Shallow crustal structure of the continental margin off Nova Scotia

1983 ◽  
Vol 20 (11) ◽  
pp. 1657-1672 ◽  
Author(s):  
Thomas M. Brocher
Keyword(s):  
Nova Scotia ◽  
Velocity Structure ◽  
Velocity Model ◽  
Passive Margin ◽  
Ocean Bottom ◽  
Stoneley Waves ◽  
Unconsolidated Sand ◽  
Ocean Bottom Seismometers ◽  
Well Data ◽  
Sand And Gravel

The nature of the upper sediments of the shelf and slope on a passive margin was investigated by using high-quality refraction profiles recorded by ocean-bottom seismometers off Nova Scotia. In agreement with previously published reflection profiles, well data, and lithospheric models for the evolution of passive margins, we found little thickening of the post-Early Cretaceous section, implying an even sedimentation rate over the outer shelf for this time period. The velocity model determined from slant stacks agreed reasonably well with well-log data, but had velocities slightly lower than those found from a nearby refraction line using first-arrival travel-time methods. Starting at the sea floor the compressional velocity–depth model consists of a gradient of roughly 0.4 s−1 to a depth of about 1.25 km, followed by another gradient of roughly 1 s−1 to a depth of about 3.5 km. Beneath this depth the velocity gradient approaches zero and can be modelled as a constant velocity layer. Stoneley waves were used to investigate the velocity structure of the upper 260 m of the sediment column. These velocities cannot be measured in the oil wells located on the shelf by conventional 3.5 kHz echo sounders or by measuring the sonic velocities of sediments collected in piston cores. A thinning of the Pleistocene–Holocene Sable Island Sand and Gravel layer was documented by pronounced differences in the propagation of Stoneley waves across the shelf. Although the origin of the thinning is uncertain, the shear-wave velocity determined for this unit, 260 m/s, is appropriate for an unconsolidated sand.

Comparison of the S/N ratios of low-frequency hydrophones and geophones as a function of ocean depth

1981 ◽  
Vol 71 (5) ◽  
pp. 1649-1659
Author(s):  
Thomas M. Brocher ◽  
Brian T. Iwatake ◽  
Joseph F. Gettrust ◽  
George H. Sutton ◽  
L. Neil Frazer
Keyword(s):  
Nova Scotia ◽  
Continental Margin ◽  
Low Frequency ◽  
Weather Conditions ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Scotian Shelf ◽  
Ocean Bottom Seismometers ◽  
Particle Velocities ◽  
Refraction Data

abstract The pressures and particle velocities of sediment-borne signals were recorded over a 9-day period by an array of telemetered ocean-bottom seismometers positioned on the continental margin off Nova Scotia. The telemetered ocean-bottom seismometer packages, which appear to have been very well coupled to the sediments, contained three orthogonal geophones and a hydrophone. The bandwidth of all sensors was 1 to 30 Hz. Analysis of the refraction data shows that the vertical geophones have the best S/N ratio for the sediment-borne signals at all recording depths (67, 140, and 1301 m) and nearly all ranges. The S/N ratio increases with increasing sensor depth for equivalent weather conditions. Stoneley and Love waves detected on the Scotian shelf (67-m depth) are efficient modes for the propagation of noise.

scholarly journals Travel Time Tomography to Delineate 3-D Regional Seismic Velocity Structure in the Banyumas Basin, Central Java, Indonesia, Using Dense Borehole Seismographic Stations

2021 ◽  
Vol 9 ◽  
Author(s):  
Hidayat Hidayat ◽  
Andri Dian Nugraha ◽  
Awali Priyono ◽  
Marjiyono Marjiyono ◽  
Januar H. Setiawan ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Crustal Layer ◽  
Central Java ◽  
Resolution Element ◽  
Passive Seismic ◽  
Dextral Strike Slip Fault ◽  
Southern Central ◽  
Well Data

