scholarly journals Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction

Geophysics ◽  
2002 ◽  
Vol 67 (1) ◽  
pp. 89-97 ◽  
Author(s):  
John H. Bradford
Keyword(s):  
Layer Thickness ◽  
Water Table ◽  
Seismic Reflection ◽  
Negative Impact ◽  
Compressional Wave ◽  
Unconsolidated Sediments ◽  
Velocity Analysis ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Saturated Layer

As seismic reflection data become more prevalent as input for quantitative environmental and engineering studies, there is a growing need to assess and improve the accuracy of reflection processing methodologies. It is common for compressional‐wave velocities to increase by a factor of four or more where shallow, unconsolidated sediments change from a dry or partially water‐saturated regime to full saturation. While this degree of velocity contrast is rare in conventional seismology, it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table. The problem is compounded in shallow reflection studies where problems primarily associated with surface‐related noise limit the range of offsets we can use to record reflected energy. For offset‐to‐depth ratios typically required to record reflections originating in this zone, the assumptions of NMO velocity analysis are violated, leading to very large errors in depth and layer thickness estimates if the Dix equation is assumed valid. For a broad range of velocity profiles, saturated layer thickness will be overestimated by a minimum of 10% if the boundary of interest is <30 m below the water table. The error increases rapidly as the boundary shallows and can be very large (>100%) if the saturated layer is <10 m thick. This degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource estimates.

Minimizing field operations in shallow 3‐D seismic reflection surveying

Geophysics ◽  
2001 ◽  
Vol 66 (6) ◽  
pp. 1761-1773 ◽  
Author(s):  
Roman Spitzer ◽  
Alan G. Green ◽  
Frank O. Nitsche
Keyword(s):  
Seismic Reflection ◽  
Cost Effective ◽  
Unconsolidated Sediments ◽  
Data Set ◽  
Seismic Reflection Data ◽  
Northern Switzerland ◽  
Shallow Subsurface ◽  
Reflection Data ◽  
Effective Manner ◽  
Original Survey

By appropriately decimating a comprehensive shallow 3‐D seismic reflection data set recorded across unconsolidated sediments in northern Switzerland, we have investigated the potential and limitations of four different source‐receiver acquisition patterns. For the original survey, more than 12 000 shots and 18 000 receivers deployed on a [Formula: see text] grid resulted in common midpoint (CMP) data with an average fold of ∼40 across a [Formula: see text] area. A principal goal of our investigation was to determine an acquisition strategy capable of producing reliable subsurface images in a more efficient and cost‐effective manner. Field efforts for the four tested acquisition strategies were approximately 50%, 50%, 25%, and 20% of the original effort. All four data subsets were subjected to a common processing sequence. Static corrections, top‐mute functions, and stacking velocities were estimated individually for each subset. Because shallow reflections were difficult to discern on shot and CMP gathers generated with the lowest density acquisition pattern (20% field effort) such that dependable top‐mute functions could not be estimated, data resulting from this acquisition pattern were not processed to completion. Of the three fully processed data subsets, two (50% field effort and 25% field effort) yielded 3‐D migrated images comparable to that derived from the entire data set, whereas the third (50% field effort) resulted in good‐quality images only in the shallow subsurface because of a lack of far‐offset data. On the basis of these results, we concluded that all geological objectives associated with our particular study site, which included mapping complex lithological units and their intervening shallow dipping boundaries, would have been achieved by conducting a 3‐D seismic reflection survey that was 75% less expensive than the original one.

Ultrashallow seismic reflection monitoring of seasonal fluctuations in the water table

2000 ◽  
Vol 6 (3) ◽  
pp. 271-277 ◽  
Author(s):  
G. S. Baker ◽  
D. W. Steeples ◽  
C. Schmeissner ◽  
K. T. Spikes
Keyword(s):  
Water Table ◽  
Seismic Reflection ◽  
Test Site ◽  
Seasonal Fluctuations ◽  
Seismic Profiles ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Velocity Information ◽  
One Year

Abstract Ultrashallow seismic-reflection data were collected at a test site in Great Bend, Kansas. The purpose of the experiment was to image seasonal submeter-scale fluctuations in the water table over a period of one year to identify the factors important in monitoring the water table when using seismic-reflection techniques. The study indicates that detailed velocity information must be used when interpreting water-table levels. Using detailed velocity information as a control when depth-converting the seismic profiles yielded correct positioning of the water table within + or -12 cm at the test site.

