Shear-wave seismic reflection studies of unconsolidated sediments in the near surface

Geophysics ◽  
2010 ◽  
Vol 75 (2) ◽  
pp. B59-B66 ◽  
Author(s):  
Seth S. Haines ◽  
Karl J. Ellefsen
Keyword(s):  
Water Table ◽  
Seismic Reflection ◽  
Love Wave ◽  
Unconsolidated Sediments ◽  
Sh Wave ◽  
Sh Waves ◽  
Near Surface ◽  
Strong Reflection ◽  
Bedrock Surface ◽  
Sand And Gravel

We have successfully applied of SH-wave seismic reflection methods to two different near-surface problems targeting unconsolidated sediments. At the former Fort Ord, where the water table is approximately [Formula: see text] deep, we imaged aeolian and marine aquifer and aquitard stratigraphy to a depth of approximately [Formula: see text]. We identified reflections from sand/clay and sand/silt interfaces and we mapped these interfaces along our transects. At an aggregate study site in Indiana, where the water table is at a depth of [Formula: see text], we imaged stratigraphy in alluvial sand and gravel, and observe a strong reflection from the [Formula: see text]-deep bedrock surface. In both cases, we exploited the high resolution potential of SH waves, their insensitivity to water content, and the possibility of reducing Love wave contamination by working along a roadway. We accomplished our results using only sledgehammer sources and simple data processing flows.

An ultrashallow SH‐wave seismic reflection experiment on a subsurface ground model

Geophysics ◽  
2001 ◽  
Vol 66 (4) ◽  
pp. 1097-1104 ◽  
Author(s):  
G. P. Deidda ◽  
R. Balia
Keyword(s):  
Water Table ◽  
Seismic Reflection ◽  
Cost Effective ◽  
Minimal Amount ◽  
Filling Material ◽  
Ground Model ◽  
Sh Wave ◽  
Low Velocity ◽  
Near Surface ◽  
Concrete Layer

An SH‐wave seismic reflection experiment was conducted to evaluate the feasibility and cost effectiveness of reflection imaging ultrashallow targets commonly encountered in engineering, groundwater, and environmental investigations. It was carried out on a purpose‐built subsurface ground model consisting of a concrete layer, at a depth from 2.85–5 m, and a low‐velocity overburden (<80 and 150 m/s for S‐ and P‐waves, respectively), constituted of filling material, with the water table 2.60 m deep. High‐quality CDP data, acquired by using a 10‐kg sledgehammer and newly designed horizontal detectors, allowed us to obtain an extremely detailed stacked section with a minimal amount of processing. Uncertainty in determining the depth and horizontal dimensions of the concrete model was estimated to be 0.2 and 0.3 m, respectively; however, the dominant frequencies lower than 150 Hz, the low‐transmission coefficient at the upper interface, and the relatively high velocity (900 m/s) of the concrete layer prevented us from resolving the layer thickness. The experiment demonstrates that when overburden materials exhibit low velocities (a common condition in near surface), the SH‐wave seismic reflection method is a reliable, detailed, and cost‐effective technique to image ultrashallow targets, even in disturbed material and below the water table.

Low‐cost geophysical investigations of a paleochannel aquifer in the Eastern Goldfields, Western Australia

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 690-700 ◽  
Author(s):  
Josef Holzschuh
Keyword(s):  
Water Table ◽  
Western Australia ◽  
Seismic Reflection ◽  
Wave Reflection ◽  
Gravity Data ◽  
P Wave ◽  
Gravity Modeling ◽  
S Wave ◽  
Sand And Gravel ◽  
Gravel Aquifer

