Sub-Basalt Imaging Using Long Offset Reflection Seismic Data

Emsley, D.; Boswell, P. G. H.; Davis, Philip A.

doi:10.3997/2214-4609.201408220

Cited by 10 publications

(10 citation statements)

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“…Wide‐angle reflections that contain higher amplitude returns are beneficial to obtain quality reflection signals (e.g., Emsley, Boswell, and Davis ). Thus, our focus on reflected signals from offsets that approach imaging depths help produce interpretable results in the upper kilometre from all sites.…”

Section: Discussionmentioning

confidence: 99%

Seismic imaging through the volcanic rocks of the Snake River Plain: insights from Project Hotspot

Liberty

Schmitt

Shervais

2015

Geophysical Prospecting

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A B S T R A C THotspot: The Snake River Geothermal Drilling Project was undertaken to better understand geothermal systems across the Snake River Plain volcanic province. A series of surface and borehole seismic profiles were obtained to provide insights into volcanic stratigraphy and test the capabilities of engineering-scale seismic imaging in such terranes. The Kimberly site drilled through 1.9 km of mostly rhyolite, with thin sedimentary interbeds in the upper part of the section. The Kimama site drilled through 1.9 km of mostly basalt with sedimentary interbeds at ß200 m depth and 1700 m depth. The Mountain Home site contained numerous sediment and volcanic rock layers. Downhole and surface vibroseis seismic results suggest sedimentary interbeds at depth correspond with low-velocity, high-temperature zones that relate to reflections on seismic profiles. Our results suggest that eruption flow volumes can be estimated and flow boundaries can be imaged with surface seismic methods using relatively high-fold and wide-angle coverage. High-frequency attenuation is observed at all sites, and this deficit may be countered by acquisition design and a focus on signal processing steps. Separation of surface and body waves was obtained by muting, and the potential for large static effects was identified and addressed in processing. An accurate velocity model and lithology contacts derived from borehole information improved the confidence of our seismic interpretations.resources are located at depth with little to no surface expression, Project Hotspot has utilized geophysical, geological, and geochemical techniques to better understand the heat distribution and to identify pathways for fluid flow. Here, we discuss surface and borehole seismic data that were acquired to identify key lithologic and hydrogeologic boundaries that are relevant to geothermal exploration in this volcanic province. We focus our study on geothermal resources located in the upper 2 km. To date, seismic imaging within such complex geological environments has been limited, in part, by the large seismic velocity and density contrasts between volcanic rocks and the often thin sediment interbeds, and the large lateral variations in volcanic flow boundaries. These

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Section: Discussionmentioning

confidence: 99%

Seismic imaging through the volcanic rocks of the Snake River Plain: insights from Project Hotspot

Liberty

Schmitt

Shervais

2015

Geophysical Prospecting

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“…a basalt flow. This is interesting because unusually large offsets have been recorded in such cases ( Emsley et al 1998; Frühn et al 1998 ), and because the traditional small‐offset approximations based on the Taner and Koehler (1969) series then work very poorly. Figure 1 shows that in this case the rays bend strongly even at small offsets.…”

Section: Discussionmentioning

confidence: 99%

“…In the presence of a thin high‐velocity layer above the reflector, this series may approximate traveltimes with too little accuracy for stacking, for offsets as small as one or two times the reflector depth, depending of the number of terms. Therefore, it cannot be used for velocity analysis and stacking of wide‐aperture reflection data, such as the surveys acquired close to the Faeroes ( Frühn et al 1998 ) and the Hebrides ( Emsley, Boswell and Davis 1998), where offsets larger than 10 km are used.…”

Section: Introductionmentioning

confidence: 99%

Large‐offset approximation to seismic reflection traveltimes

Causse

Haugen

Rommel

2000

Geophysical Prospecting

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Conventional approximations of reflection traveltimes assume a small offset‐to‐depth ratio, and their accuracy decreases with increasing offset‐to‐depth ratio. Hence, they are not suitable for velocity analysis and stacking of long‐offset reflection seismic data. Assuming that the offset is large, rather than small, we present a new traveltime approximation which is exact at infinite offset and has a decreasing accuracy with decreasing offset‐to‐depth ratio. This approximation has the form of a series containing powers of the offset from 1 to −∞. It is particularly accurate in the presence of a thin high‐velocity layer above the reflector, i.e. in a situation where the accuracy of the Taner and Koehler series is poor. This new series can be used to gain insight into the velocity information contained in reflection traveltimes at large offsets, and possibly to improve velocity analysis and stacking of long‐offset reflection seismic data.

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“…This is a simple extension to conventional seismic surveying and can be implemented in the field with only a nominal increase in cost. This approach has been suggested by many authors (Ryu, 1997;Emsley et al, 1998;Fruehn et al, 1998;Wombell et al, 1999;Fliedner and White, 2001;Fruehn et al, 2001;Hu et al, 2003;Lau et al, 2007;Spitzer et al, 2008;Gallagher and Dromgoole, 2008;Masoomzadeh et al, 2010). Long-offset recordings are less affected by shot noise and forward modeling indicates large amplitude reflection events are generated at long offsets (Wombell et al, 1999;Dobson et al, 2003).…”

Section: Introductionmentioning

confidence: 91%

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

Zhou

Hatherly

Peters³

et al. 2014

GEOPHYSICS

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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.

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Sub-Basalt Imaging Using Long Offset Reflection Seismic Data

Cited by 10 publications

References 0 publications

Seismic imaging through the volcanic rocks of the Snake River Plain: insights from Project Hotspot

Seismic imaging through the volcanic rocks of the Snake River Plain: insights from Project Hotspot

Large‐offset approximation to seismic reflection traveltimes

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

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