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Invited Columns

	Michael Asten, Professorial Fellow, Monash University, Melbourne, Australia <michael.asten@sci.monash.edu.au>

 
The microtremor wave-field consists predominantly of fundamental-mode surface waves which can provide seismic sounding information. The most common sources of micro-tremor seismic energy are road traffic, industrial machinery, and meteorological sources such as wave action. Velocity studies with the microtremor wave-field enable passive seismic investigations, which are interpreted to yield a shear-wave velocity-depth profile; the technique is an analog of the better-known electrically passive magneto-telluric exploration technique.

The microtremor array method works well in areas where conventional seismic methods are difficult to justify for reasons of cultural noise, environmental restraints and safety. Recent studies demonstrate the usefulness of the microtremor array method over a range of tasks including: estimation of thickness and shear-velocity of sands overlying lower-velocity clays in 50 to 100 m of Quaternary cover in the Perth basin, the study of a 15 m thickness of high-velocity basalt overlying river sediments in the Melbourne area, and the assessment of thickness and shear-velocity of sediments to 1,000 m depth in the Santa Clara valley, California (http://erp-web.er.usgs.gov/reports/annsum/vol46/nc/nc_vol46.htm).

Future possible exploration applications include seismic soundings for reconnaissance mapping of the thickness of regolith over prospective basement rocks, and independent estimation of sediment thickness for complementing EM sounding data in salinity studies.

	Thomas Bohlen, Assistant Professor, Kiel University, Germany <tbohlen@geophysik.uni-kiel.de>

 
For marine geotechnical applications Scholte waves are often utilized to derive shear wave velocities of shallow water marine sediments. Scholte waves have similar properties to Rayleigh waves, so that most processing and inversion schemes developed for land seismic Rayleigh waves can be applied to Scholte waves as well. Scholte waves acquired in water not deeper than 25 m are generally characterized by a few distinct modes (1–4) observed at frequencies below 30 Hz. They can be efficiently excited by airgun sources located near the water surface, and can be recorded by either ocean-bottom seismometers (OBS) deployed at the . . . [Full Text of this Article]