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.

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.

THE POTENTIAL OF DIGITAL SEISMIC RECORDINGS FOR MULTICHANNEL PROCESSING

Geophysics ◽  
1970 ◽  
Vol 35 (3) ◽  
pp. 461-470 ◽  
Author(s):  
J. P. Lindsey
Keyword(s):  
Dynamic Range ◽  
Single Channel ◽  
Recording Equipment ◽  
Field Equipment ◽  
Reflection Data ◽  
Field Recording ◽  
Amplifier Gain ◽  
Multichannel Processing ◽  
New Processing ◽  
Processing Techniques

The availability of seismic digital field recording equipment has made possible new processing techniques which achieve significant reflection data enhancement. Typical of the processes that are now used routinely are deconvolution, autocorrelation and crosscorrelation, Fourier transformation, and spectral alteration. A recording fidelity that reduces errors to 1 part in 10,000 has provided the motive for developing and using these techniques. An additional capability of digital field equipment is the recording of amplifier gain information to a precision of 0.1 percent. This appears to provide a motive for developing multichannel processes which expand further our processing capabilities beyond the essentially single channel ones now in use. The present study evaluates the multichannel processing potential afforded by present day seismic digital field recording systems. The evaluation is based on measurement and computation of the effects of channel performance deviations. Each component of the field recording system (geophone, cable, amplifier, filters, sampling skew) separately, and the system as a whole, are evaluated in this context. Results of the study indicate that whereas any given channel possesses a dynamic range of 80 db, channel‐to‐channel variations establish a dynamic range of only 15 db. The 15 db range sets a serious limit on the performance of multichannel processes and points up the need for additional improvements in field hardware capabilities.

Labeling long‐period multiple reflections

Geophysics ◽  
1989 ◽  
Vol 54 (1) ◽  
pp. 122-126 ◽  
Author(s):  
R. J. J. Hardy ◽  
M. R. Warner ◽  
R. W. Hobbs
Keyword(s):  
Seismic Reflection ◽  
High Amplitude ◽  
Data Sets ◽  
Multiple Reflections ◽  
Multiple Series ◽  
Seismic Reflection Data ◽  
Long Period ◽  
Reflection Data ◽  
Zero Offset ◽  
Marine Seismic

The many techniques that have been developed to remove multiple reflections from seismic data all leave remnant energy which can cause ambiguity in interpretation. The removal methods are mostly based on periodicity (e.g., Sinton et al., 1978) or the moveout difference between primary and multiple events (e.g., Schneider et al., 1965). They work on synthetic and selected field data sets but are rather unsatisfactory when applied to high‐amplitude, long‐period multiples in marine seismic reflection data acquired in moderately deep (700 m to 3 km) water. Differential moveout is often better than periodicity at discriminating between types of events because, while a multiple series may look periodic to the eye, it is only exactly so on zero‐offset reflections from horizontal layers. The technique of seismic event labeling described below works by returning offset information from CDP gathers to a stacked section by color coding, thereby discriminating between seismic reflection events by differential normal moveout. Events appear as a superposition of colors; the direction of color fringes indicates whether an event has been overcorrected or undercorrected for its hyperbolic normal moveout.

Deep‐water peg legs and multiples: Emulation and suppression

Geophysics ◽  
1986 ◽  
Vol 51 (12) ◽  
pp. 2177-2184 ◽  
Author(s):  
J. R. Berryhill ◽  
Y. C. Kim
Keyword(s):  
Wave Equation ◽  
Seismic Data ◽  
Water Layer ◽  
Original Data ◽  
Equation Method ◽  
Multiple Reflections ◽  
Step Method ◽  
Arrival Times ◽  
Sea Floor ◽  
Water Bottom

