Interpreting seismic amplitudes - 3-D Seismic Interpretation - page 148

aV p f Density =

Lithology a f

ss/sh avg 1.741 0.25

shale 1.75 0.265

sandstone 1.66 0.261

limestone 1.5 0.225

dolomite 1.74 0.252

anhydrite 2.19 0.16

Fig. 5.23 P velocity-density relations. V is in km/s, density in g/cm 3 .

have been given by Castagna (1993 ) , as shown in fig. 5.23 . An improved version of the
limestone relation has been given by Mavko et al . (1998) :

ρ = 1 . 359 V 0 . 386
p

,

where the density is in g/cm 3 and the velocity is in km/s.
Where there exist some wells with both sonic and density data for an interval being
studied, it is better to use them to construct a specific relationship rather than use these
general equations.

5.5.5 Shear velocity

Ideally, shear velocity should be determined by direct measurement. However, reliable
shear velocity logging is a fairly recent introduction, and many wells have P but not
S wave logs. It is therefore useful to be able to predict S velocity from P velocity, for
use as an input to fluid substitution and AVO modelling. This is also a useful check on
measured shear logs, which are sometimes of doubtful quality; shear log processing is
to some extent interpretive. Also, some types of sonic tool are not able to record shear
velocities lower than the compressional velocity in the drilling mud.
Various empirical relations have been proposed. A useful method is described by
Greenberg & Castagna (1992) . V s is predicted from V p and mineral fractions using four
V p - V s relations:

V s = 0 . 804 16 V p − 0 . 855 88 sand

V s =

.

769 69 V p −

.

shale 0 0 867 35

055 08 V p 2

limestone V s =− 0 . + 1 . 016 77 V p − 1 . 030 49

dolomite V s = 0 . 583 21 V p − 0 . 077 75