American Oil and Gas Reporter - July 2020 - 56

SpecialReport: Geophysics & Computing

CRAM Gathers Enhance 3-D Inversion
By Elive Menyoli
AUSTIN TX.-Elastic inversion from
common reflection angle migration
(CRAM) gathers can accurately capture
lithology-driven lateral variations in reservoir properties, particularly in a strongly
deformed and faulted geologic environment. Compared with angle gathers transformed from post-migration offset gathers,
the intrinsic angle gathers from CRAM
have three critical advantages:
· Reliable angle gathers greater than
40 degrees;
· A robust and reliable amplitude
and phase preservation; and
· Regularly sampled gathers.
Many practitioners have shown that
in order to obtain reliable density, compressional- and shear-wave velocities
(Vp/Vs), Poisson ratio and total organic
carbon (TOC) content from prestack simultaneous seismic inversion, seismic
gathers with sufficient angle range greater
than 40 degrees are required.
It has been shown in shale resource
plays that the TOC content has a stronger
influence on the density of the rock-and
quite often density correlates to gamma
ray curves (lithology predictor) or petrophysical properties such as porosity-than
on velocities. However, the contribution
of density to the overall seismic amplitude

generally is present only on large angles
greater than 40 degrees. Therefore, having
a migration algorithm that naturally produces reliable large-angle gathers is critical.
The conventional Kirchhoff migration
produces offset gathers that must later be
converted into angle gathers prior to seismic inversion. Computing an accurate
offset-to-angle conversion can be crucial
to predicting subsurface lithology, particularly in shale plays with consolidated
rocks that may have rapid lateral and
vertical lithology changes.
Whereas a conventional normal moveout-based transform is increasingly inaccurate with increasing angles, especially
in the presence of anisotropy, the CRAMbased method generates angle gathers directly using an intrinsic ray tracing procedure.
Generating CRAM Gathers
Unlike a conventional common shot
or common offset Kirchhoff migration,
which begins at the acquisition surface,
common reflection angle domain migrations start at the depth image point. From
each subsurface point, a fan of upgoing
rays is shot at uniform emergence angle
increments. This approach allows the
direct application of correct summation
weighting factors, resulting in continuous

FIGURE 1
VTI Migrated Offset Gathers (Left) versus
Common Angle Gathers (Right)

56 THE AMERICAN OIL & GAS REPORTER

amplitude and phase-preserved image
gathers for a wide range of reflection angles.
By shooting dense upgoing rays at
uniform emergence angle increments, a
uniform illumination is obtained directly
at the image points from all directions
(dips and azimuths), which is essential
for the accurate reconstruction of angle
image gathers that is required for seismic
inversion.
Each event in the common image
gather is constructed by summing all
seismic data reflected from the image
points with the same opening reflection
angle. The relevant contributing data
points are defined by the various ray
paths from the image point. The illumination of the image points from all directions ensures uniform data sampling
for building the image.
CRAM gathers are organized naturally
according to opening angles at each subsurface point, rather than according to
source-receiver offset at the surface. Figure 1 compares an example of CRAM
with a common offset Kirchhoff migration gather. Within the yellow-boxed
zone, CRAM gathers are much cleaner,
more continuous, and have better amplitude distribution.
CRAM gathers also are affected less
by migration stretch at the far angles
(area circled in red). This phenomenon
is intrinsic to angle gathers and is another
important advantage of CRAM migration.
Avoiding migration stretch makes such
angle gathers ideally suited for impedance
inversion of elastic properties, even in
complex geologic areas.
CRAM compensates equally for amplitude preservation and multiray path
arrivals, and accounts for wavelet phase
shift caused by caustics along the ray
paths. Accounting for the total ray phase
shift is important for keeping the wavelet
phase stationary during propagation. Maintaining a stationary phase wavelet is one
of the assumptions for seismic inversion.
It is worth noting that even in layercake geologic formations such as in South
Texas, strong lateral and vertical velocity
variations across fault boundaries may
cause multiarrivals and ray caustics.
While CRAM gathers are organized
in angles and depth, Kirchhoff gathers
are organized naturally in offsets and
depth. For inversion purposes, the Kirch-



