scholarly journals Common-reflection-surface imaging of shallow and ultrashallow reflectors

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. B177-B185 ◽  
Author(s):  
Gian Piero Deidda ◽  
Enzo Battaglia ◽  
Zeno Heilmann
Keyword(s):  
Wave Reflection ◽  
High Sensitivity ◽  
P Wave ◽  
Data Sets ◽  
Velocity Analysis ◽  
Sh Wave ◽  
Low Velocity ◽  
Near Surface ◽  
Significant Step ◽  
Common Reflection Surface

We analyzed the feasibility of the common-reflection-surface (CRS) stack for near-surface surveys as an alternative to the conventional common midpoint (CMP) stacking procedure. The data-driven, less user-interactive CRS method could be more cost efficient for shallow surveys, where the high sensitivity to velocity analysis makes data processing a critical step. We compared the results for two field data sets collected to image shallow and ultrashallow reflectors: an example of shallow P-wave reflection for targets in the first few hundred meters, and an example of SH-wave reflection for targets in the first 10 m. By processing the shallow P-wave records using the CMP method, we imaged several nearly horizontal reflectors with onsets from 60 to about 250 ms. The CRS stack produced a stacked section more suited for a subsurface interpretation, without any preliminary formal and time-consuming velocity analysis, because the imaged reflectors possessed greater coherency and lateral continuity. With CMP processing of the SH-wave records, we imaged a dipping bedrock interface below four horizontal reflectors in unconsolidated, very low velocity sediments. The vertical and lateral resolution was very high, despite the very shallow depth: the image showed the pinchout of two layers at less than 10 m depth. The numerous traces used by the CRS stack improved the continuity of the shallowest reflector, but the deepest overburden reflectors appear unresolved, with not well-imaged pinchouts. Using the kinematic wavefield attributes determined for each stacking operation, we retrieved velocity fields fitting the stacking velocities we had estimated in the CMP processing. The use of CRS stack could be a significant step ahead to increase the acceptance of the seismic reflection method as a routine investigation method in shallow and ultrashallow seismics.

scholarly journals Finite difference modelling to evaluate seismic <i>P</i> wave and shear wave field data

2014 ◽  
Vol 6 (2) ◽  
pp. 2169-2213
Author(s):  
T. Burschil ◽  
T. Beilecke ◽  
C. M. Krawczyk
Keyword(s):  
Finite Difference ◽  
Data Quality ◽  
Shear Wave ◽  
Field Data ◽  
Wave Reflection ◽  
P Wave ◽  
Sh Wave ◽  
Near Surface ◽  
Reflection Seismic ◽  
Seismic Measurements

Abstract. High-resolution reflection seismic methods are an established non-destructive tool for engineering tasks. In the near surface, shear wave reflection seismic measurements usually offer a higher spatial resolution in the same effective signal frequency spectrum than P wave data, but data quality varies more strongly. To discuss the causes of these differences, we investigated a P wave and a SH wave reflection seismic profile measured at the same location on Föhr island, and applied reflection seismic processing to the field data as well as finite difference modelling of the seismic wavefield (SOFI FD-code). The simulations calculated were adapted to the acquisition field geometry, comprising 2 m receiver distance and 4 m shot distance along the 1.5 km long P wave and 800 m long SH wave profiles. A Ricker-Wavelet and the use of absorbing frames were first order model parameters. The petrophysical parameters to populate the structural models down to 400 m depth are taken from borehole data, VSP measurements and cross-plot relations. The first simulation of the P wave wavefield was based on a simplified hydrogeological model of the survey location containing six lithostratigraphic units. Single shot data were compared and seismic sections created. Major features like direct wave, refracted waves and reflections are imaged, but the reflectors describing a prominent till layer at ca. 80 m depth was missing. Therefore, the P wave input model was refined and 16 units assigned. These define a laterally more variable velocity model (vP = 1600–2300 m s−1) leading to a much better reproduction of the field data. The SH wave model was adapted accordingly but only led to minor correlation with the field data and produced a higher signal-to-noise ratio. Therefore, we suggest to consider for future simulations additional features like intrinsic damping, thin layering, or a near surface weathering layer. These may lead to a better understanding of key parameters determining the data quality of near-surface seismic measurements.

