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.

The Design of New Low Cost Ocean Bottom Seismometers

2012 ◽  
Vol 226-228 ◽  
pp. 2107-2110
Author(s):  
Hu Sheng Guo ◽  
Bin Yan ◽  
Zhi Dong Wu
Keyword(s):  
Dynamic Range ◽  
Low Cost ◽  
High Dynamic Range ◽  
Low Noise ◽  
Seismic Exploration ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Mems Accelerometer ◽  
Mems Sensor ◽  
Ocean Bottom Seismometers

The performance of the Ocean Bottom Seismometers (OBS) in seismic wave field measurement is vital to seismic exploration. In order to improve the performance of OBS, we have been developed a new Ocean Bottom Seismometer based 3-component MEMS accelerometer sensors. In order to sample seismic data synchronously, we have been designed multichannel A/D unit under the control of MSP430.We also are involved in a handle and sophisticated equipment allows to storage sampling data in the SD card module. The system based MEMS sensor are compared with conventional analog moving coil geophones, the result shows that the new measurement system with the advantage of high dynamic range, low noise and anti-jamming that suit for the high resolution seismicity information. The paper show that the new digital OBS using MEMS accelerometer will replace the tradition OBS in oil exploration, scientific research and seabed surveys.

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.

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.

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

Development of a Compact Broadband Ocean-Bottom Seismometer

2021 ◽  
Author(s):  
Masanao Shinohara ◽  
Tomoaki Yamada ◽  
Hajime Shiobara ◽  
Yusuke Yamashita
Keyword(s):  
Low Frequency ◽  
Plate Boundaries ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Broadband Seismometer ◽  
Seismic Sensors ◽  
Slow Earthquakes ◽  
Short Period ◽  
Ocean Bottom Seismometers

Abstract Studies of very-low-frequency earthquakes and low-frequency tremors (slow earthquakes) in the shallow region of plate boundaries need seafloor broadband seismic observations. Because it is expected that seafloor spatially high-density monitoring requires numerous broadband sensors for slow earthquakes near trenches, we have developed a long-term compact broadband ocean-bottom seismometer (CBBOBS) by upgrading the long-term short-period ocean-bottom seismometer that has seismic sensors with a natural frequency of 1 Hz and is being mainly used for observation of microearthquakes. Because many long-term ocean-bottom seismometers with short-period sensors are available, we can increase the number of broadband seafloor sensors at a low cost. A short-period seismometer is exchanged for a compact broadband seismometer with a period of 20 or 120 s. Because the ocean-bottom seismometers are installed by free fall, we have no attitude control during an installation. Therefore, we have developed a new leveling system for compact broadband seismic sensors. This new leveling system keeps the same dimensions as the conventional leveling system for 1 Hz seismometers so that the broadband seismic sensor can be installed conveniently. Tolerance for leveling is less than 1°. A tilt of up to 20° is allowed for the leveling operation. A microprocessor controls the leveling procedure. Some of the newly developed ocean-bottom seismometers were deployed in the western Nankai trough, where slow earthquakes frequently occur. The data from the ocean-bottom seismometers on the seafloor were evaluated, and we confirmed that the long-term CBBOBS is suitable for observation of slow earthquakes. The developed ocean-bottom seismometer is also available for submarine volcanic observation and broadband seafloor observation to estimate deep seismic structures.

scholarly journals Seismic Velocity Structure of Kita-Yamato Trough, Japan Sea Revealed by Ocean Bottom Seismometer and Airgun Survey

2001 ◽  
Vol 53 (4) ◽  
pp. 337-355 ◽  
Author(s):  
Takeshi SATO ◽  
Masanao SHINOHARA ◽  
Kiyoshi SUYEHIRO ◽  
Nobuhiro ISEZAKI ◽  
Boris Y. KARP ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Japan Sea ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Seismic Velocity Structure

scholarly journals Crust and upper mantle structure of the Ligurian Sea revealed by ambient noise tomography using ocean bottom seismometer data

