 km, and 4.15 km, respectively. Transition in AMC width across these discontinuities is unclear due to the sparseness of cross-axis line spacing; however, a simple association of Devals with decreased magma supply is doubtful: the minimum (250 m) and maximum (4150 m) AMC widths are found near the 9-degrees-35'N and 9-degrees-17'N Devals, respectively. The reprocessing of CDP profiles included repicking stacking velocities to ensure a proper stack of the AMC reflector and its associated diffractions, imaging postcritical reflections from the base of layer 2A, finite difference time migration, ray theoretical depth migration, and travel time modeling of AMC diffraction patterns. Constraints on AMC width were derived from forward modeling techniques based on analytic raytracing. Velocity models were constructed from SeaBeam bathymetry and modified expanding spread profile (ESP) velocity-depth functions. ESP velocity models were altered to compensate for off-axis thickening of layer 2A as imaged in the CDP reflection data. Continuous two-dimensional velocity models constructed from modified ESP velocity-depth functions and SeaBeam bathymetry should account for ray bending at the seafloor/basalt interface and any lateral velocity gradients induced by a thickening layer 2A. Stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration. Reflector positions were input into our forward modeling scheme to produce a zero-offset travel time response of our migrated solution. The travel time response was then overlain on the stacked section to ensure an adequate match, especially to diffractions generated at the AMC edge. Forward modeling of AMC diffraction patterns reveals that original AMC width estimates were too large. The under-migration of the AMC reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual CMP gathers, thus resulting in improperly collapsed diffraction hyperbolae. Ship wandering relative to the AMC edge can account for variations in AMC reflector amplitude and dropout on the along-axis CDP line. The continuity of the AMC appears unbroken across several ridge axis discontinuities between the Clipperton transform and the 9-degrees-17'N Deval which suggests an AMC whose along-axis dimension exceeds 75 km. Reflectivity modeling of CMP gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush. Such a thin layer of melt might inhibit along-axis mixing of magmas and thereby account for variations in magma composition along the rise crest. This melt lens model for the AMC would also produce strong diffraction patterns as imaged in the CDP data.

Kent, GM, Harding AJ, Orcutt JA.  1993.  Distribution of magma beneath the East Pacific rise near the 9-degrees-03' n overlapping spreading center from forward modeling of common depth point data. Journal of Geophysical Research-Solid Earth. 98:13971-13995.   10.1029/93jb00706   AbstractWebsite

We have reprocessed six cross-axis and three along-axis common depth point (CDP) profiles near the 9-degrees-03'N overlapping spreading center (OSC) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Travel time modeling of the AMC reflector reveals an asymmetric distribution of melt across the 9-degrees-03'N OSC. The variation of modeled AMC width beneath either OSC limb is minimal, but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4.15 km near the 9-degrees-17'N ridge axis discontinuity. Additionally, melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis/neovolcanic zone but is displaced toward the western rise flank. Depth migration, based on a continuum velocity model consistent with postcritical reflections from the base of layer 2A, places the skewed AMC reflector beneath a nearly constant thickness sheeted dike section which dips approximately 10-degrees away from the rise axis. To confirm AMC continuity beneath the western rise flank, we use the Maslov synthetic seismogram method to show that amplitude enhancement of the AMC reflector is consistent with a continuous melt body underlying a thickening extrusive layer. Analysis of along-strike CDP profiles indicates an AMC which is neither overlapping nor discontinuous when projected onto the along-strike plane. Identifying intracrustal events on along-axis CDP lines. however, requires extreme caution; we have modeled out-of-plane scattering using a Kirchhoff formulation, and we show that a coherent event identified beneath the overlap basin results from diffraction off the AMC which lies nearly 3 km to the west of the profile. We attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust In this model, melt ascends upward (buoyancy forces) until deflected by the impermeable sheeted dike complex; melt then migrates updip, beneath the base of the sheeted dikes, toward the neovolcanic zone where fissuring produces a temporary conduit for emplacement. Discrete jumps in modeled AMC width toward the overlap basin represent a further displacement/defocusing of melt supply (western AMC edge) relative to the neovolcanic zone (eastern AMC edge). The asymmetric pattern of melt therefore represents a gradual, en-echelon accommodation of melt supply across the 9 km of ridge axis offset at 9-degrees-03'N. Thus for asymmetric configurations, AMC width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone. Here we present a new model for OSC development which invokes a significant component of cross-axis melt migration. Moreover, abrupt changes in AMC width near ridge axis discontinuities (e.g., 9-degrees-17'N deviation in axial linearity) suggest that any along-axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models. The transition of melt supply beneath the overlap basin might favor a continuous low-velocity zone underlying this feature; if true, basin development may be related to the subsidence of a mechanically weak crustal lid. The proposed model for OSC development therefore views ridge axis discontinuities as the surficial response of misalignment and/or defocusing of melt supply in the uppermost mantle.