9. Shallow/Near-Surface Structural Applications
Identification of Near-Surface Mud Volcano Structure Using Surface Wave
Jamhir Safani(1), Adam O'Neill(2), Toshifumi Matsuoka(1), Kyosuke Onishi(1), Yoshinori Sanada(1), Toshiyuki Yokota(3), Tomochika Tokunaga(4) and Kazuhiro Tanaka(5)
(1) Kyoto University, Dept. Civil and Earth Resources Engineering, Japan. (2) DownUnder GeoSolutions Pty Ltd, Australia. (3) Institute for Geo-Resources and Environment, AIST, Japan. (4) University of Tokyo, Dept. Environmental Studies, Japan. (5) Yamaguchi University, Earth Sciences, Japan.
Abstract
Reliable shear-wave velocity models of shallow subsurface structure are important for engineering site evaluations, lithological site characterization, and seismic exploration studies. The 2D shear-wave velocity structure of shallow subsurface site is inferred from lateral variation of surface wave dispersion. Surface waves are gathered by moving land-streamer and shot points array in a constant shot spacing along the survey line. The dispersion curve resulted at each shot point is inverted to provide 1D shear-wave velocity model which represents the average structure below the recording array. All the inverted 1D models are then plotted continuously at each spread midpoint to produce 2D shear-velocity section.Three 2D shear-wave velocity profiles from real data acquired at different seismic lines over a 'mud volcano' in Matsudai, Niigata, Japan are shown. The Line-1 profile shows the presence of soft zones (Vs = 50 - 110 m/s) between positions 27 - 60 m and 72 - 86 m at depth, possibly as main conduits for mud venting. The Line-2 profile shows the laterally sharper boundaries between stiff and soft zones at position 22 - 33 m at depth, which again represents the possibly main conduit. In the Line-3 profile, soft zone is present at depth from position 17 m to 44 m, bounded by a stiff zone in the beginning part of the profile. This last profile has well correlation with a resistivity image data along a parallel line. The stiff zones (Vs = 200 - 340 m/s) existed at all profiles represent a possibly weathered basement. The more significant result from surface wave investigation is delineation of thin soft zones at 1 - 4 m depth, which is difficult to be identified from resistivity result. The existences of the shallow soft zones are confirmed with the information obtained from drilling data, which were identified as a clay-water-gas mix. Thus, surface wave method has potential to be used for accurately estimating 2D/3D subsurface lateral variation, although stiffness reversals between layers exist.
Last modified: Mon May 29 10:31:37 2006