INTEGRATED TECHNIQUES FOR DETERMINING THE VELOCITY FUNCTION IN THE SEISMIC REFLECTION: EXAMPLE OF LOW SIGNAL-TO-NOISE DATA

2016 ◽  
Vol 34 (2) ◽  
Author(s):  
Rodrigo Francis Revorêdo ◽  
Carlos César Nascimento da Silva
Keyword(s):  
Seismic Reflection ◽  
Signal To Noise Ratio ◽  
Hydrocarbon Exploration ◽  
Velocity Analysis ◽  
Signal To Noise ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Noise Data ◽  
Quality And Reliability ◽  
Seismic Images

ABSTRACT. In the hydrocarbon exploration activities, the reprocessing of old seismic reflection data, acquired with few channels and with low signal-to-noise ratio, is commonly undertaken to ameliorate the quality and reliability of the seismic images...Keywords: seismic processing, velocity analysis, CVS. RESUMO. É comum na exploração de hidrocarbonetos o reprocessamento de dados sísmicos antigos, por vezes com um baixo número de canais e baixa razão sinal/ruído, com o objetivo de gerar uma imagem de melhor qualidade e confiabilidade quando comparada àquelas já existentes...Palavras-chave: processamento sísmico, análise de velocidades, CVS. 

Shear-wave seismic reflection processing - the importance of velocity analysis

2021 ◽  
Author(s):  
Barbara Dietiker ◽  
André J.-M. Pugin ◽  
Matthew P. Griffiths ◽  
Kevin Brewer ◽  
Timothy Cartwright
Keyword(s):  
Shear Wave ◽  
Seismic Reflection ◽  
Velocity Analysis ◽  
P Waves ◽  
Seismic Reflection Data ◽  
Time Frequency ◽  
Wave Velocities ◽  
Reflection Data ◽  
Shear Wave Velocities ◽  
Local Shift

&lt;p&gt;Based on our experience, one of the most important steps in processing shear-wave seismic reflection data is the velocity analysis. In unconsolidated materials a very fine velocity analysis is more essential for S-waves than for P-waves because shear-wave velocities vary over several orders of magnitude and can change very quickly laterally and with depth. Velocities between 100m/s in glaciolacustine/marine deposits (clay-sized silts) and 1200m/s in stiff diamicton (till) were encountered in recent surveys. Shear-wave velocities have the large advantage of not being changed by the phase of the pore content such as the groundwater table.&lt;/p&gt;&lt;p&gt;We present two fundamentally different methods for velocity determination: 1) velocity semblance analysis based on hyperbolic reflection move-out on common midpoint (cmp) gathers and 2) Local Phase &amp;#8211; Local Shift (LPLS) method which automatically estimates the reflection slope (local static shift) in the time-frequency domain of cmp gathers. Published in 2020, the latter method can be used for automated processing and substantially saves processing time.&lt;/p&gt;&lt;p&gt;Processing steps in preparation for velocity analysis (independent of the chosen method) include frequency filtering, trace equalizing and muting. We show velocity semblance images from different geological settings (glacial, postglacial) and from different shear components and discuss differences. Information gained besides shear velocities include mapped reflectors and located diffractions. Using those examples, we demonstrate how combining all information using visualisation techniques enhances interpretation of such data sets.&lt;/p&gt;

scholarly journals Interferometric velocity analysis using physical and nonphysical energy

Geophysics ◽  
2011 ◽  
Vol 76 (1) ◽  
pp. SA35-SA49 ◽  
Author(s):  
Simon King ◽  
Andrew Curtis ◽  
Travis L. Poole
Keyword(s):  
Green’S Function ◽  
Layer Thickness ◽  
Green's Function ◽  
Velocity Analysis ◽  
Seismic Reflection Data ◽  
Reflected Waves ◽  
Interval Velocity ◽  
Reflection Data ◽  
Receiver Array ◽  
Marine Seismic

In controlled-source seismic interferometry, waves from a surrounding boundary of sources recorded at two receivers are crosscorrelated and summed to synthesize the interreceiver Green’s function. Deviations of physically realistic source and receiver geometries from those required by theory result in errors in the Green’s function estimate. These errors are manifested as apparent energy that could not have propagated between receiver locations — so-called nonphysical energy. We have developed a novel method of velocity analysis that uses both the physical and nonphysical wavefield energy in the crosscorrelated data generated between receiver pairs. This method is used to constrain the root-mean-square (rms) velocity and layer thickness of a locally 1D medium. These estimates are used to compute the piece-wise constant interval velocity. Instead of suppressing multiple energy as in conventional common midpoint velocity analysis, the method uses the multiply reflected wavefield to further constrain the rms velocity and layer-thickness estimates. In particular, we determined that the nonphysical energy contains useful physical information. By using the nonphysical energy associated with the truncation of the source boundary and the crosscorrelation of reflected waves, a better-defined estimate of the rms velocity and layer thickness is achieved. Because this energy is excited far from the receiver pair, the technique may be ideally suited to long-offset seismic reflection data. We found that interferometric velocity analysis works best to characterize the first few layers beneath a receiver array. We have considered an acquisition configuration that can be used in a marine seismic setting.

The style of transverse faulting in the Dead Sea basin from seismic reflection data: The Amazyahu fault

2006 ◽  
Vol 55 (3) ◽  
pp. 129-139 ◽  
Author(s):  
Avihu Ginzburg ◽  
Moshe Reshef ◽  
Zvi Ben-Avraham ◽  
Uri Schattner
Keyword(s):  
Seismic Reflection ◽  
Dead Sea ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Dead Sea Basin ◽  
The Dead
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