AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
(1) GDPD, Saudi Aramco, Dhahran, Saudi Arabia.
Understanding the geology is vital in modeling the near-surface. The geophysical implications are rapidly varying vertical and lateral velocities, complete with velocity inversions. Modeling strategies include elevation correction with sand dune term, and use of refraction statics and tomography. The modeling challenges include shingling, acquisition geometry, long wavelength and imaging of complex structures.
Shingling is a near-surface generated en-echelon effect due to shallow inversions, outcropping refractors, multiple fast thin refractors, or interference. Shingling creates medium and long wavelength anomalies due to poor definition of the layer velocity field. Strategies for model building include picking a consistent refractor and averaging all shallow layers as a single weathering layer.
The recording of near offsets is very important not only for data processing, but also for refraction tomography. Should the near offsets be unavailable then the under sampled lateral velocity variations will be propagated down into the model. For refraction statics the loss of those offsets creates poor models unless middle or far offsets are picked for time arrivals. Where receivers are laid around an obstruction, the receiver offsets have larger travel times which can be interpreted as time delays due to low velocity material.
Time sections may display near-surface features which increase in magnitude with depth. In this paper a time section will display four major features with an apex at 800 msec. At each of these apexes there is the characteristic “bow tie,” of a buried focus point. These anomalies are not corrected by near-surface static corrections. Examination of a source gather shows no additional travel times on the first arrivals. A dynamic correction is required which depends on offset. Prestack migration and analysis involving techniques such as redatuming or common focal point processing is the solution for this issue.
Near-surface problems are caused by unconsolidated material and geology. Understanding the causes of the challenges is essential in knowing how to solve them. There is not a single solution to modeling the near-surface successfully everywhere. The physics of the regolith must dictate strategic modeling decisions, such as the avoidance of refraction methods where there are velocity inversions. Finally, even with a perfect near-surface model, there may still be time imaging problems due to buried paleo-geomorphology.