Estimating quality factor and mean grain size of sediments from high-resolution marine seismic data
Estimating quality factor and mean grain size of sediments from high-resolution marine seismic data
Seismic quality factor has the potential to characterize sediment properties but seldom is used by the industry for offshore site investigations because of practical difficulties with reflection seismology (e.g., restricted bandwidth) and because of uncertainties in rock-physics models. A spectral-ratio analysis of high-resolution marine seismic data can determine a quality factor to within a 95% confidence of ±10 within the uppermost 30 m of unconsolidated marine sediments. Our spectral-ratio technique does not require assumptions on how attenuation scales with frequency. Emphasis is placed on interpretation of spectral signatures before applying an iteratively reweighted robust least-squares regression to subdue the effects of noise and local heterogeneities when determining the quality factor of a sediment package. We combined data from boomer and chirp sources toexamine attenuation over four octaves of frequency (0.5–8.0kHz) and to demonstrate that expanding the frequency range improves the precision and accuracy of quality-factor fits. We obtain frequency-independent quality factors with 95% confidence intervals of 135 (+12; ?10) and 107 (+6; ?5) for silty clays with mean grain sizes of 7.7 and 6.9 phi, respectively, and 63 (+10; ?7) for a modern sand deposit with mean grain size 2.5 phi, from the Solent (U. K.). Sediments with higher quality factors require more independent observations to achieve a desirable 95% confidence. We required only 45 traces over sands and 1250 traces over the lowest attenuating silty clays. By constructing an empirical model of quality factor against mean grain size from published sediment studies, the mean grain sizes of our Solent sediments can be located, and we find that quality factor can be used to distinguish between coarse grain-dominated and clay-dominated sediments.
geophysical techniques, grain size, offshore installations, rocks, sediments, seismology
G19-G28
Pinson, Luke J.W.
754e7438-c374-4c2a-946b-aab1b11bc998
Henstock, Timothy J.
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Dix, Justin K.
efbb0b6e-7dfd-47e1-ae96-92412bd45628
Bull, Jonathan M.
974037fd-544b-458f-98cc-ce8eca89e3c8
July 2008
Pinson, Luke J.W.
754e7438-c374-4c2a-946b-aab1b11bc998
Henstock, Timothy J.
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Dix, Justin K.
efbb0b6e-7dfd-47e1-ae96-92412bd45628
Bull, Jonathan M.
974037fd-544b-458f-98cc-ce8eca89e3c8
Pinson, Luke J.W., Henstock, Timothy J., Dix, Justin K. and Bull, Jonathan M.
(2008)
Estimating quality factor and mean grain size of sediments from high-resolution marine seismic data.
Geophysics, 73 (4), .
(doi:10.1190/1.2937171).
Abstract
Seismic quality factor has the potential to characterize sediment properties but seldom is used by the industry for offshore site investigations because of practical difficulties with reflection seismology (e.g., restricted bandwidth) and because of uncertainties in rock-physics models. A spectral-ratio analysis of high-resolution marine seismic data can determine a quality factor to within a 95% confidence of ±10 within the uppermost 30 m of unconsolidated marine sediments. Our spectral-ratio technique does not require assumptions on how attenuation scales with frequency. Emphasis is placed on interpretation of spectral signatures before applying an iteratively reweighted robust least-squares regression to subdue the effects of noise and local heterogeneities when determining the quality factor of a sediment package. We combined data from boomer and chirp sources toexamine attenuation over four octaves of frequency (0.5–8.0kHz) and to demonstrate that expanding the frequency range improves the precision and accuracy of quality-factor fits. We obtain frequency-independent quality factors with 95% confidence intervals of 135 (+12; ?10) and 107 (+6; ?5) for silty clays with mean grain sizes of 7.7 and 6.9 phi, respectively, and 63 (+10; ?7) for a modern sand deposit with mean grain size 2.5 phi, from the Solent (U. K.). Sediments with higher quality factors require more independent observations to achieve a desirable 95% confidence. We required only 45 traces over sands and 1250 traces over the lowest attenuating silty clays. By constructing an empirical model of quality factor against mean grain size from published sediment studies, the mean grain sizes of our Solent sediments can be located, and we find that quality factor can be used to distinguish between coarse grain-dominated and clay-dominated sediments.
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Published date: July 2008
Keywords:
geophysical techniques, grain size, offshore installations, rocks, sediments, seismology
Organisations:
Ocean and Earth Science
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Local EPrints ID: 59047
URI: http://eprints.soton.ac.uk/id/eprint/59047
ISSN: 0016-8033
PURE UUID: aba8a049-491d-4076-abb5-4bf698582b35
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Date deposited: 21 Aug 2008
Last modified: 16 Mar 2024 03:13
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Author:
Luke J.W. Pinson
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