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Pitfalls in seismic data flattening

Anadarko Petroleum Corporation, Houston, Texas, U.S.

Corresponding author: R. Lee, richard_lee@anadarko.com

The first 20% of the full text of this article appears below.

Determination of bed thickness and reconstruction of paleosurfaces are important parts of seismic interpretation. This tutorial provides technical support for the "15° rule of thumb" and a straightforward method for calculating bed thickness for higher geologic dips.

When geologic dips are less than 15°, it is common to ignore the correction and accept an error of about 5%. However, when improved accuracy is required, the astute user must have a clear understanding of the issues involved and of the appropriate mathematical corrections. This tutorial is aimed at helping the interpreter understand these corrections for determining true bed thickness.

	Background

 
Geophysicists have long been flattening seismic data for the purposes of (1) computing interval thickness (IT) and (2) reconstructing the paleosurface (also known as restoration thickness or RT). These calculations are generally done without depth conversion or application of any corrections. Often no distinction is made between IT and RT. These estimates are usually reasonably successful because, in most geologic situations, vertical thickness (VT) very closely approximates IT and RT.

The relationship between VT and IT or RT is a function of azimuth angles (_A, _B) and dip angles (_A, _B) of the two bounding surfaces of the interval. Alternatively, the relationships can be defined using three independent angle variables, namely (Figure 1a): (1) the average dip angle of the interval, 1/2(_B + _A); (2) the wedge angle of the interval, WA = (_B – _A), and (3) the azimuth difference, AD = (_B – _A).

View larger version (15K): [in this window] [in a new window]   Figure 1. (a) Illustrations of dip angle, wedge angle, and azimuth difference (left to right). (b) Block model with dip. (c) Wedge model with no dip. (d) Wedge model with dip.