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3: Velocity Analysis and Statics Corrections

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Published:
January 01, 2001

3.0 INTRODUCTION

A sonic log represents direct measurement of the velocity with which seismic waves travel in the earth as a function of depth. Seismic data, on the other hand, provide an indirect measurement of velocity. Based on these two types of information, the exploration seismologist derives a large number of different types of velocity – interval, apparent, average, root-mean-square (rms), instantaneous, phase, group, normal moveout (NMO), stacking, and migration velocities. However, the velocity that can be derived reliably from seismic data is the velocity that yields the best stack.

Assuming a layered media, stacking velocity is related to normal-moveout velocity. This, in turn, is related to the root-mean-squared (rms) velocity, from which the average and interval velocities are derived. Interval velocity is the average velocity in an interval between two reflectors.

Several factors influence interval velocity within a rock unit with a certain lithologic composition:

(a) Pore shape,

(b) Pore pressure,

(c) Pore fluid saturation,

(d) Confining pressure, and

(e) Temperature.

These factors have been investigated extensively under laboratory conditions. Figure 3.0-1 shows laboratory measurements of velocity as a function of the confining pressure in a Bedford limestone sample with pores in the form of microcracks. The experiment was conducted using enclosed samples to control the pore fluid pressure independent of the confining pressure.

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Investigations in Geophysics

Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data

Öz Yilmaz
Öz Yilmaz
Search for other works by this author on:
Society of Exploration Geophysicists
Volume
10
ISBN electronic:
9781560801580
Publication date:
January 01, 2001

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