scholarly journals Finite-difference modelling to evaluate seismic P-wave and shear-wave field data

Solid Earth ◽  
2015 ◽  
Vol 6 (1) ◽  
pp. 33-47 ◽  
Author(s):  
T. Burschil ◽  
T. Beilecke ◽  
C. M. Krawczyk
Keyword(s):  
Wave Field ◽  
Shear Wave ◽  
Field Data ◽  
P Wave ◽  
Sh Wave ◽  
Surface Shear ◽  
Near Surface ◽  
Reflection Seismic ◽  
Surface Shear Wave ◽  
Seismic Measurements

Abstract. High-resolution reflection seismic methods are an established non-destructive tool for engineering tasks. In the near surface, shear-wave reflection seismic measurements usually offer a higher spatial resolution in the same effective signal frequency spectrum than P-wave data, but data quality varies more strongly. To discuss the causes of these differences, we investigated a P-wave and a SH-wave seismic reflection profile measured at the same location on the island of Föhr, Germany and applied seismic reflection processing to the field data as well as finite-difference modelling of the seismic wave field. The simulations calculated were adapted to the acquisition field geometry, comprising 2 m receiver distance (1 m for SH wave) and 4 m shot distance along the 1.5 km long P-wave and 800 m long SH-wave profiles. A Ricker wavelet and the use of absorbing frames were first-order model parameters. The petrophysical parameters to populate the structural models down to 400 m depth were taken from borehole data, VSP (vertical seismic profile) measurements and cross-plot relations. The simulation of the P-wave wave-field was based on interpretation of the P-wave depth section that included a priori information from boreholes and airborne electromagnetics. Velocities for 14 layers in the model were derived from the analysis of five nearby VSPs (vP =1600–2300 m s-1). Synthetic shot data were compared with the field data and seismic sections were created. Major features like direct wave and reflections are imaged. We reproduce the mayor reflectors in the depth section of the field data, e.g. a prominent till layer and several deep reflectors. The SH-wave model was adapted accordingly but only led to minor correlation with the field data and produced a higher signal-to-noise ratio. Therefore, we suggest to consider for future simulations additional features like intrinsic damping, thin layering, or a near-surface weathering layer. These may lead to a better understanding of key parameters determining the data quality of near-surface shear-wave seismic measurements.

Rapid map and inversion of P-SV waves

Geophysics ◽  
1991 ◽  
Vol 56 (6) ◽  
pp. 859-862 ◽  
Author(s):  
Robert R. Stewart
Keyword(s):  
Spatial Resolution ◽  
Seismic Data ◽  
P Wave ◽  
Rock Properties ◽  
Low Velocity ◽  
P Waves ◽  
Near Surface ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
And Inversion

Multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional P‐wave sections and to find and use rock properties associated with shear waves (e.g. Dohr, 1985; Danbom and Dominico, 1986). Mode‐converted (P-SV) waves hold a special interest for several reasons: They are generated by conventional P‐wave sources and have only a one‐way travel path as a shear wave through the typically low velocity and attenuative near surface. For a given frequency, they will have a shorter wavelength than the original P wave, and thus offer higher spatial resolution; this has been observed in several vertical seismic profiling (VSP) cases (e.g., Geis et al., 1990). However, for surface seismic data, converted waves are often found to be of lower frequency than P-P waves (e.g., Eaton et al., 1991).

From migration to inversion velocity analysis

Geophysics ◽  
2017 ◽  
Vol 82 (3) ◽  
pp. S207-S223 ◽  
Author(s):  
Hervé Chauris ◽  
Emmanuel Cocher
Keyword(s):  
Objective Function ◽  
Synthetic Data ◽  
Velocity Model ◽  
Specific Power ◽  
Model Parameters ◽  
Data Sets ◽  
Velocity Analysis ◽  
Low Velocity ◽  
Image Domain ◽  
New Approach

