High‐frequency acoustic backscattering from a coarse shell ocean bottom

1989 ◽  
Vol 85 (1) ◽  
pp. 125-136 ◽  
Author(s):  
S. Stanic ◽  
K. B. Briggs ◽  
P. Fleischer ◽  
W. B. Sawyer ◽  
R. I. Ray
Keyword(s):  
High Frequency ◽  
Ocean Bottom ◽  
Acoustic Backscattering

Shear Modulus of Soft Marine Clays

1989 ◽  
Vol 111 (4) ◽  
pp. 265-272 ◽  
Author(s):  
S. Pamukcu
Keyword(s):  
Cyclic Loading ◽  
Shear Modulus ◽  
High Frequency ◽  
Dynamic Testing ◽  
Strain Range ◽  
Ocean Bottom ◽  
Static Data ◽  
Measured Strain ◽  
Cyclic Degradation ◽  
Marine Clays

Instabilities occur frequently in ocean-bottom sediments where the deposition is faster than the consolidation of the material. Cyclic loading of waves contribute to the existing pore pressures within the sediment reducing the effective stresses. The sediment can lose strength and stability and flow in gullies of depth up to 30 m, on slopes less than 0.5 deg. One theory and some related experiments indicate that, depending on the depositional conditions and state of stress, the failure mechanism for such soft saturated marine clays may not necessarily require large straining of the material. Laboratory determination of low-strain shear behavior or shear modulus of soft marine clays can be complicated if high-frequency dynamic testing methods are utilized. Cyclic loading can promote fast degradation of moduli for these soils even at low strain amplitudes. A monotonic torsional shear device, namely a triaxial vane device, was equipped with a computer-aided data acquisition system to detect low-strain shear deformations under quasi-static loading conditions. The average range of electronically measured strain range was 10−4 to 1 percent, which was compatible with that of a high-frequency, low-strain dynamic testing method, namely, resonant column. Comparison of the dynamic and static moduli reduction curves of artificially prepared soft kaolinite specimens demonstrated the cyclic degradation effects on such clays. The relatively continuous, high-resolution low-strain static data indicated further gain in understanding of low-strain nonlinearity and yielding behavior of soft marine clays.

High‐frequency bistatic reverberation from a smooth ocean bottom

1993 ◽  
Vol 93 (5) ◽  
pp. 2633-2638 ◽  
Author(s):  
S. Stanic ◽  
E. Kennedy ◽  
R. I. Ray
Keyword(s):  
High Frequency ◽  
Ocean Bottom

scholarly journals Acoustic backscattering at low grazing angles from the ocean bottom. I. Bottom backscattering strength

1984 ◽  
Vol 75 (S1) ◽  
pp. S30-S30 ◽  
Author(s):  
H. Boehme ◽  
N. P. Chotiros ◽  
L. D. Rolleigh ◽  
S. P. Pitt ◽  
A. L. Garcia ◽  
...  
Keyword(s):  
Ocean Bottom ◽  
Acoustic Backscattering ◽  
Backscattering Strength ◽  
Grazing Angles

Hybrid long-period volcanic events observed in off Nicobar region, the Andaman Sea from a passive OBS experiment

2020 ◽  
Author(s):  
Karanam Kattil Aswini ◽  
Pawan Dewangan ◽  
Kattoju Achuta Kamesh Raju ◽  
Yatheesh Vadakkeyakath ◽  
Pabitra Singha ◽  
...  
Keyword(s):  
High Frequency ◽  
Source Region ◽  
Strike Slip ◽  
Andaman Sea ◽  
Earthquake Swarms ◽  
Ocean Bottom ◽  
Upward Movement ◽  
Ocean Bottom Seismometer ◽  
Long Period ◽  
Back Arc

<p>The off Nicobar region in the Andaman Sea is witnessing frequent earthquake swarms after December 2004 Tsunamigenic earthquake in January 2005, March and October 2014, November 2015 and April 2019. In this study, we present the geophysical evidence of active volcanism in the Off Nicobar back-arc region on 21<sup>st</sup> and 22<sup>nd</sup> March 2014 based on a passive Ocean Bottom Seismometer (OBS) experiment. We detected a series of hybrid earthquake events characterized by the onset of high–frequency signal (1-10 Hz) which is followed by a long period waveform of up to 600s having a range of 0.1-1 Hz. The waveforms appear to be emergent and lack the onset of a distinct S-phase. We also observed a very high frequency (10-40 Hz) hydro-acoustic phase in the coda of long-period events.  These hybrid events are considered to be volcano-tectonic (VT) events that may trigger magmatic activities in the Off Nicobar region. We have identified and located 141 high-frequency events on 21<sup>st</sup> and 22<sup>nd</sup> March 2014 using hypocent v.3.2 program and they are distributed along NW-SE direction aligning with the submarine volcanoes defining the volcanic arc as observed in the high-resolution bathymetry data. The fault plane solution of the major high-frequency events suggests strike-slip faulting with the strike, dip and rake values of 334<sup>°</sup>, 89<sup>°</sup> and 171<sup>°</sup>, respectively along the direction of the prevalent sliver strike-slip faulting in the Andaman back-arc region. We propose that the upward movement of magma is a plausible mechanism which can explain the frequent occurrence of earthquake swarms in the off Nicobar region. The stress generated from magma movement may initially trigger shallow VT events such as faulting or dike intrusions and later generate long period ringing associated with the resonance of the magma chamber. The shallow nature of the events also generates a hydroacoustic wave which is detected in the OBS experiment as the source region is in the SOFAR channel.</p>

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