Compressional (P) wave and shear (S) wave seismic reflection techniques were used to delineate the sand and gravel aquifer within a highly saline clay‐filled paleochannel in the Eastern Goldfields of Western Australia. The seismic refraction and gravity methods were also used to investigate the paleochannel. The unsaturated loose fine‐grained sand up to 10 m in depth at the surface is a major factor in degrading subsurface imaging. The seismic processing needed to be precise, with accurate static corrections and normal moveout corrections. Deconvolution enhanced the aquifer and other paleochannel reflectors. P‐wave reflection and refraction layer depths had good correlation and showed a total of six boundaries: (1) water table, (2) change in velocity (compaction) in the paleochannel sediments, (3) sand and gravel aquifer, (4) red‐brown saprolite and green saprolite boundary, (5) weathered bedrock, and (6) unweathered bedrock. P‐wave explosive and hammer sources were found to have similar signal characteristics, and the aquifer and bedrock were both imaged using the hammer source. The deep shots below the water table have the most broadband frequency response for reflections, but stacking clear reflections was difficult. The S‐wave reflection results showed high lateral and vertical resolution of the basal saprolite clay, the sand and gravel aquifer, and very shallow clays above the aquifer. The S‐wave reflection stacking velocities were 10–20% of the P‐waves, increasing the resolution of the S‐wave section. The gravity data were modelled to fit the known drilling and P‐wave seismic reflection depths. The refraction results did not identify the top of bedrock, so refraction depths were not used for the gravity modeling in this highly weathered environment. The final gravity model mapped the bedrock topography beyond the lateral extent of the seismic and drilling data.

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.

SH-wave velocity in a fiber-reinforced anisotropic layer overlying a gravitational heterogeneous half-space

2015 ◽  
Vol 11 (3) ◽  
pp. 386-400 ◽  
Author(s):  
Rajneesh Kakar
Keyword(s):  
Dispersion Equation ◽  
Half Space ◽  
Love Wave ◽  
Elastic Half Space ◽  
Wave Number ◽  
Fiber Reinforced ◽  
Sh Wave ◽  
Dimensionless Wave Number ◽  
Sh Waves ◽  
Content Type

Purpose – The purpose of this paper is to investigate the existence of SH-waves in fiber-reinforced layer placed over a heterogeneous elastic half-space. Design/methodology/approach – The heterogeneity of the elastic half-space is caused by the exponential variations of density and rigidity. As a special case when both the layers are homogeneous, the derived equation is in agreement with the general equation of Love wave. Findings – Numerically, it is observed that the velocity of SH-waves decreases with the increase of heterogeneity and reinforced parameters. The dimensionless phase velocity of SH-waves increases with the decreases of dimensionless wave number and shown through figures. Originality/value – In this work, SH-wave in a fiber-reinforced anisotropic medium overlying a heterogeneous gravitational half-space has been investigated analytically and numerically. The dispersion equation for the propagation of SH-waves has been observed in terms of Whittaker function and its derivative of second degree order. It has been observed that on the removal of heterogeneity of half-space, and reinforced parameters of the layer, the derived dispersion equation reduces to Love wave dispersion equation thereby validates the solution of the problem. The equation of propagation of Love wave in fiber-reinforced medium over a heterogeneous half-space given by relevant authors is also reduced from the obtained dispersion relation under the considered geometry.

Seismic reflection investigations of a bedrock surface buried under alluvium

Geophysics ◽  
1992 ◽  
Vol 57 (9) ◽  
pp. 1217-1227 ◽  
Author(s):  
Tom Goforth ◽  
Chris Hayward
Keyword(s):  
Water Table ◽  
Seismic Reflection ◽  
Body Wave ◽  
Shear Waves ◽  
Compressional Wave ◽  
Analytical Models ◽  
Baylor University ◽  
Order Of Magnitude ◽  
Ground Roll ◽  
Bedrock Surface

