scholarly journals Borehole seismic‐source radiation in layered isotropic and anisotropic media: Boundary element modeling

Geophysics ◽  
1995 ◽  
Vol 60 (3) ◽  
pp. 735-747 ◽  
Author(s):  
Wenjie Dong ◽  
Michel Bouchon ◽  
M. Nafi Toksöz
Keyword(s):  
Boundary Element ◽  
Anisotropic Media ◽  
Seismic Source ◽  
Borehole Wall ◽  
Cement Interface ◽  
Secondary Sources ◽  
Source Radiation ◽  
Guided Modes ◽  
Borehole Seismic ◽  
Ti Media

In modeling waves radiated from a borehole seismic source in layered isotropic or anisotropic media, the commonly used numerical methods (e.g., finite difference and finite element) encounter difficulties because of the large scale difference between the borehole diameter and the formation extent. To get around this problem, we apply the indirect boundary element method to establish a general algorithm for modeling source radiation from open and cased boreholes in layered transversely isotropic (TI) media. The essence of the algorithm is to use discrete secondary sources (unknowns) on both sides of the borehole wall (formation/cement interface) to represent the influence of the interface on wave scattering, so that wave propagation inside and outside the borehole can be carried out by Green’s functions. The discrete distribution of the secondary sources is determined by matching boundary conditions on the borehole wall. Comparison with the discrete wavenumber method validates the implementation. Applications to fluid‐filled open and cased boreholes in three‐layer media demonstrate the creation of guided modes in low velocity layers. Presence of anisotropy complicates the guided modes as a result of dispersion and P‐ and SV‐waves coupling in homogeneous TI media. Presence of casing and cement enhances the visibility of the guided modes.

Borehole seismic‐source radiation pattern in transversely isotropic media

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 29-42 ◽  
Author(s):  
Wenjie Dong ◽  
M. Nafi Toksöz
Keyword(s):  
Radiation Effects ◽  
Low Frequency ◽  
Transversely Isotropic ◽  
Seismic Source ◽  
Wave Pattern ◽  
Radiation Patterns ◽  
S Wave ◽  
Source Radiation ◽  
Air Gun ◽  
Borehole Seismic

We extend previous discussions on crosswell tomography in anisotropic formations by deriving the radiation patterns of three typical downhole seismic sources (impulsive air gun or dynamite, wall‐clamped vertical vibrators, and cylindrical bender) inside a fluid‐filled borehole embedded in a transversely isotropic (TI) formation. The method of steepest descents, in conjuncture with the low‐frequency and far‐field assumptions, is applied to the exact displacement integrals of these sources to obtain their radiation patterns asymptotically. In spite of complications caused by quasi‐P‐ and quasi‐SV‐wave coupling and wavefront triplication in homogeneous TI media, the final results can still be expressed in slowness components determined by a ray direction, which is desired when source radiation effects are to be accounted for by ray‐based tomography techniques. Tests with the radiation patterns show that while the effect of anisotropy on P‐waves is moderate, its effect on the S‐wave pattern is significant even for slightly anisotropic formations. One can predict the S‐wave pattern from the sign of the Thomsen’s measure δ*.

Crosswell seismic imaging in a contaminated basalt aquifer

Geophysics ◽  
2004 ◽  
Vol 69 (1) ◽  
pp. 16-24 ◽  
Author(s):  
Thomas M. Daley ◽  
Ernest L. Majer ◽  
John E. Peterson
Keyword(s):  
Seismic Source ◽  
P Wave ◽  
Low Velocity ◽  
S Waves ◽  
High Attenuation ◽  
Well Spacing ◽  
Simultaneous Acquisition ◽  
Environmental Laboratory ◽  
New Type ◽  
Borehole Seismic

Multiple seismic crosswell surveys have been acquired and analyzed in a fractured basalt aquifer at Idaho National Engineering and Environmental Laboratory. Most of these surveys used a high‐frequency (1000–10,000 Hz) piezoelectric seismic source to obtain P‐wave velocity tomograms. The P‐wave velocities range from less than 3200 m/s to more than 5000 m/s. Additionally, a new type of borehole seismic source was deployed as part of the subsurface characterization program at this contaminated groundwater site. This source, known as an orbital vibrator, allows simultaneous acquisition of P‐ and S‐waves at frequencies of 100 to 400 Hz, and acquisition over larger distances. The velocity tomograms show a relationship to contaminant transport in the groundwater; zones of high contaminant concentration are coincident with zones of low velocity and high attenuation and are interpreted to be fracture zones at the boundaries between basalt flows. The orbital vibrator data show high Vp/Vs values, from 1.8 to 2.8. In spite of the lower resolution of orbital vibrator data, these data were sufficient for constraining hydrologic models at this site while achieving imaging over large interwell distances. The combination of piezoelectric data for closer well spacing and orbital vibrator data for larger well spacings has provided optimal imaging capability and has been instrumental in our understanding of the site aquifer's hydrologic properties and its scale of heterogeneity.

Performance evaluation of a high‐freqency borehole seismic source

1991 ◽  
Author(s):  
R. F. Ballard ◽  
R. D. Rechtien ◽  
K. L. Hambacker
Keyword(s):  
Performance Evaluation ◽  
Seismic Source ◽  
Borehole Seismic

scholarly journals A numerical simulation of the acoustic and elastic wavefields radiated by a source on a fluid‐filled borehole embedded in a layered medium

Geophysics ◽  
1993 ◽  
Vol 58 (4) ◽  
pp. 475-475 ◽  
Author(s):  
Michel Bouchon
Keyword(s):  
Layered Medium ◽  
Borehole Wall ◽  
Stoneley Wave ◽  
Wave Reflections ◽  
Volume Source ◽  
Linear System Of Equations ◽  
Method Of Calculation ◽  
Secondary Sources ◽  
Element Technique ◽  
Discrete Wavenumber Method

We present a method of calculation to simulate the propagation of acoustic and elastic waves generated by a borehole source embedded in a layered medium. The method is formulated as a boundary element technique where the Green’s functions are calculated by the discrete wavenumber method. The restrictive assumptions are that the borehole is cylindrical and that its axis runs normal to the layer interfaces. The physics of the method rely on Huygens’s principle that states that a diffracting boundary—the borehole wall in the present case—can be represented as a distribution of secondary sources. The borehole is discretized into small cylindrical elements and each element is represented by three sources: a volume source representing the wavefield diffracted in the fluid and two surface forces that give rise to the elastic wavefield radiated outside the borehole. The strength of each source is obtained by solving the linear system of equations that describes the boundary conditions at the borehole wall. The method is used to generate synthetic acoustic logs and to investigate the wavefield radiated into the formation. The simulations considered display the Stoneley wave reflections at the bed boundaries and show the importance of the diffraction that takes place where the borehole wall intersects the layer interfaces.

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