PRESSURE AMPLITUDES OF SEISMIC SIGNALS IN OFFSHORE LOUISIANA

Geophysics ◽  
1977 ◽  
Vol 42 (1) ◽  
pp. 3-16
Author(s):  
M. E. Arnold
Keyword(s):  
Field Tests ◽  
Signal Amplitude ◽  
Test Area ◽  
Seismic Signals ◽  
Code Generators ◽  
Single Detector ◽  
First Arrival ◽  
Reflected Signal ◽  
Refracted Waves ◽  
Seismic Lines

Pressure amplitudes were determined for various kinds of seismic signals observed on special test records obtained during field tests conducted along a 14,000-ft seismic lines in Eugene Island Block 184, offshore Louisiana. Vibrators attached to a Seismograph Service Corp. (SSC) boat generated swept‐frequency and monofrequency signals. Signals from detectors on a streamer cable towed by the boat were recorded by an SSC recording system. Signals from a vertical spread of detectors were recorded by a DFS/9000 recorder on the Transco 184 platform centrally located in the test area. Location of the boat was determined by analysis of time relations of signals from responders located at established positions some distance from the test area. Clock times from manually referenced timing code generators were recorded by both the SSC and DFS recorders to permit synchronization between separately recorded signals. The signals analyzed were separated into three classes: [Formula: see text] includes direct and refracted waves; [Formula: see text] consists of primary reflections; and [Formula: see text] includes signals diffracted from scatterers. The average level of first‐arrival signal [Formula: see text] and reflected signal [Formula: see text] for frequency sets 25, 40, 42.2, 50, and 70.4 Hz in the range of 1414 and 2143 ft, which encompasses streamer cable single‐detector groups, is 337 and 29.6 microbars, respectively. The amplitude of signals [Formula: see text], believed to be diffracted from the contact between key reflectors and a salt dome, ranges from 13 to 20 microbars and is 10 to 100 times the amplitudes of towing and ambient noise, respectively. The observed decay of first‐arrival signal amplitude is approximately proportional to the square root of range distance, or about 2 dB/1000 ft. The observed decay of reflected signal amplitude with range distance is approximately 1 dB/1000 ft.

scholarly journals On the Impact and Mitigation of Signal Crosstalk in Ground-Based and Low Altitude Airborne GNSS-R

2021 ◽  
Vol 13 (6) ◽  
pp. 1085
Author(s):  
Corentin Lubeigt ◽  
Lorenzo Ortega ◽  
Jordi Vilà-Valls ◽  
Laurent Lestarquit ◽  
Eric Chaumette
Keyword(s):  
Specular Reflection ◽  
Signal Amplitude ◽  
Satellite System ◽  
Mitigation Strategies ◽  
Variance Estimator ◽  
Single Source ◽  
Dual Source ◽  
Reflected Signal ◽  
The Impact ◽  
Signal Crosstalk

Global Navigation Satellite System Reflectometry (GNSS-R) is a powerful way to retrieve information from a reflecting surface by exploiting GNSS as signals of opportunity. In dual antenna conventional GNSS-R architectures, the reflected signal is correlated with a clean replica to obtain the specular reflection point delay and Doppler estimates, which are further processed to obtain the GNSS-R product of interest. An important problem that may appear for low elevation satellites is signal crosstalk, that is the direct line-of-sight signal leaks into the antenna dedicated to the reflected signal. Such crosstalk may degrade the overall system performance if both signals are very close in time, similar to multipath in standard GNSS receivers, the reason why mitigation strategies must be accounted for. In this article: (i) we first provide a geometrical analysis to justify that the estimation performance is only affected for low height receivers; (ii) then, we analyze the impact of crosstalk if not taken into account, by comparing the single source conditional maximum likelihood estimator (CMLE) performance in a dual source context with the corresponding Cramér–Rao bound (CRB); (iii) we discuss dual source estimators as a possible mitigation strategy; and (iv) we investigate the performance of the so-called variance estimator, which is designed to eliminate the coherent signal part, compared to both the CRB and non-coherent dual source estimators. Simulation results are provided for representative GNSS signals to support the discussion. From this analysis, it is found that: (i) for low enough reflected-to-direct signal amplitude ratios (RDR), the crosstalk has no impact on standard single source CMLEs; (ii) for high enough signal-to-noise ratios (SNR), the dual source estimators are efficient irrespective of the RDR, then being a promising solution for any reflected signal scenario; (iii) non-coherent dual source estimators are also efficient at high SNR; and (iv) the variance estimator is efficient as long as the non-coherent part of the signal is dominant.