Seismic reflection techniques were used to characterize a bedrock surface buried under alluvium near a construction site on the campus of Baylor University in Waco, Texas. One of the objectives of the study was to determine if either compressional or shear seismic profiling could be used to reduce the number of engineering boreholes required to determine the bedrock depth and relief prior to building construction. The upper few meters of the alluvium is dry but the lower portion is below the water table, making the bedrock surface a difficult target for compressional waves. The compressional reflection coefficient at the water table is an order of magnitude greater than that at the bedrock surface, and the dry alluvium reduces the signal bandwidth such that the two reflections cannot be distinguished. Also, the multimode Rayleigh ground roll, traveling along the surface at about half the speed of the compressional wave, swamps the reflections. By using shear waves to profile the alluvium/bedrock interface, it was possible to avoid the water table and ground roll problems associated with compressional profiling. Walkaway survey results and analytical models presented demonstrate that shear waves do not “see” the water table, and masking of the bedrock target by the reflection at the dry/wet alluvium interface does not occur. Nor was ground roll a problem because the Love “ground roll,” traveling at a velocity almost as fast as the shear body wave, moves across the geophone spread before the return of the shallow reflections. Common depth point (CDP) and optimum offset shear profiles are presented. Uncertainty in determining the depth to bedrock from the seismic data was estimated to be 3 ft (0.9 m), which is sufficiently accurate to be useful in reducing the number of preconstruction boreholes required in the Brazos floodplain.

Coal seismic surveying over near-surface basalts: Experience from Central Queensland, Australia

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. B109-B122 ◽  
Author(s):  
Binzhong Zhou ◽  
Peter Hatherly ◽  
Troy Peters ◽  
Weijia Sun
Keyword(s):  
Surface Waves ◽  
Seismic Reflection ◽  
Unconsolidated Sediments ◽  
Successful Outcome ◽  
Low Velocity ◽  
Prestack Depth Migration ◽  
Seismic Line ◽  
Near Surface ◽  
Depth Migration ◽  
Seismic Surveying

Seismic reflection surveying in basalt-covered areas often fails to image underlying reflectors. To gain insights into the nature of the problem and obtain potential solutions, we have conducted experimental 2D seismic reflection and offset VSP surveys at two coal mines in the Bowen Basin of Australia. At the first mine, the basalt is relatively deep (114 m) and relatively thin (20 m). Conventional seismic acquisition and processing of a 2D seismic line provide poor results. However, upgoing reflections from layers below the basalt are clearly evident in the VSP survey and prestack depth migration is able to improve the continuity of the reflectors beneath the basalt. At the second mine, the 360 m wide basalt is at a depth of 40 m and has a thickness of about 40 m. It is fresh and unweathered and consists of multiple flows which are interlayered with unconsolidated sediments. Long-offset data acquisition combined with prestack depth migration was expected to produce satisfactory results but this is not the case. The associated VSP survey suggests that the problems at this mine are due to (1) the generation of complex downgoing and upgoing wave-fields within the basalt and (2) significant scattering of surface waves from outside the basalt at the margins of the basalt. Another problem is that the target coal seams are at about 300 m depth and the muting required to remove refraction events limited the effectiveness of the prestack depth migration. Reducing the strength of the surface waves through selection of an appropriate source and placement of shots at the base of the low-velocity zone (as had been the case at the first mine) will therefore improve the chances for a successful outcome. A Vibroseis survey subsequently undertaken at the second mine, which produced shot records with reduced surface waves, shows this to be the case.

Scattering of SH Wave on Periodic Cracks in an Infinite Plane of Piezoelectric/Piezomagnic Composite Materials

2012 ◽  
Vol 166-169 ◽  
pp. 3364-3368
Author(s):  
Wei Shi ◽  
Li Xia Ma
Keyword(s):  
Stress Intensity Factor ◽  
Integral Equation ◽  
Composite Materials ◽  
Piezoelectric Materials ◽  
Infinite Plane ◽  
Sh Wave ◽  
Scattering Problems ◽  
Sh Waves ◽  
Periodic Cracks ◽  
The Fourier Transform

In this paper, the scattering problems of SH waves on periodic cracks in an infinite of piezoelectric/piezomagnic composite materials bonded to an infinite of homogeneous piezoelectric materials is investigated, the Fourier transform techniques are used to reduce the problem to the solution of Hilbert singular integral equation, the latter is solved by Lobotto-Chebyshev and Gauss integral equation, at last, numerical results showed the effect of the frequency of wave, sizes and so on upon the normalized stress intensity factor.

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