Ground viscosity and stiffness measurements for near surface seismic velocity
Robert Ley, Willem Adolfs, Ralph Bridle, Mohammad Al-Homaili, Aldo Vesnaver and Paul Ras
Journal name: Geophysical Prospecting
Issue: Vol 54, No 6, November 2006 pp. 751 - 762
Special topic: Section I – Madrid Workshop on Near-Surface 2005
Info: Article, PDF ( 942.66Kb )
In land surveys, the weathering layer can often distort the seismic signal due to it passing through rapid velocity and density changes, dispersion, scattering and inelastic absorption. In a simple spring-dashpot model for the earth response, an equivalent medium groups these complex phenomena into two parameters only; these are called ground viscosity and ground stiffness. The most recent controllers for vibrators can estimate both parameters. To validate these measurements, Saudi Aramco conducted an experiment measuring ground viscosity and stiffness from two different vibrator control systems over an area of varying terrain conditions, including unconsolidated sand and limestone outcrop. The two systems measured different values, but detected similar trends that correlated well with weathering conditions and surface geology, e.g. lower viscosity values on the outcrop than on the sand.
The ratio of ground viscosity to ground stiffness can approximate the shallow S-wave velocity, which we converted into P-wave velocity through calibration with sparse uphole data. Static corrections incorporating this velocity information somewhat improved the focusing of seismic time sections. This new approach does not require additional acquisition efforts, and can model shallow complex formations in arid areas where classical static methods often fail.