Effect of hydrophone arrays on offshore Texas seismic signals

Geophysics ◽  
1978 ◽  
Vol 43 (6) ◽  
pp. 1083-1098
Author(s):  
M. E. Arnold
Keyword(s):  
Dynamic Range ◽  
High Amplitude ◽  
Early Event ◽  
Seismic Signals ◽  
Multiple Reflections ◽  
Surface Reflection ◽  
Field Recording ◽  
First Arrival ◽  
Slope Line ◽  
Water Bottom

The effect of hydrophone arrays in the recording of seismic signals during offshore Texas seismic marine experiments is judged by comparing traces of spatially tapered hydrophone array signals with traces that are combinations of simultaneously recorded wavetest hydrophone signals. Each spatially tapered hydrophone group array consists of 26 hydrophones nonuniformly spaced over 212 ft. The wavetest streamer section consists of 36 groups of two hydrophones, each pair connected in parallel and with hydrophones back‐to‐back for acceleration cancellation, with 5-ft spacing between groups. Reflection from deep subsurface interfaces are negligibly affected by hydrophone arrays except for very long arrays and/or long‐range distances. Consequently, the report is primarily concerned with the effects of simulated and real hydrophone arrays on first‐arrival signal and early subbottom reflections. Comparison of theoretical and actual seismic traces from an Aquapulse source for near range distances (835 ft) used in normal operations indicates that (1) near‐simultaneous arrival of the direct wave and surface reflection result in their virtual cancellation, (2) the early event with largest amplitude is associated with constructive interference between source and receiver ghost reflections, and (3) the “pseudo‐bubble” period effectively fixed the predominant frequency of all seismic events at values near 28 Hz. At medium range distances (4755 ft), such comparisons indicate that (1) first arrivals are refracted waves traveling in subbottom layers; (2) the water‐bottom reflection is beyond critical angle and is, therefore, complex; (3) the early events with largest amplitude are multiple reflections; and (4) at least two orders of water‐bottom multiples are identified. The attenuation of the high‐amplitude, first‐arrival signal that includes the water‐bottom reflection permits greater dynamic range in field recording and higher levels of “true” amplitude for later reflections without overload distortion of early events on playback. However, if improved resolution of reflection from moderate depths (∼4000 ft) is important, then arrays of length studied in this report (∼200 ft) should not be used to record signals at range distances greater than about 2000 ft because frequencies above 50 Hz are attenuated severely. Spectral analysis of wavetest records in the absence of signals shows that the wavenumber distribution of the noise is located along a slope line equivalent to 5000 ft/sec between wavenumbers that imply a spectral distribution of 30 to 100 Hz. Theoretical array response studies show that both the 36‐element Chebyshev array and the 26‐element spatially tapered array are superior to a 36‐element uniformly weighted array in rejection of seismic noise in the spectral range of 30 to 100 Hz.

scholarly journals Points to be considered in the establishment of long-term field trials

1964 ◽  
Vol 36 (1) ◽  
pp. 51-55
Author(s):  
Yrjö Pessi
Keyword(s):  
Field Tests ◽  
Soil Surface ◽  
Field Trials ◽  
Soil Characteristics ◽  
Test Area ◽  
Test Plot ◽  
Experimental Station ◽  
Term Field

On the basis of the experience gained in the long-term field tests at the Experimental Station of Leteensuo, some of the factors have been examined which have to be taken into consideration when tests of this kind are established. It is noted that in the course of time the soil may become increasingly inhomogeneous, e.g. owing to sludge brought in by inundations, and owing to the wear of the peat on cultivated peat land. An initial shaping of the soil surface is essential in the case of cultivated peat lands because non-uniform settling of the soil may occur in the course of time in the test area. The soil surface of the different test members may also settle in different degrees, depending on the treatment involved in the test. Because of soil transportation from one test plot to another, caused by the tilling operations, the location and shape of the test plots are of significance in long-term tests intended to clarify questions associated with soil characteristics.