2020 ◽  
Author(s):  
Felix Noah Wolf ◽  
Dietrich Lange ◽  
Heidrun Kopp ◽  
Anke Dannowski ◽  
Ingo Grevemeyer ◽  
...  
Keyword(s):  
Phase Velocity ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Structure ◽  
Signal To Noise Ratio ◽  
Time Windows ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Ligurian Sea ◽  
Crust And Upper Mantle

<p>The Liguro-Provencal-basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block at roughly 32–24 Ma is associated with rifting, shaping the Ligurian Sea. It is highly debated though, whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin.</p><p>To investigate the velocity structure of the Ligurian Sea a network (LOBSTER) of 29 broadband Ocean Bottom Seismometer (OBS) was installed jointly by GEOMAR (Germany) and ISTerre (France). The LOBSTER array measured continuously for eight months between June 2017 and February 2018 and is part of the AlpArray seismic network. AlpArray is a European initiative to further reveal the geophysical and geological properties of the greater Alpine area.</p><p>We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea.<br>Because of additional noise sources in the ocean, causing instrument tilt or seafloor compliance, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data to improve the signal-to-noise ratio. Then, we calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong events. The cross-correlations of these are dominated by earthquake signals and allow us to derive surface wave group velocities for longer periods than using ambient noise only. Finally, phase velocity maps are obtained by inverting Green’s functions derived from cross-correlation of ambient noise and teleseismic events, respectively. The phase velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). Causes for these include varying sediment thickness, fault zones and magmatism. For longer periods (20-80 s) the velocity structure smoothens and reveals mantle velocities north-northwest of the basin centre. This might hint on an asymmetric opening of the basin. We do not see strong indications for an oceanic spreading centre in the Ligurian basin.</p>

scholarly journals Seafloor sediment thickness beneath the VoiLA broad-band ocean-bottom seismometer deployment in the Lesser Antilles from P-to-S delay times

2020 ◽  
Vol 223 (3) ◽  
pp. 1758-1768
Author(s):  
Ben Chichester ◽  
Catherine Rychert ◽  
Nicholas Harmon ◽  
Robert Allen ◽  
Jenny Collier ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Confidence Region ◽  
Broad Band ◽  
Lesser Antilles ◽  
Earth Model ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Sediment Thickness ◽  
Delay Times ◽  
Crystalline Crust

SUMMARY Broad-band ocean-bottom seismometer (OBS) deployments present an opportunity to investigate the seafloor sediment thickness, which is important for constraining sediment deposition, and is also useful for subsequent seismological analyses. The Volatile Recycling in the Lesser Antilles (VoiLA) project deployed 34 OBSs over the island arc, fore- and backarc of the Lesser Antilles subduction zone for 15 months from 2016 to 2017. Using the amplitudes and delay times of P-to-S (Ps) scattered waves from the conversion of teleseismic earthquake Pwaves at the crust–sediment boundary and pre-existing relationships developed for Cascadia, we estimate sediment thickness beneath each OBS. The delay times of the Ps phases vary from 0.20 ± 0.06 to 3.55 ± 0.70 s, generally increasing from north to south. Using a single-sediment and single-crystalline crust earth model in each case, we satisfactorily model the observations of eight OBSs. At these stations we find sediment thicknesses range from 0.43 ± 0.45 to 5.49 ± 3.23 km. To match the observations of nine other OBSs, layered sediment and variable thickness crust is required in the earth model to account for wave interference effects on the observed arrivals. We perform an inversion with a two-layer sediment and a single-layer crystalline crust in these locations finding overall sediment thicknesses of 1.75 km (confidence region: 1.45–2.02 km) to 7.93 km (confidence region: 6.32–11.05 km), generally thinner than the initial estimates based on the pre-existing relationships. We find agreement between our modelled velocity structure and the velocity structure determined from the VoiLA active-source seismic refraction experiment at the three common locations. Using the Ps values and estimates from the VoiLA refraction experiment, we provide an adjusted relationship between delay time and sediment equations for the Lesser Antilles. Our new relationship is ${{H}} = {{1.42}}{{\rm d}}{{{t}}^{ {1.44}}}$ , where H is sediment thickness in kilometres and dt is mean observed Ps delay time in seconds, which may be of use in other subduction zone settings with thick seafloor sediments.

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