Online Journal for E&P Geoscientists

Bridle, Ralph M.¹; Al-Saad, Abdulaziz ^*1
(1) GDPD, Saudi Aramco, Dhahran, Saudi Arabia.

Building of a multiple layer model starts with the interpretation of uphole survey. Based on these interpretations an initial model is built for each 2D line. After gridding of these 2D velocity models, a contour map of the average velocity is created. Given the thickness from surface to datum, the static correction for any X/Y location within the map is calculated.

Several assumptions are made to produce a multiple layer model. The main assumption is that the uphole data and interpretations are accurate. Other assumptions are that the lateral velocity variations are small, and that the base of the deepest layer defines the base of the model. The velocity used from base of the model to datum is the same velocity as the deepest layer in the valid upholes.

The multiple layer model has limitations: 1) Sufficient upholes adequately define the base of the model. 2) The model does not define any variation in depth and velocity between uphole locations. 3) Where an uphole does not reach datum, information from the base of the uphole model to the datum is lacking. 4) The velocity within a layer is assumed to be vertically constant and 5) The model is built using individual 2D lines, which requires additional effort to ensure that the individual 2D models tie.

Three 2D seismic lines that were stacked using statics derived from multiple layer near-surface models are shown. In the first example the model focuses the time horizons. In the second example the multiple layer model produces the same static solution as a single layer model. While in the third example the multiple layer model does not solve the medium wavelength anomalies. Unsampled anomalies can occur due to variation of thickness or velocity the weathering layer between uphole locations. Those anomalies will not be reflected in the near surface model. Examples of this type of anomaly include buried wadi and dissolution of shallow anhydrite layers such as the Hyth collapse in Saudi Arabia.

Despite the limitations of multiple layer modeling, with valid upholes interpretation, it creates near surface static and velocity initial models. These models create statics in which focusing and long period is more realistic time image than the single layer model.