• Journal of the Korean earth science society
• /
• v.25 no.4
• /
• pp.270-276
• /
• 2004

Multi-channel Analysis of Surface Waves (MASW) and Seismic Cone Penetration Test (SCPT) have been recently developed to obtain S-wave velocity profiles which were conventionally investigated by a down-hole seismic survey. For unconsolidated sedimentary sites, we studied these three methods, and compared the results with a drilling log. All the methods showed that the changes in the S-wave velocities were consistent with the changes in the sedimentary facies. In addition, the SCPT was most sensitive to changes in sedimentary facies among the three profiles. The results of the SCPT showed that there exists a low velocity zone, which is mainly composed of clayey sand, at the depth of 8${\sim}$12m in the sediments.

• 한국지구물리탐사학회:학술대회논문집
• /
• 2005.05a
• /
• pp.287-291
• /
• 2005

Two-dimensional velocity tomograms of P- and S-waves were obtained by inverting traveltimes of first arrivals. The two sections of shear-wave velocity show similar features as a whole, with smaller values on the section from surface wave dispersions. Difficulties in picking SH-wave phases due to noise and later arrivals than P waves and PS converted waves are experienced. In addition, a flat layer model based on the surface wave inversion prohibits applications of the method where sgear wave velocities vary strongly in the lateral direction.

• Geophysics and Geophysical Exploration
• /
• v.12 no.1
• /
• pp.132-142
• /
• 2009

The Tamar rift valley runs through the City of Launceston, Tasmania. Damage has occurred to city buildings due to earthquake activity in Bass Strait. The presence of the ancient valley, the Tamar valley, in-filled with soft sediments that vary rapidly in thickness from 0 to 250mover a few hundreds metres, is thought to induce a 2D resonance pattern, amplifying the surface motions over the valley and in Launceston. Spatially averaged coherency (SPAC), frequency-wavenumber (FK) and horizontal to vertical spectrum ratio (HVSR) microtremor survey methods are combined to identify and characterise site effects over the Tamar valley. Passive seismic array measurements acquired at seven selected sites were analysed with SPAC to estimate shear wave velocity (slowness) depth profiles. SPAC was then combined with HVSR to improve the resolution of these profiles in the sediments to an approximate depth of 125 m. Results show that sediments thicknesses vary significantly throughout Launceston. The top layer is composed of as much as 20m of very soft Quaternary alluvial sediments with a velocity from 50 m/s to 125 m/s. Shear-wave velocities in the deeper Tertiary sediment fill of the Tamar valley, with thicknesses from 0 to 250m vary from 400 m/s to 750 m/s. Results obtained using SPAC are presented at two selected sites (GUN and KPK) that agree well with dispersion curves interpreted with FK analysis. FK interpretation is, however, limited to a narrower range of frequencies than SPAC and seems to overestimate the shear wave velocity at lower frequencies. Observed HVSR are also compared with the results obtained by SPAC, assuming a layered earth model, and provide additional constraints on the shear wave slowness profiles at these sites. The combined SPAC and HVSR analysis confirms the hypothesis of a layered geology at the GUN site and indicates the presence of a 2D resonance pattern across the Tamar valley at the KPK site.

• Journal of the Korean Geophysical Society
• /
• v.8 no.2
• /
• pp.67-74
• /
• 2005

Two-dimensional S-wave velocity sections from SH-wave refraction tomography and surface wave dispersions were obtained by inverting traveltimes of first arrivals and surface wave dispersions, respectively. For the purpose of comparison, a P-wave velocity tomogram was also obtained from a P-wave refraction profiling. P and Rayleigh waves generated by vertical blows on a plate with a sledgehammer were received by 100- and 4.5-Hz geophones, respectively. SH-waves generated by horizontal blows on both sides of a 50 kg timber were received by 8 Hz horizontal geophones. The shear-wave signals were enhanced subtracting data of left-side blows from ones of the right-side blows. Shear-wave velocities from tomography inversion of first-arrival times were compared with ones from inverting dispersion curves of Rayleigh waves. Although the two velocity sections look similar to each other in general, the one from the surface waves tends to have lower velocities. First arrival picking of SH waves is troublesome since P and PS-converted waves arrive earlier than SH waves. Application of the surface wave method, on the other hand, is limited where lateral variation of subsurface tructures is not mild.