Page 142 - Introduction to Petroleum Engineering

SEISMIC DATA ACQUISITION, PROCESSING, AND INTERPRETATION 129
horizon. The resolution associated with reservoir geophysics is more quantitative
than the resolution associated with exploration geophysics.
Wittick (2000) and Pennington (2001) observed that the difference in resolution
between exploration and reservoir geophysics is due to the role of calibration.
Ordinarily, no wells are available during the exploration process, so it is not possible
to calibrate seismic data measurements with well log measurements. By contrast,
well log data can be acquired during the development process to provide measure-
ments in the wellbore that can be used to calibrate seismic surveys conducted at the
surface. Consequently, reservoir geophysics can have more information available to
improve quantitative estimates of reservoir properties if appropriate well log data is
used to calibrate seismic data.
The process of using calibrated seismic information to predict reservoir prop-
erties is called seismic inversion. Seismic inversion is an attempt to correlate
seismic attributes like acoustic impedance to rock properties. Seismic attributes
are cross‐plotted against groupings of rock properties. Examples of cross‐plots
for a formation with permeability K, oil saturation S , and net thickness h
net
o
include acoustic impedance versus porosity ϕ, seismic amplitude versus flow
capacity (Kh ) or rock quality (ϕKh ), and seismic amplitude versus oil produc-
net
net
tive capacity (S ϕKh ).
o
net
One of the first examples of seismic inversion that included a field test was provided
by De Buyl et al. (1988). They predicted reservoir properties at two wells using
seismic inversion and then compared actual results to predicted results. A similar
comparison was made between measurements at wells and predictions prepared using
only well logs. Predictions made with seismic inversion were at least as accurate as
predictions made with well log data only and were more accurate in some cases.

7.4 SEISMIC DATA ACQuISITION, PROCESSING,
AND INTERPRETATION

Subsurface geologic features are mapped in reflection seismology by measuring the
time it takes an acoustic signal to travel from the source to a seismic reflector and
then to a receiver. Three steps are required to analyze seismic measurements: data
acquisition, data processing, and interpretation.

7.4.1 Data Acquisition
Seismic surveys are conducted to acquire seismic data. A 2‐D seismic survey uses a
vibrational source and a single line of receivers to prepare a cross‐sectional image
of the subsurface. A 3‐D seismic survey uses a line of sources with a 2‐D array of
receivers to prepare a 3‐D image of the subsurface.
The recorded seismic trace is a function of travel time and combines source signal
with the sequence of seismic reflectors known as reflectivity sequence. Reflectors
are determined by changes in acoustic impedance and the corresponding reflection