Non-uniqueness with refraction inversion - the Mt Bulga shear zone

ABSTRACT The tau‐p inversion algorithm is widely employed to generate starting models with many computer programs that implement refraction tomography. However, this algorithm can frequently fail to detect even major lateral variations in seismic velocities, such as a 50 m wide shear zone, which is...

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Veröffentlicht in:Geophysical Prospecting Jg. 58; H. 4; S. 561 - 575
1. Verfasser: Palmer, Derecke
Format: Journal Article
Sprache:Englisch
Veröffentlicht: Oxford, UK Blackwell Publishing Ltd 01.07.2010
Blackwell
Schlagworte:
Algorithms
Applied geophysics
Computer programs
Data processing
Earth sciences
Earth, ocean, space
Exact sciences and technology
Internal geophysics
Inversion
Inversions
Mathematical models
Prospecting
Refraction
Seismic phenomena
Seismics
Shear zone
Tomography
Velocity analysis
reversals
waves
algorithms
models
inverse problem
computer programs
geologic sections
spectral analysis
accuracy
weathering
velocity
seismic profiles
velocity analysis
lateral variations
boreholes
amplitude
tomography
shear zones
travel time
reduction
ISSN:0016-8025, 1365-2478
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Zusammenfassung:ABSTRACT The tau‐p inversion algorithm is widely employed to generate starting models with many computer programs that implement refraction tomography. However, this algorithm can frequently fail to detect even major lateral variations in seismic velocities, such as a 50 m wide shear zone, which is the subject of this study. By contrast, the shear zone is successfully defined with the inversion algorithms of the generalized reciprocal method. The shear zone is confirmed with a 2D analysis of the head wave amplitudes, a spectral analysis of the refraction convolution section and with numerous closely spaced orthogonal seismic profiles recorded for a later 3D refraction investigation. Further improvements in resolution, which facilitate the recognition of additional zones with moderate reductions in seismic velocity, are achieved with a novel application of the Hilbert transform to the refractor velocity analysis algorithm. However, the improved resolution also requires the use of a lower average vertical seismic velocity, which accommodates a velocity reversal in the weathering. The lower seismic velocity is derived with the generalized reciprocal method, whereas most refraction tomography programs assume vertical velocity gradients as the default. Although all of the tomograms are consistent with the traveltime data, the resolution of each tomogram is comparable only with that of the starting model. Therefore, it is essential to employ inversion algorithms that can generate detailed starting models, where detailed lateral resolution is the objective. Non‐uniqueness can often be readily resolved with head wave amplitudes, attribute processing of the refraction convolution section and additional seismic traverses, prior to the acquisition of any borehole data. It is concluded that, unless specific measures are taken to address non‐uniqueness, the production of a single refraction tomogram that fits the traveltime data to sufficient accuracy does not necessarily demonstrate that the result is either correct, or even the most probable.
Bibliographie:ark:/67375/WNG-MTRVG010-Z
ArticleID:GPR855
istex:0684436E8918737A2A81AABBC10B7E82F42D7597
ObjectType-Article-2
SourceType-Scholarly Journals-1
ObjectType-Feature-1
content type line 23
ISSN:0016-8025
1365-2478
DOI:10.1111/j.1365-2478.2009.00855.x