Ultrasonic borehole velocity imaging

Geophysics ◽  
2006 ◽  
Vol 71 (3) ◽  
pp. F25-F30 ◽  
Author(s):  
Kenneth W. Winkler ◽  
Ralph D’Angelo
Keyword(s):  
High Stress ◽  
Physical Property ◽  
Ultrasonic Waves ◽  
Thin Layers ◽  
Stress Concentrations ◽  
Velocity Image ◽  
Velocity Images ◽  
First Arrival ◽  
A New Technique ◽  
Refracted Waves

We describe a new technique for making high-resolution velocity images of rocks using refracted ultrasonic waves. The use of refracted waves makes this technique potentially suitable for imaging borehole walls. In the laboratory, we use a single-transmitter, two-receiver, first-arrival method for making velocity measurements, with a spatial resolution on the order of [Formula: see text]. Our acoustic pulses are centered near [Formula: see text]. Scans of a borehole wall reveal dipping thin layers and fractures. When external stress is applied perpendicular to the borehole, stress concentrations appear on our images as axial bands of high and low velocities. Breakouts created by high stress also can be imaged. On a planar sample, a velocity image reveals shale laminations and carbonate stringers. For field applications, this technique offers the potential for imaging in both conductive and nonconductive muds and provides images based on a physical property (velocity) that currently is not used for fine-scale borehole imaging.

THE EFFECT OF VELOCITY OF DETONATION ON THE EFFICIENCY OF EXPLOSIVES USED IN SEISMIC PROSPECTING

Geophysics ◽  
1946 ◽  
Vol 11 (3) ◽  
pp. 350-361 ◽  
Author(s):  
J. Taylor ◽  
G. Morris ◽  
T. C. Richards
Keyword(s):  
Field Tests ◽  
Small Decrease ◽  
Seismic Prospecting ◽  
Significant Difference ◽  
Velocity Of Detonation ◽  
Refracted Waves

Field tests have shown that no significant difference in amplitude or frequency of the first arrivals of refracted waves at distances from 6,000 to 20,000 feet, is found from explosives having velocities of detonation in the range 7,500 to 1,100 metres/second or powers from 61% to 85% blasting gelatine. When a deflagrating explosive was employed there was only a very small decrease in the amplitude of the refracted wave.

Static corrections on the southeastern Piedmont of the United States

Geophysics ◽  
1982 ◽  
Vol 47 (11) ◽  
pp. 1540-1549 ◽  
Author(s):  
Michael S. Bahorich ◽  
Cahit Coruh ◽  
Edwin S. Robinson ◽  
John K. Costain
Keyword(s):  
United States ◽  
The United States ◽  
Weathered Zone ◽  
Common Depth Point ◽  
Position Data ◽  
First Arrival ◽  
Static Corrections ◽  
Metamorphic Terrane ◽  
Thickness Variations ◽  
Refracted Waves

On the Piedmont of the southeastern United States, seismic reflection statics at different points in a typical common‐depth‐point (CDP) gather can differ from one another by more than 50 msec because of topographic relief in excess of 50 m, variations in the thickness of the weathered zone that can extend deeper than 50 m, and variation of velocity in the weathered zone of more than 100 m/sec. The ABCD method for computing statics is introduced to account for these velocity and thickness variations as well as the topographic effect. This method combines elevations and positions of source‐receiver points with times of first arriving refracted waves read from reflection correlograms or seismograms. It was tested in central Virginia where typical piedmont conditions are encountered. At four locations, ABCD statics are close to control values determined independently from refraction experiments. At 99 source‐detector points, ABCD statics differ by an average of 4 msec, (maximum of 19 milliseconds) from conventional elevation statics that do not account for local velocity thickness variations in the weathered zone. Where source‐receiver points were in line, modified ABCD statics were obtained from first arrival traveltimes without using elevation and position data. In this metamorphic terrane where clear reflections are difficult to record, ABCD statics appear to be more effective than conventional elevation statics for enhancing reflections on a seismic record section.

scholarly journals Single-block rockfall dynamics inferred from seismic signal analysis

2017 ◽  
Author(s):  
Clément Hibert ◽  
Jean-Philippe Malet ◽  
Franck Bourrier ◽  
Floriane Provost ◽  
Frédéric Berger ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Potential Energy ◽  
Scaling Laws ◽  
Scaling Law ◽  
Soft Rock ◽  
Signal Amplitude ◽  
Seismic Signal ◽  
Seismic Signals ◽  
Video Cameras ◽  
The Impact

