Ground viscosity and stiffness measurements for near surface seismic velocity

Abstract

A B S T R A C TIn 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.

Geostatistical integration of near‐surface geophysical data

“…Recent uphole measurements elsewhere in the Arabian Peninsula showed that seasonal variations may cause shallow velocity changes up to 400 m/s. A similar dependence was observed by Ley et al (2006) while processing vibrator-plate attributes. Interpreting a variogram like this one, in order to choose an optimal range for kriging, is often quite questionable.…”

A Self-Adjustable Input Genetic Algorithm for the Near-Surface Problem in Geophysics

“…Strong wave field distortions give rise to amplitude changes, time shifts, and scattered noise degrading the seismic image quality. This so-called near-surface problem has been recognized since the early days of seismic exploration [6]- [11]. Fig.…”

Recent Technologies for Land Seismic Imaging Over a Complex Near Surface Overburden

“…The basic idea of exploiting the vibrator plate response dates back to the 80's, introduced by the pioneering papers of Lerwill (1981) and Sallas (1984), but only more recently Ali et al (2003) and Ley et al (2006) applied it on a larger scale, validating its practical value. The Earth response is approximated by a simple model composed of a spring and a dash-pot in parallel, characterized by a stiffness and viscosity factor, respectively.…”

“…Indeed, this approach overcomes only partially the problem affecting refraction statics, i.e., the so-called Љhidden layer(s)Љ, while it works optimally when lateral and vertical velocity variations are mild. When lateral changes are sharp instead, other recent methods can be used and are reviewed in this paper: for example, the integration of remote sensing in the near-surface modelling (Vesnaver et al 2009), and the use of geostatistics (co-kriging) for extracting information from the Vibroseis response, such as the stiffness and viscosity of the ground (Ley et al 2006).…”