This paper discusses a two‐step method for predicting and attenuating multiple and peg‐leg reflections in unstacked seismic data. In the first step, an (observed) seismic record is extrapolated through a round‐trip traversal of the water layer, thus creating an accurate prediction of all possible multiples. In the second step, the record containing the predicted multiples is compared with and subtracted from the original. The wave‐equation method employed to predict the multiples takes accurate account of sea‐floor topography and so requires a precise water‐bottom profile as part of the input. Information about the subsurface below the sea floor is not required. The arrival times of multiple reflections are reproduced precisely, although the amplitudes are not accurate, and the sea floor is treated as a perfect reflector. The comparison step detects the similarities between the computed multiples and the original data, and estimates a transfer function to equalize the amplitudes and account for any change in waveform caused by the sea‐floor reflector. This two‐step wave‐equation method is effective even for dipping sea floors and dipping subsurface reflectors. It does not depend upon any assumed periodicity in the data or upon any difference in stacking velocity between primaries and multiples. Thus it is complementary to the less specialized methods of multiple suppression.

scholarly journals Vertical Distribution of the Mirror Image Returns Observed by TRMM PR and Estimated for a 35-GHz Radar

2005 ◽  
Vol 22 (11) ◽  
pp. 1829-1837 ◽  
Author(s):  
Ji Li ◽  
Kenji Nakamura
Keyword(s):  
Vertical Distribution ◽  
Dynamic Range ◽  
Tropical Rainfall Measuring Mission ◽  
Rain Attenuation ◽  
Mirror Image ◽  
Direct Image ◽  
Surface Reflection ◽  
Bistatic Scattering ◽  
Trmm Precipitation ◽  
The Difference

Abstract The vertical distribution of Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR)-observed precipitation reflectivity and their mirror image (MI) reflectivity are outlined in this paper. The purpose of this study is to investigate the possibility and the limitation of the MI method, which can be used to estimate rain attenuation. Because the MI returns are attenuated much more greatly than the direct image returns, and also because the MI return is affected by the surface reflection and surface scattering, the MI returns are much smaller and more complex than the direct image (DI) returns. However, because the MI returns might be contaminated by the surface or contributed by bistatic scattering near the surface, there are many strong mirror returns between the surface and below surface at 1 km. The ratio of detectable MI return pixels to detected DI return pixels depends on rain rate, target height, and storm height. In addition, differences also exist between the convective and stratiform rain. The reason for this might be because the difference of the surface cross section and the difference of the storm height between the two types of rainfall. Furthermore, the direct and the mirror returns for a 35-GHz radar are also estimated. The virtue of the MI method of the Ka-band radar may reside in expanding the dynamic range of the MI method from 4–30 to 0.6–30 mm h−1.

Downhole seismic logging for high‐resolution reflection surveying in unconsolidated overburden

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1371-1384 ◽  
Author(s):  
J. A. Hunter ◽  
S. E. Pullan ◽  
R. A. Burns ◽  
R. L. Good ◽  
J. B. Harris ◽  
...  
Keyword(s):  
High Resolution ◽  
Seismic Velocity ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Dynamic Shear Modulus ◽  
S Wave ◽  
Interface Shear ◽  
Similar Frequency ◽  
Surface Reflection ◽  
First Arrival

Downhole seismic velocity logging techniques have been developed and applied in support of high‐resolution reflection seismic surveys. For shallow high‐resolution reflection surveying within unconsolidated overburden, velocity‐depth control can sometimes be difficult to achieve; as well, unambiguous correlation of reflections with overburden stratigraphy is often problematic. Data obtained from downhole seismic logging can provide accurate velocity‐depth functions and directly correlate seismic reflections to depth. The methodologies described in this paper are designed for slimhole applications in plastic‐cased boreholes (minimum ID of 50 mm) and with source and detector arrays that yield similar frequency ranges and vertical depth resolutions as the surface reflection surveys. Compressional- (P-) wave logging uses a multichannel hydrophone array with 0.5-m detector spacings in a fluid‐filled borehole and a high‐frequency, in‐hole shotgun source at the surface. Overlapping array positions downhole results in redundant first‐arrival data (picked using interactive computer techniques), which can be processed to provide accurate interval velocities. The data also can be displayed as a record suite, showing reflections and directly correlating reflection events with depths. Example applications include identification of gas zones, lithological boundaries within unconsolidated sediments, and the overburden‐bedrock interface. Shear- (S-) wave logging uses a slimhole, well‐locked, three‐component (3-C) geophone pod and a horizontally polarized, hammer‐and‐loaded‐plate source at ground surface. The pod is moved in successive 0.5- or 1-m intervals downhole with no redundancy of overlapping data as in the P-wave method. First‐arrival data can be obtained by picking the crossover onset of polarized energy or by closely examining particle‐motion plots using all three components of motion. In unconsolidated sediments, shear‐wave velocity contrasts can be associated with changes in material density or dynamic shear modulus, which in turn can be related to consolidation. Example applications include identification of a lithological boundary for earthquake hazard applications and mapping massive ice within permafrost materials.

