Effect of local high-velocity body

Problems in Exploration Seismology and their Solutions

Series	Geophysical References Series
Title	Problems in Exploration Seismology and their Solutions
Author	Lloyd P. Geldart and Robert E. Sheriff
Chapter	9
Pages	295 - 366
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	SEG Online Store

Problem 9.24

The horizon velocity analysis for horizon A in Figure 9.24a indicates higher stacking velocities on opposite sides of the salt pillows B and C and low stacking velocities immediately below the salt pillow D. Does this have geological significance? The streamer was about 4 km long. Note that the upper 2 s of the section have been cut off.

Figure 9.24a.  (i) Seismic section; (ii) horizon velocity analysis along event $A$.

Background

A horizon velocity analysis is a more-or-less continuous series of velocity analyses along a single reflection event or along a series of more-or-less parallel events lying within a narrow time window that contains an event.

A salt pillow is a small dome formed by salt flowing from the surrounding region. Often the growth of a salt pillow is cut off when the salt supply (mother salt) is cut off.

Solution

With a 4-km streamer, the long-offset traces of CMP gathers immediately to the left and right of the salt pillows B and C are apt to have one leg of their long-offset travel paths through the salt whereas their short-offset traces may not involve salt travel. Because the salt is higher velocity than the sediments, this will flatten the gather hyperbolas and thus they will indicate fictitious high velocity. Over the center of the salt dome (D) the opposite will be the case, indicating fictitious low velocity.

The uplift in reflection $A$, which is below the pillow, is probably a velocity pull-up because of the overlying high-velocity salt. The first 2.5+ km to the right probably involve little salt and show fairly uniform and good quality velocity data probably not involving salt travel, so that the average velocity above horizon $A$, about 3 km/s, is probably a good average for the overlying sediments. The data to the left of the dome are not as clear, indicating other complications, perhaps more salt in this region. Horizon $A$ may be faulted under the center of the dome, which may explain why the dome developed here.

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Reflection field methods	Geologic interpretation of reflection data
Table of Contents (book)

Also in this chapter

• Fourier series
• Space-domain convolution and vibroseis acquisition
• Fourier transforms of the unit impulse and boxcar
• Extension of the sampling theorem
• Alias filters
• The convolutional model
• Water reverberation filter
• Calculating crosscorrelation and autocorrelation
• Digital calculations
• Semblance
• Convolution and correlation calculations
• Properties of minimum-phase wavelets
• Phase of composite wavelets
• Tuning and waveshape
• Making a wavelet minimum-phase
• Zero-phase filtering of a minimum-phase wavelet
• Deconvolution methods
• Calculation of inverse filters
• Inverse filter to remove ghosting and recursive filtering
• Ghosting as a notch filter
• Autocorrelation
• Wiener (least-squares) inverse filters
• Interpreting stacking velocity
• Effect of local high-velocity body
• Apparent-velocity filtering
• Complex-trace analysis
• Kirchhoff migration
• Using an upward-traveling coordinate system
• Finite-difference migration
• Effect of migration on fault interpretation
• Derivative and integral operators
• Effects of normal-moveout (NMO) removal
• Weighted least-squares

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