The Banyumas Basin is a tertiary sedimentary basin located in southern Central Java, Indonesia. Due to the presence of volcanic deposits, 2-D seismic reflection methods cannot provide a good estimation of the sediment thickness and the subsurface geology structure in this area. In this study, the passive seismic tomography (PST) method was applied to image the 3-D subsurface Vp, Vs, and Vp/Vs ratio. We used 70 seismograph borehole stations with a recording duration of 177 days. A total of 354 events with 9, 370 P and 9, 368 S phases were used as input for tomographic inversion. The checkshot data of a 4, 400-meter deep exploration well (Jati-1) located within the seismic network were used to constrain the shallow crustal layer of the initial 1-D velocity model. The model resolution of the tomographic inversions was assessed using the checkerboard resolution test (CRT), the diagonal resolution element (DRE), and the derivative weight sum (DWS). Using the obtained Vp, Vs, and Vp/Vs ratio, we were able to sharpen details of the geological structures within the basin from previous geological studies, and a fault could be well-imaged at a depth of 4 km. We interpreted this as the main dextral strike-slip fault that controls the pull apart process of the Banyumas Basin. The thickness of the sediment layers, as well as its layering, were also could be well determined. We found prominent features of the velocity contrast that aligned very well with the boundary between the Halang and Rambatan formations as observed in the Jati-1 well data. Furthermore, an anticline structure, which is a potential structural trap for the petroleum system in the Banyumas Basin, was also well imaged. This was made possible due to the dense borehole seismographic stations which were deployed in the study area.

scholarly journals P-wave velocity structure in the northern part of the central Japan Basin, Japan Sea with ocean bottom seismometers and airguns

2004 ◽  
Vol 56 (5) ◽  
pp. 501-510 ◽  
Author(s):  
Takeshi Sato ◽  
Masanao Shinohara ◽  
Boris Y. Karp ◽  
Ruslan G. Kulinich ◽  
Nobuhiro Isezaki
Keyword(s):  
Wave Velocity ◽  
Velocity Structure ◽  
Japan Sea ◽  
P Wave ◽  
Ocean Bottom ◽  
Japan Basin ◽  
Central Japan ◽  
P Wave Velocity ◽  
Ocean Bottom Seismometers ◽  
P Wave Velocity Structure

Exploration in the Shetland‐Faeroe Basin using densely spaced arrays of ocean‐bottom seismometers

Geophysics ◽  
1998 ◽  
Vol 63 (2) ◽  
pp. 490-501 ◽  
Author(s):  
Stephen Hughes ◽  
Penny J. Barton ◽  
David Harrison
Keyword(s):  
Velocity Structure ◽  
Physical Property ◽  
Seismic Exploration ◽  
Gravity Modeling ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Near Surface ◽  
Oil Reserves ◽  
Ocean Bottom Seismometers ◽  
Basaltic Layer

Recent exploration activity in the peripheral regions of the Shetland‐Faeroe Basin, offshore northwest Scotland, has led to the discovery of some of the largest oil reserves on the United Kingdom (UK) continental shelf. We present results from two ocean‐bottom seismometer profiles acquired by Mobil North Sea Ltd. across the center of the Shetland‐Faeroe Basin. These data provide a powerful tool for delineating long‐wavelength velocity variations and thus have potential for reducing the nonuniqueness associated with conventional seismic exploration methods. Analysis of the first‐arrival traveltime data using both forward and inverse ray‐based techniques produces a well constrained velocity‐depth model of the basin fill. We estimate that the uncertainty in the velocity structure is ±5% from a series of trial and error perturbations applied to the final models. The velocity structure of the Faeroe Basin has three principal layers: (1) a near‐surface layer with velocities in the range 1.6 to 2.2 km/s, (2) a 3.0–3.2 km/s layer which is characterized by a northwards structural pinch out in the center of the basin, and (3) a deeper laterally heterogeneous layer with velocities in the range 3.8 to 4.2 km/s. In the northwestern portion of the basin, a high velocity (5.0 km/s) basaltic layer is imaged dipping toward the southeast at a depth of 2–3 km. The basement is mapped at a depth of 7–9 km in the center of the basin. Gravity modeling provides independent corroboration of our models through the application of a velocity‐density relationship obtained from a synthesis of physical property measurements. Reflections from the Moho indicate a crustal thickness of 18 ± 3 km, suggesting that the basin is underlain by highly attenuated continental crust, but the velocities in the basement are closer to those of the Faeroe Islands basalts than the expected Lewisian gneiss, suggesting that it may be highly intruded.