Migration velocity analysis (MVA) is a technique defined in the image domain to determine the background velocity model controlling the kinematics of wave propagation. In the presence of discontinuous interfaces, the velocity gradient used to iteratively update the velocity model exhibits spurious oscillations. For more stable results, we replace the migration part by an inversion scheme. By definition, migration is the adjoint of the Born modeling operator, whereas inversion is its asymptotic inverse. We have developed new expressions in 1D and 2D cases based on two-way wave-equation operators. The objective function measures the quality of the images obtained by inversion in the extended domain depending on the subsurface offset. In terms of implementation, the new approach is very similar to classic MVA. A 1D analysis found that oscillatory terms around the interface positions can be removed by multiplying the inversion result with the velocity at a specific power before evaluating the objective function. Several 2D synthetic data sets are discussed through the computation of the gradient needed to update the model parameters. Even for discontinuous reflectivity models, the new approach provides results without artificial oscillations. The model update corresponds to a gradient of an existing objective function, which was not the case for the horizontal contraction approach proposed as an alternative to deal with gradient artifacts. It also correctly handles low-velocity anomalies, contrary to the horizontal contraction approach. Inversion velocity analysis offers new perspectives for the applicability of image-domain velocity analysis.

scholarly journals Reanalysis of and attribution to near-surface ozone concentrations in Sweden during 1990–2013

2017 ◽  
Author(s):  
Camilla Andersson ◽  
Heléne Alpfjord ◽  
Lennart Robertson ◽  
Per Erik Karlsson ◽  
Magnuz Engardt
Keyword(s):  
Transport Model ◽  
Linear Trend ◽  
Surface Ozone ◽  
Vegetation Indices ◽  
High Sensitivity ◽  
Anthropogenic Emissions ◽  
Data Sets ◽  
Chemistry Transport Model ◽  
Near Surface ◽  
Surface O3

Abstract. We have constructed two data sets of hourly resolution reanalyzed near-surface ozone (O3) concentrations for the period 1990–2013 for Sweden. Long-term simulations from a chemistry-transport model (CTM) covering Europe were combined with hourly ozone concentration observations at Swedish and Norwegian background measurement sites using data assimilation. The reanalysis data sets show improved performance than the original CTM when compared to independent observations. In one of the reanalyzes we included all available hourly near-surface O3 observations, whilst in the other we carefully selected time-consistent observations in order to avoid introducing artificial trends. Based on the second reanalysis we investigated statistical aspects of the near-surface O3 concentration, focusing on the linear trend over the 24 year period. We show that high near-surface O3 concentrations are decreasing and low O3 concentrations are increasing, which is mirrored by observed improvement of many health and vegetation indices (apart from those with a low threshold). Using the chemistry-transport model we also conducted sensitivity simulations to quantify the causes of the observed change, focusing on three processes: change in hemispheric background, meteorology and anthropogenic emissions (Swedish and other European). The rising low concentrations of near-surface O3 in Sweden are caused by a combination of all three processes, whilst the decrease in the highest O3 concentrations is caused by O3 precursor emissions reductions. While studying the relative impact of anthropogenic emissions changes, we identified systematic differences in the modelled trend compared to observations that must be caused by incorrect trends in the utilised emissions inventory or by too high sensitivity of our model to emissions changes.

Depth‐domain velocity analysis in VTI media using surface P-wave data: Is it feasible?

Geophysics ◽  
2001 ◽  
Vol 66 (3) ◽  
pp. 897-903 ◽  
Author(s):  
Yves Le Stunff ◽  
Vladimir Grechka ◽  
Ilya Tsvankin
Keyword(s):  
Vertical Velocity ◽  
Heterogeneous Media ◽  
Wave Reflection ◽  
P Wave ◽  
Model Parameters ◽  
Velocity Analysis ◽  
Depth Migration ◽  
Velocity Models ◽  
Trade Offs ◽  
Vti Media

