A search for layering in the oceanic crust

Collins, J. A.

doi:10.1575/1912/5453

Cited by 1 publication

(4 citation statements)

References 46 publications

(90 reference statements)

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“…17) suggests that the depth to the extrusives/sheeted dike transition at Hole 504B might be more readily mapped with the wideangle reflection/refraction technique than with conventional MCS techniques. Wide-angle reflection/refraction data acquired at Hole 504B show amplitude focusing at ranges of 5-7 km (Collins, 1989), as predicted by the synthetic seismogram modeling. The horizontal range of this amplitude high has been shown to be a sensitive indicator of the depth to the Layer 2/ Layer 3 velocity transition (Bratt and Purdy, 1984).…”

Section: Wide-aperture Synthetic Cmp Gatherssupporting

confidence: 62%

“…Consequently, the dike succession at Hole 504B is interpreted as a sheeted dike sequence. Velocity-depth profiles determined from wide-angle reflection/refraction experiments (Little and Stephen, 1985;Collins, 1989) suggest that the current bottom of the drill hole is near the Layer 2/Layer 3 transition (Fig. 2B).…”

Section: Study Areamentioning

confidence: 99%

“…B. The observed sonic velocity (model OBS) profile for Hole 504B plotted alongside the preferred velocity profiles of Little and Stephen (1985) (dashed line) and Collins (1989) (heavy solid line), based on borehole and sonobuoy seismic-refraction data, respectively.…”

Section: Mcs Datamentioning

confidence: 99%

“…Parts of lines 490 and 485, stacked with depth-varying velocities, are shown in Figures 7 and 8. Stacking velocities were calculated from the velocity-depth model derived from analysis of wide-angle reflection/refraction data collected at the drill site (Collins, 1989). This velocity-depth model predicts that a stacking velocity of about 1.9 km/s is appropriate for a reflection event with an intracrustal traveltime of 0.25 s. Both profiles show a reflection event with an intracrustal traveltime of 0.25-0.3 s. This event is the same as the event identified in the constant-velocity stacked section (Fig.…”

Section: Mcs Datamentioning

confidence: 99%

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Seismic-Reflection Structure of the Upper Oceanic Crust: Implications from DSDP/ODP Hole 504B, Panama Basin

Collins¹,

Brocher²,

Purdy³

1989

Proceedings of the Ocean Drilling Program

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We investigate the seismic reflectivity structure of the upper oceanic crust by comparing multichannel seismic (MCS) reflection data collected at DSDP/ODP Hole 504B to the results of downhole logging. At this site drilling shows a welldefined change in physical properties at depths within the basement of about 0.5-0.6 km, corresponding to the downward transition from volcanic rocks to dikes. Extensive processing of the MCS data, required to remove high-amplitude side-scattered arrivals, revealed no conclusive evidence for laterally coherent reflection events generated within the upper 1-2 km of the crust. The crustal traveltime to the volcanic/dike boundary (about 0.25 s) is similar to the traveltimes of shallow reflection events observed in other areas. In an attempt to understand the lack of reflectivity from the volcanic/ dike boundary at Hole 504B, we calculated synthetic reflection seismograms for a series of velocity-depth profiles constructed from the logged downhole variations in physical properties. These seismograms were calculated with the source signature of the 1785-in. 3 (29.3-L) air gun array used to acquire the MCS data. The synthetic seismograms demonstrate that the reflections from the shallow crust are low in amplitude and may be obscured by source reverberation and by sediment-column multiples. Nonetheless, the limited available data also suggest that the upper crustal structure at Hole 504B may in fact differ from that at other crustal sites where high-amplitude reflections from within the shallow crust have been observed.

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Section: Wide-aperture Synthetic Cmp Gatherssupporting

confidence: 62%

Section: Study Areamentioning

confidence: 99%