• journal_title：Geophysics
• Contributor：Maria-Daphne Mangriotis ; James W. Rector III ; E. Frederic Herkenhoff ; John C. Neu
• Publisher：Society of Exploration Geophysicists
• Date：2013-03-01
• Format：text/html
• Language：en
• Identifier：10.1190/geo2012-0174.1
• journal_abbrev：Geophysics
• issn：0016-8033
• volume：78
• issue：2
• firstpage：B49
• section：Case Histories

We studied scattering versus intrinsic attenuation estimates in the vadose zone from a shallow VSP experiment conducted in the Lawrence Livermore National Laboratory (LLNL) facility. Using permanent downhole geophones and a vertical impact source, we estimated effective attenuation of the downgoing transmitted P-wave. We compared theoretical scattering attenuation estimates and finite-difference synthetics to the measured field <mml:math display="inline"><mml:mrow><mml:mi>Q</mml:mi></mml:mrow></mml:math>Q values (<mml:math display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mi>Q</mml:mi></mml:mrow></mml:math>1/Q being a measure of attenuation). Using a selected range of impedance profiles of variance typical for a sedimentary basin, our estimates of <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mtext>scattering</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>Qscattering are in the order of 20–85. Given the short propagation pathlengths involved (<mml:math display="inline"><mml:mrow><mml:mo form="prefix">∼</mml:mo><mml:mn>30</mml:mn><mml:mrow><mml:mo> </mml:mo></mml:mrow><mml:mrow><mml:mo> </mml:mo></mml:mrow><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>∼30  m), we show that attenuation due to lateral heterogeneity is not significant. We analyzed additional distorting factors, including near-field presence, local impedance effects, and interference from reflections originating beneath receivers, and found that they may significantly impact attenuation measurement in near-surface studies, and result in biased <mml:math display="inline"><mml:mrow><mml:mi>Q</mml:mi></mml:mrow></mml:math>Q values. From a comparison of the analyzed ranges of <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mtext>scattering</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>Qscattering to the measured <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mtext>effective</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>Qeffective values, we deduced <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mtext>intrinsic</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>Qintrinsic, which is consistently low, but whose inferred frequency dependence depends strongly on the scattering model assumed. The ranges for the <mml:math display="inline"><mml:mrow><mml:mi>Q</mml:mi></mml:mrow></mml:math>Q factor resulting from scattering and distorting factors, and the intrinsic <mml:math display="inline"><mml:mrow><mml:mi>Q</mml:mi></mml:mrow></mml:math>Q value were estimated as, respectively, 4–7, and 7–4. We identified two potential mechanisms which could lead to low <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mtext>intrinsic</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>Qintrinsic values in the vadose zone: patchy saturation and squirt flow. We found through viscoelastic 3D synthetic modeling using a standard linear solid (SLS), that the field <mml:math display="inline"><mml:mrow><mml:mi>Q</mml:mi></mml:mrow></mml:math>Q frequency dependence can be reproduced, although nonuniquely, for the studied range of impedance variance.