Reply by the authors to the discussion by A. J. Calvert

Geophysics ◽  
1998 ◽  
Vol 63 (1) ◽  
pp. 318-318
Author(s):  
A. J. Calvert
Keyword(s):  
Sea Floor ◽  
Scattered Energy ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

The authors of “Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment” chose not to reply to the discussion by.

On: “Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment” by G. M. Kent, I. I. Kim, A. J. Harding, R. S. Detrick, and J. A. Orcutt (May‐June 1996 GEOPHYSICS, 61, 821–834)

Geophysics ◽  
1998 ◽  
Vol 63 (1) ◽  
pp. 316-318
Author(s):  
A. J. Calvert
Keyword(s):  
Sea Floor ◽  
Counter Example ◽  
Scattered Energy ◽  
Mid Ocean Ridge ◽  
Out Of Plane ◽  
History Of ◽  
The Common ◽  
Velocity Filtering ◽  
Ocean Ridge ◽  
Water Depths

In their paper, Kent et al. (1996) present an excellent case history of the use of dip moveout (DMO) and velocity‐filtering in the common midpoint (CMP) domain for the suppression of out‐of‐plane arrivals scattered from a deep sea‐floor. However, they imply that as a result of a “small offset approximation” the use of DMO in this way is limited to surveys recorded in water depths of at least a few kilometers with conventional streamer offsets. This is incorrect. I argue here that the application of DMO will reduce to water velocity the stacking velocity of arrivals scattered from the upper surface of the seafloor without any restriction on water depth. Furthermore, I argue that this use of DMO is simply an example of the equivalence between 2-D and 3-D DMO for marine surveys where all source‐receiver azimuths are equal, and that no “small offset approximation” is required. I first present a counter‐example to the claim of Kent et al. (1996) that seafloor scattering cannot be suppressed using DMO in shallow water, and then consider in more detail their argument for the application of DMO to out‐of‐plane scattering. In the discussion that follows, I only consider DMO in the context of a constant velocity medium.

Suppression of sea‐floor‐scattered energy using a dip‐moveout approach—Application to the mid‐ocean ridge environment

Geophysics ◽  
1996 ◽  
Vol 61 (3) ◽  
pp. 821-834 ◽  
Author(s):  
Graham M. Kent ◽  
Isaac I. Kim ◽  
Alistair J. Harding ◽  
Robert S. Detrick ◽  
John A. Orcutt
Keyword(s):  
Deep Water ◽  
East Pacific Rise ◽  
Processing Scheme ◽  
Sea Floor ◽  
East Pacific ◽  
Scattered Energy ◽  
Mid Ocean Ridge ◽  
Out Of Plane ◽  
Dip Angle ◽  
Ocean Ridge

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 mid‐ocean 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. Moveover, 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.

A near-ridge origin for seamounts at the southern terminus of the Pratt-Welker Seamount Chain, northeast Pacific Ocean

1999 ◽  
Vol 36 (6) ◽  
pp. 1021-1031 ◽  
Author(s):  
Brian Cousens ◽  
Jarda Dostal ◽  
T S Hamilton
Keyword(s):  
Gulf Of Alaska ◽  
Trace Element Composition ◽  
Primitive Mantle ◽  
Sea Floor ◽  
Northeast Pacific ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Juan De Fuca ◽  
Seamount Chain ◽  
Ocean Ridge

Three seamounts close to the south end of the Pratt-Welker Seamount Chain, Gulf of Alaska, have been sampled to test whether or not mantle plume-related volcanism extends south of Bowie Seamount. Lavas recovered from Oshawa, Drifters, and Graham seamounts are weathered, Mn-encrusted pillow lavas and sheet-flow fragments, commonly with glassy rims. The glasses and holocrystalline rocks are tholeiitic basalts, with light rare earth element depleted to flat primitive mantle normalized incompatible element patterns and radiogenic isotope compositions within the ranges of mid-ocean ridge and near-ridge seamount basalts from the Explorer and northern Juan de Fuca ridges. Chemically, the seamount lavas strongly resemble older, "shield-phase" tholeiitic rocks dredged from the flanks of southern Pratt-Welker seamounts, but are distinct from the younger alkaline intraplate lavas that cap Pratt-Welker edifices. The weathered, encrusted basalts were most likely erupted in a near-ridge environment, adjacent to Explorer Ridge, between 11 and 14 Ma. No evidence of plume-related activity is found in this area. Compared with northeast Pacific mid-ocean ridge and alkaline intraplate basalts, Graham seamount lavas have anomalously high 206Pb/204Pb, which does not appear to be a function of sea-floor alteration, magma contamination, or mixing between previously identified mantle components. All near-ridge seamounts in the northeast Pacific exhibit isotopic heterogeneity that does not correlate with major or trace element composition, suggesting that the mantle sources of all near-ridge seamounts have been variably depleted by prior, but recent melting events.

PETROGENESIS OF GABBRO FROM ARCTIC GAKKEL MID-OCEAN RIDGE

2019 ◽  
Author(s):  
Yung Ping Lee ◽  
◽  
Jonathan E. Snow ◽  
Yongjun Gao
Keyword(s):  
Mid Ocean Ridge ◽  
Ocean Ridge

scholarly journals Melt addition to mid-ocean ridge peridotites increases spinel Cr# with no significant effect on recorded oxygen fugacity

2021 ◽  
Vol 566 ◽  
pp. 116951
Author(s):  
Suzanne K. Birner ◽  
Elizabeth Cottrell ◽  
Jessica M. Warren ◽  
Katherine A. Kelley ◽  
Fred A. Davis
Keyword(s):  
Oxygen Fugacity ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

scholarly journals Sunda arc mantle source δ18O value revealed by intracrystal isotope analysis

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Frances M. Deegan ◽  
Martin J. Whitehouse ◽  
Valentin R. Troll ◽  
Harri Geiger ◽  
Heejin Jeon ◽  
...  
Keyword(s):  
Mantle Source ◽  
Secondary Ion Mass Spectrometry ◽  
Isotope Composition ◽  
Isotope Analysis ◽  
Sunda Arc ◽  
Mid Ocean Ridge ◽  
Plumbing Systems ◽  
Ocean Ridge ◽  
Secondary Ion ◽  
Magma Plumbing

AbstractMagma plumbing systems underlying subduction zone volcanoes extend from the mantle through the overlying crust and facilitate protracted fractional crystallisation, assimilation, and mixing, which frequently obscures a clear view of mantle source compositions. In order to see through this crustal noise, we present intracrystal Secondary Ion Mass Spectrometry (SIMS) δ18O values in clinopyroxene from Merapi, Kelut, Batur, and Agung volcanoes in the Sunda arc, Indonesia, under which the thickness of the crust decreases from ca. 30 km at Merapi to ≤20 km at Agung. Here we show that mean clinopyroxene δ18O values decrease concomitantly with crustal thickness and that lavas from Agung possess mantle-like He-Sr-Nd-Pb isotope ratios and clinopyroxene mean equilibrium melt δ18O values of 5.7 ‰ (±0.2 1 SD) indistinguishable from the δ18O range for Mid Ocean Ridge Basalt (MORB). The oxygen isotope composition of the mantle underlying the East Sunda Arc is therefore largely unaffected by subduction-driven metasomatism and may thus represent a sediment-poor arc end-member.

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