Abstract. We conducted controlled releases of single blocks within a soft-rock (black marls) gully of the Rioux Bourdoux torrent (French Alps). 28 blocks, with masses ranging from 76 kg to 472 kg, were used for the experiment. An instrumentation combining video cameras and seismometers was deployed along the traveled path. The video cameras allow to reconstruct the trajectories of the blocks and to estimate their velocities at the time of the different impacts with the slope. These data are compared to the recorded seismic signals. As the distance between the falling block and the seismic sensors at the time of each impact is known, we were able to determine the associated seismic signal amplitude corrected from propagation and attenuation effects. We compared the velocity, the loss of potential energy, the kinetic energy and the momentum of the block at each impact to the true amplitude and the energy of the corresponding part of the seismic signal. Our results suggest that the amplitude of the seismic signal scales with the momentum of the block at the impact. We also found a scaling law between the potential energy lost, the kinetic energy and the energy of the seismic radiation generated by the impacts. By combining these scaling laws, we inferred the mass and the velocity before impact of each block directly from the seismic signal. Despite high uncertainties, the values found are close to the true values of the mass and the velocities of the blocks. These relationships also provide new insights to understand the source of high-frequency seismic signals generated by rockfalls.

scholarly journals Multi-Azimuth Ground Penetrating Radar Surveys to Improve the Imaging of Complex Fractures

Geosciences ◽  
2018 ◽  
Vol 8 (11) ◽  
pp. 425 ◽  
Author(s):  
Federico Lombardi ◽  
Maurizio Lualdi
Keyword(s):  
Ground Penetrating Radar ◽  
Signal Amplitude ◽  
Relative Orientation ◽  
Complex Fractures ◽  
Reflected Signal ◽  
Single Polarization ◽  
Ground Penetrating ◽  
Imaging Performance ◽  
Inclined Planes ◽  
Relative Geometry

Ground Penetrating Radar (GPR) images are affected, to some degree, by the relative orientation of antennas and subsurface targets. This is particularly true not only for targets that show a significant directivity, but also for inclined planes, such as fractures and faults. Depending on the relative geometry between the antennas and the orientation of the target, radar waves can be preferentially scattered, which causes changes in the reflected signal amplitude. Therefore, traditional single polarization and single azimuth surveys may produce inadequate results. The work presented here examines the use of a multi-azimuth GPR survey to increase the imaging performance of inclined fractures, showing the shortcomings of single-profile surveying and highlighting the benefits that such a strategy has on detection and characterization.

scholarly journals Single-block rockfall dynamics inferred from seismic signal analysis

2017 ◽  
Vol 5 (2) ◽  
pp. 283-292 ◽  
Author(s):  
Clément Hibert ◽  
Jean-Philippe Malet ◽  
Franck Bourrier ◽  
Floriane Provost ◽  
Frédéric Berger ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Potential Energy ◽  
Soft Rock ◽  
Seismic Energy ◽  
Signal Amplitude ◽  
Seismic Signal ◽  
Seismic Signals ◽  
Video Cameras ◽  
Radiated Seismic Energy ◽  
The Impact

Abstract. Seismic monitoring of mass movements can significantly help to mitigate the associated hazards; however, the link between event dynamics and the seismic signals generated is not completely understood. To better understand these relationships, we conducted controlled releases of single blocks within a soft-rock (black marls) gully of the Rioux-Bourdoux torrent (French Alps). A total of 28 blocks, with masses ranging from 76 to 472 kg, were used for the experiment. An instrumentation combining video cameras and seismometers was deployed along the travelled path. The video cameras allow reconstructing the trajectories of the blocks and estimating their velocities at the time of the different impacts with the slope. These data are compared to the recorded seismic signals. As the distance between the falling block and the seismic sensors at the time of each impact is known, we were able to determine the associated seismic signal amplitude corrected for propagation and attenuation effects. We compared the velocity, the potential energy lost, the kinetic energy and the momentum of the block at each impact to the true amplitude and the radiated seismic energy. Our results suggest that the amplitude of the seismic signal is correlated to the momentum of the block at the impact. We also found relationships between the potential energy lost, the kinetic energy and the seismic energy radiated by the impacts. Thanks to these relationships, we were able to retrieve the mass and the velocity before impact of each block directly from the seismic signal. Despite high uncertainties, the values found are close to the true values of the masses and the velocities of the blocks. These relationships allow for gaining a better understanding of the physical processes that control the source of high-frequency seismic signals generated by rockfalls.

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