Page 432 - Acquisition and Processing of Marine Seismic Data

9. VELOCITY ANALYSIS 423

• Frequency of the seismic signal ρ ¼ 0:23V 0:25 (9.1)
P
• Anisotropy and fractures
• Clay content where V P and ρ are P wave velocity and bulk
• Consolidation density, respectively.
Shapes and sizes of the grains also affect the
The influences of these factors on seismic velocity in sedimentary rocks. For instance, seis-
velocities and attenuation of P and S waves in mic velocity decreases in the case of poor grain
sedimentary rocks are the major exploration field contact, while cementation results in a signifi-
of rock physics research (King, 2005). The seismic cant increase in the velocity. Shape of the pores
velocity of the subsurface layers is not constant also influences the seismic velocity, and the
for most cases and varies along both time and rocks with flat pores are of lower velocities
space directions, even within a specific medium. Wang (2001). Effects of anisotropy are more pro-
This is because the local variations in several fac- nounced for the shales, which are generally con-
tors, including compaction, overburden pres- sidered as cap rocks for the reservoirs.
sure, cementation, saturation, etc., play an Sometimes fracturing causes anisotropic effects,
important role in the velocity of a subsurface especially on the P wave velocity.
layer. Wang (2001) suggests that dolomite has V P does change with the porosity and fluid sat-
the highest P wave velocity in reservoir rocks, fol- uration of the sedimentary rocks. Increasing
lowed by limestone and sandstone, while the S porosity decreases the seismic velocities; how-
wave velocity decreases, with a possible order ever, the V P /V S ratio is not strongly affected by
of sandstone, dolomite and limestone. Because porosity change. As the fluid saturation
S wave velocity is normally not sensitive to the increases, P wave velocity slightly decreases first,
change of fluids in the pore spaces, while P wave then starts to increase sharply, while S wave
velocities are strongly affected by fluid type and velocity decreases, which ultimately causes an
saturation, P to S wave velocity (V P /V S ) ratio var- increase in the V P /V S ratio. Incorporating gas
iations may also provide valuable information for into the saturated liquid, however, V P signifi-
lithology discrimination, which is particularly cantly decreases while V S slightly increases,
important in amplitude vs. offset (AVO) analysis. resulting in a lower V P /V S ratio. Increasing the
For instance, shales are of higher V P /V S ratios viscosity and density of the pore fluids increases
than sands within a reservoir. The V P /V S ratio the velocity of the saturated rock.
is also sensitive to pore fluids and is lower in In addition, velocity increases with increasing
the case of gas saturation. depth (or overburden pressure), thus the seismic
Although the most important factor control- velocities are generally higher for deeper reflec-
ling the rock velocity is lithology, experimental tions on the seismic data because the pores of the
studies indicate that the other factors listed here rocks are closed as the overburden pressure
also strongly affect the velocity distribution increases. This increase in the velocity is much
through the subsurface in 3D. Fig. 9.1B summa- more pronounced for shallow sediments since
rizes these effects in schematic illustrations. the number of pore spaces to close progressively
There are a number of empirical relationships decreases towards the deeper sediments; thus,
between bulk density and rock velocity, which increasing pressure does not excessively influ-
suggest that the seismic velocity increases with ence the velocity in deeper sediments
increasing bulk density, although it is not neces- (Fig.9.1B). Depth or pressure also results in com-
sary for velocity to increase with the bulk den- paction of the rocks, causing higher seismic
sity of the rocks. The most common one is velocities because of the better contact among
known as Gardner’s equation, given by the grains. An older rock has a higher velocity