The main difficulties in anisotropic velocity analysis and inversion using surface seismic data are associated with the multiparameter nature of the problem and inherent trade‐offs between the model parameters. For the most common anisotropic model, transverse isotropy with a vertical symmetry axis (VTI media), P-wave kinematic signatures are controlled by the vertical velocity V0 and the anisotropic parameters ε and δ. However, only two combinations of these parameters—NMO velocity from a horizontal reflector Vnmo(0) and the anellipticity coefficient η—can be determined from P-wave reflection traveltimes if the medium above the reflector is laterally homogeneous. While Vnmo(0) and η are sufficient for time‐domain imaging in VTI media, they cannot be used to resolve the vertical velocity and build velocity models needed for depth migration. Here, we demonstrate that P-wave reflection data can be inverted for all three relevant VTI parameters (V0, ε and δ) if the model contains nonhorizontal intermediate interfaces. Using anisotropic reflection tomography, we carry out parameter estimation for a two‐layer medium with a curved intermediate interface and reconstruct the correct anisotropic depth model. To explain the success of this inversion procedure, we present an analytic study of reflection traveltimes for this model and show that the information about the vertical velocity and reflector depth was contained in the reflected rays which crossed the dipping intermediate interface. The results of this work are especially encouraging because the need for depth imaging (such as prestack depth migration) arises mostly in laterally heterogeneous media. Still, we restricted this study to a relatively simple model and constrained the inversion by assuming that one of the layers is isotropic. In general, although lateral heterogeneity does create a dependence of P-wave reflection traveltimes on the vertical velocity, there is no guarantee that for more complicated models all anisotropic parameters can be resolved in a unique fashion.

Plane‐wave reflection coefficients for gas sands at nonnormal angles of incidence

Geophysics ◽  
1984 ◽  
Vol 49 (10) ◽  
pp. 1637-1648 ◽  
Author(s):  
W. J. Ostrander
Keyword(s):  
Reflection Coefficient ◽  
Poisson’S Ratio ◽  
Wave Reflection ◽  
P Wave ◽  
Poisson's Ratio ◽  
Angle Of Incidence ◽  
High Porosity ◽  
Low Velocity ◽  
Poisson's Ratios ◽  
Poisson’S Ratios

The P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson’s ratio in the two media. For moderate angles of incidence, the relative change in reflection coefficient is particularly significant when Poisson’s ratio differs greatly between the two media. Theory and laboratory measurements indicate that high‐porosity gas sands tend to exhibit abnormally low Poisson’s ratios. Embedding these low‐velocity gas sands into sediments having “normal” Poisson’s ratios should result in an increase in reflected P-wave energy with angle of incidence. This phenomenon has been observed on conventional seismic data recorded over known gas sands.

Imaging the upper part of the Red Lake greenstone belt, northwestern Ontario, with 3-D traveltime tomography

2006 ◽  
Vol 43 (7) ◽  
pp. 849-863 ◽  
Author(s):  
Fafu Zeng ◽  
Andrew J Calvert
Keyword(s):  
Fault Zone ◽  
Greenstone Belt ◽  
Hanging Wall ◽  
Shear Zones ◽  
P Wave ◽  
Low Velocity ◽  
Seismic Line ◽  
Near Surface ◽  
Metasedimentary Rocks ◽  
Red Lake

Seismic reflection line 2B was shot across the Archean Red Lake greenstone belt and Sydney Lake fault zone that marks the northern boundary of the English River metasedimentary belt, as part of the Western Superior Lithoprobe transect. Three-dimensional tomographic inversion of first arrival traveltimes recorded in this survey delineate the subsurface to depths as great as 1.5 km around this crooked two-dimensional seismic line. Within the Red Lake greenstone belt, P-wave velocities of 6.2–7.0 km s–1 occur at 500 m depth in the Mesoarchean Balmer assemblage, clearly distinguishable from the lower velocities of 5.1–6.1 km s–1 of the Neoarchean Confederation assemblage. Although the overall range of velocities in the metasedimentary rocks of the English River subprovince is similar to that found in the Confederation assemblage, lower velocities of 5.1–5.4 km s–1 are found in the upper 300 m of the metasedimentary rocks. In particular, two 2–3 km wide, east-northeast-striking zones of low velocity are associated with the Sydney Lake fault zone and the Pakwash Lake fault zone. Correlation of the velocities with the coincident reflection section suggests that these two faults delineate a fault-bounded block in the hanging wall of a more northerly fault zone that crops out within the Uchi subprovince. Anomalous regions of low velocity, which occur at the boundary between the Confederation and Balmer assemblages, and within the Balmer assemblage, may also be related to shear zones that have minimal near-surface expression, felsic lithologies, or hydrothermal alteration of the basalts.

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