A STUDY OF CASED AND OPEN HOLES FOR DEEP‐HOLE SEISMIC DETECTION

Geophysics ◽  
1963 ◽  
Vol 28 (1) ◽  
pp. 8-13 ◽  
Author(s):  
D. R. Van Sandt ◽  
F. K. Levin
Keyword(s):  
Seismic Signal ◽  
High Amplitude ◽  
Borehole Wall ◽  
Detectable Effect ◽  
Seismic Signals ◽  
Deep Hole ◽  
Simultaneous Measurements ◽  
Seismic Detection

A study is made of the relative merits of recording seismic signals in cased and open boreholes. Simultaneous measurements of seismic signals in adjacent cased and open holes are compared. It is shown that, in general, the same natural earth noise is recorded in both holes, and the response to the high‐amplitude unidirectional signal is the same in both holes. The conclusion is that the casing in a borehole has no detectable effect upon a seismic signal if the casing is cemented to the borehole wall and wall‐coupled geophones are used.

scholarly journals Extending a Lippmann style seismometer's dynamic range by using a non-linear feedback circuit

2013 ◽  
Vol 36 ◽  
pp. 27-30 ◽  
Author(s):  
G. Romeo ◽  
G. Spinelli
Keyword(s):  
Electronic Circuit ◽  
Dynamic Range ◽  
Strong Motion ◽  
Linear Feedback ◽  
Seismic Signals ◽  
Epicentral Area ◽  
Velocity Feedback ◽  
Single Coil ◽  
Non Linear ◽  
Feedback Method

Abstract. A Lippmann style seismometer (Lippmann and Gebrande, 1983) uses a single-coil velocity-feedback method in order to extend toward lower frequencies a geophone's frequency response. Strong seismic signals may saturate the electronics, sometimes clipping the signal or producing the characteristic whale-shaped recording. Adding a non linear feedback in the electronic circuit may avoid saturation, allowing the strong-motion use of the seismometer without affecting the usual performance. Such a seismometer will allow unsaturated data in epicentral area while offering nice low signal recording for far events.

scholarly journals Free-surface and internal multiple elimination in one step without adaptive subtraction

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. A7-A11 ◽  
Author(s):  
Lele Zhang ◽  
Evert Slob
Keyword(s):  
Free Surface ◽  
Velocity Model ◽  
Original Data ◽  
Time Instant ◽  
Scattering Media ◽  
Multiple Reflections ◽  
Data Set ◽  
Surface Reflection ◽  
Wide Range ◽  
One Step

We have derived a scheme for retrieving the primary reflections from the acoustic surface-reflection response by eliminating the free-surface and internal multiple reflections in one step. This scheme does not require model information and adaptive subtraction. It consists only of the reflection response as a correlation and convolution operator that acts on an intermediate wavefield from which we compute and capture the primary reflections. For each time instant, we keep one value for each source-receiver pair and store it in the new data set. The resulting data set contains only primary reflections, and from this data set, a better velocity model can be built than from the original data set. A conventional migration scheme can then be used to compute an artifact-free image of the medium. We evaluated the success of the method with a 2D numerical example. The method can have a wide range of applications in 3D strongly scattering media that are accessible from one side only.

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