Short- and long-term evolution of the seismicity associated with the New Volcanic Edifice offshore Mayotte island

2021 ◽  
Author(s):  
Aude Lavayssière ◽  
Lise Retailleau
Keyword(s):  
Velocity Model ◽  
Ocean Bottom ◽  
Time Data ◽  
New Developments ◽  
Ocean Bottom Seismometers ◽  
Time Variations ◽  
Short And Long Term ◽  
A New Technique ◽  
Seismic Crisis

<p>In May 2018, a seismic crisis started in the Comoros archipelago, East of Mayotte, which was widely felt on the Island. The related discovery of a new, 800-m high, submarine edifice 50 km East of Mayotte showed that the seismicity was caused by the birth of a volcano. The eruption is still on going at the time of writing and has sparked a large interest in the scientific community.</p><p>The seismicity is still active and is being continuously monitored thanks to several seismic stations installed on the island of Mayotte. The oceanographic campaigns that were carried out since the beginning of the crisis deployed a number of ocean bottom seismometers directly above the seismicity, to accurately understand the crisis and particularly its location. A new technique of automatic detection based on Machine Learning enabled to considerably increase the number of earthquakes that can be used to constrain the extent of the seismicity. Furthermore, the development of a new velocity model for the region allowed a precise location of these earthquakes.</p><p>These new developments permitted to reconstruct the seismicity evolution during two years of this seismic crisis and to complete the seismicity map associated with the new seismic activity. These results provide more details on the active structures to study the evolution in time as well as their precise spacial variations, allowing the analysis of the daily-to-yearly timescales of this unprecedented eruption. This is crucial to understand the dynamics of the volcanic and magmatic processes beneath Mayotte island. Linking these spatial and time variations with the real-time data, as well as the deformation and petrology evolutions, will provide crucial details on the dynamics of submarine eruptions.</p>

P-wave velocity structure of the margin of the southeastern Tsushima Basin in the Japan Sea using ocean bottom seismometers and airguns

2006 ◽  
Vol 412 (3-4) ◽  
pp. 159-171 ◽  
Author(s):  
Takeshi Sato ◽  
Toshinori Sato ◽  
Masanao Shinohara ◽  
Ryota Hino ◽  
Minoru Nishino ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Velocity Structure ◽  
Japan Sea ◽  
P Wave ◽  
Ocean Bottom ◽  
P Wave Velocity ◽  
The Japan Sea ◽  
Ocean Bottom Seismometers ◽  
Tsushima Basin ◽  
P Wave Velocity Structure

scholarly journals Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers

2001 ◽  
Vol 106 (B12) ◽  
pp. 30689-30699 ◽  
Author(s):  
Kei Katsumata ◽  
Toshinori Sato ◽  
Junzo Kasahara ◽  
Naoshi Hirata ◽  
Ryota Hino ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Triple Junction ◽  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometers ◽  
The Indian Ocean

Wave conversions from earthquakes and a new velocity model for the South Carolina coastal plain

1987 ◽  
Vol 77 (6) ◽  
pp. 2143-2151
Author(s):  
Susan Rhea
Keyword(s):  
Surface Layer ◽  
South Carolina ◽  
Coastal Plain ◽  
Velocity Structure ◽  
Velocity Model ◽  
Seismogenic Fault ◽  
Impedance Boundary ◽  
High Impedance ◽  
Hypocentral Parameters ◽  
Converted Phases

Abstract Phase conversions from P to SV and from SV to P occur at a high impedance boundary near the surface in Charleston, South Carolina. Four arrivals (P, converted P, converted S, and S) are observed on three-component records of earthquakes in this area. Using arrival-time differences between paired arrivals of direct and converted phases, a shallow surface layer Vp/Vs ratio of 2.9 was determined. Applying the Wadati method to travel times derived at the base of the surface layer yields a Vp/Vs ratio in deeper layers of 1.73. Relocating earthquakes using this more appropriate velocity structure for direct and converted shear waves alters hypocentral parameters such that epicenters diverge and depths converge. It is inferred that these relocated earthquakes are not exclusively associated with a single seismogenic fault.

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