Construction of 2D Pluto1.5 Elastic Earth Model – April-July 1998
Assignment of Layer Geometries
BHP provided digitized curves for the seafloor, tops and bottoms of all salt bodies, various faults,
and ten geological interfaces, all extracted from a multiply-iterated Chevron depth migration
volume. BHP also provided the residual depth-stretching function necessary to correct for
anisotropy, etc., and bring the layer depths into agreement with several inline well checkshots. It is
called the BHP Pluto model. This basic structural model was further modified in three ways: slight
increase of top salt rugosity by about +/-200 ft, two extra faults below the Pluto salt, and a seafloor
with 500 more feet of relief. The model dimensions are 105k feet wide by 30k feet deep. I used
this resulting stratigraphic fabric to guide the further population of the model with a mix of
stratigraphically parallel and pinching-out reflectors, which average 300-400 feet in thickness and
honor all faults. Special care was taken in the geometric specification of two exploration grade gas
sands penetrated by wells. The first is at X,Z = 32k,15k, and the second at 35k,18k. Two nonexistent geological features were added to the model for synthetic seismic S/N benchmarking.
The first is a series of “point” diffracting disks 200 feet in diameter, spaced 5k ft apart in X from
15k to 90k at two Z levels of 17k and 25k (but absent directly above the M38 gas sand). The
second is a horizontal “mother salt” layer occupying the bottom 500 feet of the model. The
resulting stratigraphic/geometric model is named PLUTO1.5.
Assignment of Vp
Two checkshot surveys in the plane of the section provided a consistent measure of P velocity on
a gross scale, in agreement with the prestack depth migration velocities above salt, but different
by up to 10% below salt. The best linear velocity fit down to below the target horizon is V = 5000 +
0.3*(Z-Zseafloor) ft/s, with individual layers fluctuating by up to several hundred ft/s about this
trend, especially depending on whether the facies was shale or sand rich. Accordingly, I created
seven color-coded lithologies with the same k accelerator but different Vo values. When the
colors are randomly assigned in the depth section, there results a velocity profile characterized by
small random fluctuations about a linear velocity trend. Of the seven facies, five are shale and two
are sand, in rough correspondence with their relative log population. The higher Vo for the sands
corresponds to a 5-10% higher checkshot velocity in the sand-rich zone vs the shale-rich zone.
The Vp values for both gas sands and their associated water legs and capping shales were
derived from sonic logs taken in these intervals, and the salt velocity is from the checkshot data.
(see Table below and PLUTO1.5 Vp, Vs and Den figures.)
Color Facies
Vp0
Eight pseudofacies:
blue
shale
4700
brown shale
4800
cyan
shale
4900
green shale
5000
orange shale
5100
red
sand
5200
yellow sand
5300
white diffractor disks 5000
True facies:
blue
water
4920
wheat salt
14800
wheat mother salt
14800
yellow shale sheath0 4600
dkyell shale sheath1 4700
mustrd shale sheath2 4800
magnta m15 cap shale 5130
black m15 gas sand 7692
gray
m15 water sand 5130
magnta m38 cap shale 5640
black m38 gas sand 8333
gray
m38 water sand 5640
magnta m53 shale
5100
Kp
Vs0
Ks
Vp/Vs Den0
Kd
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
2043
2087
2130
2174
2217
2600
2650
2500
0.1304
0.1304
0.1304
0.1304
0.1304
0.15
0.15
0.15
2.3
2.3
2.3
2.3
2.3
2.0
2.0
2.0
1.9
1.9
1.9
1.9
1.9
1.64
1.64
3.0
.000035
.000035
.000035
.000035
.000035
.000035
.000035
0
0
0
0
0.3
0.3
0.3
0.3
0
0.3
0.3
0
0.3
0.3
0
8559
8559
2000
2043
2087
2230
3846
2565
2564
4167
2820
2217
0
0
0
0.1304
0.1304
0.1304
0.1304
0
0.15
0.1364
0
0.15
0.1304
---1.729
1.729
2.3
2.3
2.3
2.3
2.0
2.0
2.2
2.0
2.0
2.3
1.0
0
2.16
0
3.0*
0
1.9
.000035
1.9
.000035
1.9
.000035
1.9985* .000035
2.15
0
1.8135* .000035
1.918* .000035
2.00
0
1.618* .000035
1.9
.000035
* listed model density is slightly modified to brighten seismic reflection
Assignment of Vs
One of the inplane wells (MC674#2) provided Vp, Vs and Density trends from a 10kft zone below
salt. This gave an estimate of Vp/Vs ratio of 2.3 in the shale-dominated zone and 2.0 in the sanddominated zone, which I used to convert from Vp to Vs. The salt Vs is from an estimate of Vp/Vs
for salt of 1.729 (see PLUTO1.5 Vs figure).
Assignment of Density
The same detailed logs below base salt provided good estimates of density trends between 10k’
and 20k’ below mudline, which I extrapolated both up and down. Salt density was obtained from
the literature. The M15, M38, and M53 horizons were assigned mildly perturbed densities to
slightly increase their reflection amplitude on the seismic simulations.
Random Perturbations
In order to mimic the occasional hummocky, laterally variable seismic facies (sand) sandwiched
between laterally smooth facies (shale), which are commonly observed on depth migrated
sections, I chose the cyan pseudo-facies to be perturbed as follows. At a probability of 4%, any
node within the cyan shale facies is changed to Vp = 5200+0.3*(Z-Zseafloor), Vs = 0.5*Vp, and
Den = 1.64+0.000035*(Z-Zseafloor), i.e. the red sand facies. This produces more realistic
diffractions in the time data, and when migrated with the true velocity gives realistic seismic facies.
Some of these point perturbed facies are visible in the various PLUTO1.5 figures. In a separate
perturbation, every node of the allochthonous salt masses (so excluding the pseudo mother salt)
is perturbed with a slowness fluctuation uniformly distributed between -1% to 1% of the reference
salt slowness. The salt Vs is perturbed in lockstep according to the Vp/Vs for salt, and salt density
is not perturbed. Unlike the cyan facies perturbation, this one does not produce coherent
diffractions but instead may provide some mild, extrinsic scattering attenuation in salt.