Ralph Bridle

Development of near surface models in Saudi Arabia for low relief structures and complex near surface geology

In the past the geologic information of the near surface in Saudi Arabia was not as critical an issue due to the fact that the prospective geologic structures tended to be much larger than the near surface statics variation. Today, the majority of the prospects are lower relief structures. The static corrections in the near surface statics can include medium to long wavelength statics up to +/80 ms in most prospective areas in Saudi Arabia. Several approaches have been developed to resolve these near surface issues and to better image these low relief structures. These methods are the frozen model, layer modeling and geostatistics. Using these various methods the static uncertainty is reduced in the near surface which in turn gives a more accurate sub-surface image.

Gaining a Geostatistical Advantage In Near-surface Modeling

The near-surface geology of Saudi Arabia is very complex and challenging. To resolve the near surface issues, several methods have been developed which reduce the uncertainty of static corrections. Two layer models correct for vertical time to base of weathering and the sub-weathering to datum. The refractor depth and velocity is initially derived from the intercept-time analysis of source gathers (Bridle et al., 2003).

Near-surface layer model using first break arrivals applied to 3D blocks

Summary A single-velocity layer statics model is available throughout the kingdom of Saudi Arabia. While this simple model suffices for most areas and large structures, it fails in situations where the surface topography is complex, the seismic datum crops out above the surface, or where the time structural closure is less than the static correction. In such cases, multi-layered models, using velocities derived from first-break arrivals of seismic shot records, have proven to be successful. The additional velocity information obtained from these first breaks (direct as well as refracted arrivals) vastly improves the velocity-depth structure of the near surface, regardless of the topography. Static corrections computed from these detailed nearsurface velocity models have significantly enhanced subsurface image focusing, thereby reducing the uncertainty in the closure of target structures. For regional studies the local layer model is merged with the simple single layer model.

Delay-time refraction methods applied to a 3D seismic block

Two-layer velocity models, which sum the vertical time in the weathering layer and the vertical time from base of weathering to datum, produce an initial basis for 3D seismic near-surface static corrections in Saudi Arabia. The thickness and velocity of the weathering layer are highly variable so the near-surface modeling is extended with delay times calculated by the plus-minus and the generalized reciprocal method (GRM) technique.

Painting the near surface using geology, geophysics, and satellites

A smooth interpolation of a sparse uphole survey may be the main tool available for modeling the near surface in desert areas. An uphole survey does not properly define sharp, near-surface velocity changes associated with lithologic and topographic variations such as cliffs, canyons, or outcrops, which may be delineated by satellite imagery instead. We present a patchy interpolation technique to build a consistent near-surface model. The algorithm is based on K -means clustering to integrate geological, geophysical, and remote-sensing information. A somewhat arbitrary aspect of the method is the choice of how many clusters are used in segregating the data. Linking this parameter to surface geologic formations may be inadequate because P- or S-wave velocities are unlikely to have a sufficiently unique correlation with geologic age and lithology. We suggest an empirical criterion: improvement in the seismic stack section achieved by processing with parameters derived by various clustering choices and data-t...

Tomostatics for half a million traces

Summary Traveltime inversion can estimate complex structures and sharp velocity changes, which may occur in the nearsurface layers. Tomostatics were recently proven to be an effective technology in difficult areas; however, the inversion quality depends to a large degree on the accuracy of traveltime picking and on the identification of related events. Identification and picking of events in pre-stack gathers, when dealing with large data sets, can become very cumbersome. Unfortunately, the constant increase of channel number in 3-D and multi-component surveys, along with their time-lapse repetition, exacerbates this problem. A criterion to compare different tomostatics approaches is their possible suitability for automation. On this basis, we compared some methods by computing tomostatics for a high-fold profile in a geologically complex area in Saudi Arabia. We found that our joint inversion of shallow reflectors and refractors yielded similar results to a multilayer up-hole time inversion, but at a cheaper cost.

Near-surface GRM model updating using reduced travel times

Summary Two layer models which sum the vertical time in weathering and the vertical time from base of weathering to datum have produced an initial basis for 3D seismic nearsurface static corrections in Saudi Arabia. In certain 3D blocks there exist large magnitude nearsurface time anomalies due to thick deposition in gorges, in which both the velocity of the shallow weathering layer and its depth are highly variable. The generalized reciprocal method (GRM) technique has been applied to solve these local areas. Generalized linear inversion technique was tested but failed to resolve the large high frequency time shifts. Tomography could be considered but has problems in the time tie of the models using the regional sloping datum.

Near-surface data clustering for imaging enhancement

Summary Popular approaches for near-surface modeling, as tomostatics and refraction methods, often fail when dealing with sharp variations of both elevation and P velocity. We present here a new method for integrating high-resolution geophysical data by clustering. Complex geological models can be built in a cost-effective way, providing better static corrections. This method is applied to a challenging 3D seismic survey from the Arabian Peninsula and compared to other current techniques.

Evolution of a Near-Surface Model in an Area of Complex Topography

Summary This paper describes a case study of the evolution of the near-surface statics model for an area of complex topography giving large velocity contrasts in a short distance. The 2D lines were modeled with a single layer velocity model and local anomalies investigated. This technique is inadequate for the later 3D acquisition. A twolayer model was built using existing upholes; however local anomalies were still evident. This initial two-layer model was further refined by adding extra contol points to define the near-surface geology much more accurately than before. While this model defined the regional trends better than previous models, further work was to be done to confirm the structure.