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.
Seismic imaging characteristics of a prototype electrically driven, linear synchronous motor (LSM) based, vertical-force seismic vibrator (“e-vib”) were evaluated at a site in the Netherlands. The system weighs 1.65 t and excites seismic signals with a peak force of 6.7 kN. Data were recorded along two collocated geophone based-nodal and landstreamer MEMS-based (micro-electromechanical sensors) 2D seismic profiles. To obtain a broad bandwidth dataset, the e-vib operated with a 1-200 Hz linear sweep. Shot gathers of the merged nodal-landstreamer dataset show good quality seismic data of a broadband nature. The processed merged dataset demonstrates high-resolution reflections of the stratigraphic members from approximately 200 m to 2 km, with visible reflections as deep as 2.5 to 2.9 km. As a reference, we also processed a legacy 3D microspread dataset acquired at the same site with a magnitude stronger (14.1 t, 67.5 kN) hydraulic vibrator. Comparison of our nodal-landstreamer seismic section versus 2D slices extracted from the processed microspread volume suggests similar signal penetration depth and same key marker horizons seen in both. Analysis of the reaction mass and base-plate accelerometer signals recorded with the e-vib source operating both on grass and asphalt surfaces shows that the e-vib has low total harmonic distortion. The results obtained indicate that, although relatively small, the e-vib is capable of generating high-quality, broadband seismic data.
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