Synthetic seismograms for the case of the receiver within the reflectivity zone

Stephen, Ralph A.

doi:10.1111/j.1365-246x.1977.tb04195.x

Cited by 32 publications

(36 citation statements)

References 4 publications

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“…There are first arrivals only up to 5.0 km because Layer 2 is only 1.3 km thick. If the thickness of Layer 2 were 1.7 km (the mean thickness from Ludwig et al, 1970), the direct wave would be a first arrival to about 9.0 km (Stephen, 1977b).…”

Section: Study Of Direct Wave Arrivals and Small-scale Topographymentioning

confidence: 99%

“…In addition, the background acoustic noise on Glomar Challenger is so high that the fixed-gain preamplifier is overloaded and testing is difficult. Consequently, we adapted the Geo Space seismometer to a three-component unit with variable gain preamplifiers (Stephen, 1977a). The gain could be changed at the surface to 0, 12, 24, 36, or 48 db.…”

Section: Data Acquisition Equipmentmentioning

confidence: 99%

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The Oblique Seismic Experiment on Deep Sea Drilling Project Leg 52

Stephen¹,

Louden²,

Matthews³

1980

Initial Reports of the Deep Sea Drilling Project

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SUMMARYThe first successful Oblique Seismic Experiment (OSE) in oceanic crust was carried out in Hole 417D. The OSE had been proposed to supplement the IPOD crustal borehole as a means of investigating seismic Layer 2. Specific objectives were to determine the lateral extent of the structures intersected by the borehole, to analyze the role of large cracks in the velocity structure, to look for anisotropy which may be caused by cracks with a preferred orientation, and to measure attenuation.The best velocity model for the crust at Site 417, based on travel time and amplitude studies of the seismograms, consists of a constant velocity gradient throughout Layer 2 of 1.2 s ' and 0.8 s~1 for P andS waves, respectively. Layer 2 is 1.3 km thick. The velocities at the bottom of Layer 2 correspond to those for uncracked basalt.Anisotropy in either Layers 2 or 3 is not required by the data. Since the large fissures observed in the FAMOUS area should produce noticeable anisotropy, it appears that large fissures are not present in the studied crust (110 m.y.). The results agree with the theory that large fissures close with age and cracks close with depth. The experiment should be run again in younger crust for comparison with these results.

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Section: Study Of Direct Wave Arrivals and Small-scale Topographymentioning

confidence: 99%

Section: Data Acquisition Equipmentmentioning

confidence: 99%

The Oblique Seismic Experiment on Deep Sea Drilling Project Leg 52

Stephen¹,

Louden²,

Matthews³

1980

Initial Reports of the Deep Sea Drilling Project

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“…The amplitude analysis (Figure 8) was carried out using the reflectivity synthetic seismogram method (Fuchs and Müller, 1971;Stephen, 1977). From the results of such analyses (e.g., Figure 9), it is clear that the velocity model which best reproduces the oblique seismic experiment data for Layer 2 at Site 417 is one with a uniform increase in compressional wave velocity from 4.8 km/s at the top of the basement to 6.4 km/s at a depth of 1.3 km and a similar increase in shear wave velocity from 2.6 to 3.6 km/s over the same interval.…”

Section: Upper Levels Of Cretaceous Oceanic Crustmentioning

confidence: 99%

The Physical State of the Upper Levels of Cretaceous Oceanic Crust from the Results of Logging, Laboratory Studies, and the Oblique Seismic Experiment at Deep Sea Drilling Project Sites 417 and 418

Salisbury¹,

Stephen²,

Christensen³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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The combined results of logging, physical properties studies, and the oblique seismic experiment conducted during DSDP Legs 51 through 53 at Sites 417 and 418 in Cretaceous crust at the southern end of the Bermuda Rise make possible the first detailed evaluation of the physical state of the upper levels of old oceanic crust.From an analysis of the results of the oblique seismic experiment, it appears that the P-wave velocity increases linearly from 4.8 ±0.2 km/s at the top of Layer 2 to 6.4 ±0.2 km/s at a sub-basement depth of 1.3 km. The P-wave velocity of Layer 3 at approximately 1.5 km is 6.7 ±0.2 km/s. The 5-wave velocity is 2.6 ±0.1 km/s at the top of Layer 2 and 3.7 ±0.1 km/s at the top of Layer 3.The average value of Vp (4.8 km/s) measured by logging in the uppermost basement in Hole 417D is in excellent agreement with the value obtained from the oblique seismic experiment, but is lower than the formation velocity (5.3 to 5.6 km/s) reconstructed for the site from laboratory measurements of velocity through recovered core material. This requires that the formation contains cracks on a scale finer than the resolution of the logging and oblique seismic experiments, but greater than that of laboratory samples. On the basis of these results and petrologic constraints imposed by the core, the upper crust in Hole 417D consists of 90 per cent basalt with an average grain boundary porosity of 8 per cent, less than 1 per cent interpiUow limestone, 5 per cent smectite consisting of about 50 per cent water, and 5 per cent open cracks filled with standing water. The formation porosity thus resides in two domains, grain boundaries and open cracks, and totals 13 to 14 per cent. This value is confirmed by electrical resistivity logs which indicate, in addition, that the cracks are interconnected, giving the formation an average permeability in the thousands of darcies, with lower values in the less fractured massive basalts.Comparison of these results with logging data obtained in young crust in Hole 396B on the Mid-Atlantic Ridge indicates that, although the porosity and permeability of the upper levels of the crust at Site 417 are much lower than at the ridge crest, the formation is not entirely sealed. Although water circulation is thus still possible in old crust, it may be limited by the presence of massive basalts and the absence of shallow sources of heat.

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“…Truncated expansions have been used to study multiple reflections and to limit attention to portions of the seismic wave field by Kennett (1975Kennett ( ,1979a and Stephen (1977). For studies of complex multiples it is particularly convenient to suppress all internal reverberations in a region by using the approximation…”

Section: Interpretation Of Addition Rulesmentioning

confidence: 99%