Page 141 - Introduction to Petroleum Engineering

128 RESERVOIR GEOPHYSICS
Depth in terms of velocity and two‐way travel time Δt is z = v Δt/2, and the dominant
i
wavelength is related to dominant frequency by λ = v /f . Substituting these relations
i d
d
in Equation 7.16 gives
/
v ∆ t 1  12
r = i  + 2  (7.17)
2  f d f 4 d 
The radius r of the first‐order Fresnel zone is obtained by keeping only first‐
F
order terms in dominant frequency to obtain an estimate of maximum horizontal
resolution δz :
H
v t ∆
r = i = δ z H (7.18)
F
2 f d
The Fresnel radius r is about 775 ft for wavelet velocity of 10 000 ft/s, dominant
F
frequency of 25 Hz, and two‐way travel time Δt of 0.6 s. Thus, two horizontal fea-
tures can be resolved if they are separated by at least 775 ft. If we increase the domi-
nant frequency, we can resolve horizontal features on a smaller separation because
the maximum horizontal resolution decreases.

Example 7.5 Seismic Resolution of Two Reflective Surfaces

A. A wave propagating at 1800 m/s has a frequency of 60 Hz. What is the
wavelength of the wave?
b. Assume two reflective surfaces can be distinguished when they are sep-
arated by a distance of at least ¼ wavelength. Use this criterion and the
wavelength in Part A to estimate the minimum separation needed to
distinguish two reflective surfaces.

Answer

1800 m/s
A. λ = = 30 m
60 Hz
λ 30 m
b. Separation = = = 75 m
.
4 4

7.3.3 Exploration Geophysics and Reservoir Geophysics
Historically, the primary role of geophysics in the oil and gas industry was explora-
tion. Today, geophysical technology has value during both the exploration and
development stages of the life of a reservoir. Exploration geophysics is used to pro-
vide a picture of subsurface geologic features such as stratigraphy and structure.
Reservoir geophysics is conducted in fields where wells have penetrated the target