American Oil and Gas Reporter - July 2020

Table of Contents for the Digital Edition of American Oil and Gas Reporter - July 2020

American Oil and Gas Reporter - July 2020 - Intro
American Oil and Gas Reporter - July 2020 - 1
American Oil and Gas Reporter - July 2020 - 2
American Oil and Gas Reporter - July 2020 - 3
American Oil and Gas Reporter - July 2020 - 4
American Oil and Gas Reporter - July 2020 - 5
American Oil and Gas Reporter - July 2020 - 6
American Oil and Gas Reporter - July 2020 - 7
American Oil and Gas Reporter - July 2020 - 8
American Oil and Gas Reporter - July 2020 - 9
American Oil and Gas Reporter - July 2020 - 10
American Oil and Gas Reporter - July 2020 - 11
American Oil and Gas Reporter - July 2020 - 12
American Oil and Gas Reporter - July 2020 - 13
American Oil and Gas Reporter - July 2020 - 14
American Oil and Gas Reporter - July 2020 - 15
American Oil and Gas Reporter - July 2020 - 16
American Oil and Gas Reporter - July 2020 - 17
American Oil and Gas Reporter - July 2020 - 18
American Oil and Gas Reporter - July 2020 - 19
American Oil and Gas Reporter - July 2020 - 20
American Oil and Gas Reporter - July 2020 - 21
American Oil and Gas Reporter - July 2020 - 22
American Oil and Gas Reporter - July 2020 - 23
American Oil and Gas Reporter - July 2020 - 24
American Oil and Gas Reporter - July 2020 - 25
American Oil and Gas Reporter - July 2020 - 26
American Oil and Gas Reporter - July 2020 - 27
American Oil and Gas Reporter - July 2020 - 28
American Oil and Gas Reporter - July 2020 - 29
American Oil and Gas Reporter - July 2020 - 30
American Oil and Gas Reporter - July 2020 - 31
American Oil and Gas Reporter - July 2020 - 32
American Oil and Gas Reporter - July 2020 - 33
American Oil and Gas Reporter - July 2020 - 34
American Oil and Gas Reporter - July 2020 - 35
American Oil and Gas Reporter - July 2020 - 36
American Oil and Gas Reporter - July 2020 - 37
American Oil and Gas Reporter - July 2020 - 38
American Oil and Gas Reporter - July 2020 - 39
American Oil and Gas Reporter - July 2020 - 40
American Oil and Gas Reporter - July 2020 - 41
American Oil and Gas Reporter - July 2020 - 42
American Oil and Gas Reporter - July 2020 - 43
American Oil and Gas Reporter - July 2020 - 44
American Oil and Gas Reporter - July 2020 - 45
American Oil and Gas Reporter - July 2020 - 46
American Oil and Gas Reporter - July 2020 - 47
American Oil and Gas Reporter - July 2020 - 48
American Oil and Gas Reporter - July 2020 - 49
American Oil and Gas Reporter - July 2020 - 50
American Oil and Gas Reporter - July 2020 - 51
American Oil and Gas Reporter - July 2020 - 52
American Oil and Gas Reporter - July 2020 - 53
American Oil and Gas Reporter - July 2020 - 54
American Oil and Gas Reporter - July 2020 - 55
American Oil and Gas Reporter - July 2020 - 56
American Oil and Gas Reporter - July 2020 - 57
American Oil and Gas Reporter - July 2020 - 58
American Oil and Gas Reporter - July 2020 - 59
American Oil and Gas Reporter - July 2020 - 60
American Oil and Gas Reporter - July 2020 - 61
American Oil and Gas Reporter - July 2020 - 62
American Oil and Gas Reporter - July 2020 - 63
American Oil and Gas Reporter - July 2020 - 64
American Oil and Gas Reporter - July 2020 - 65
American Oil and Gas Reporter - July 2020 - 66
American Oil and Gas Reporter - July 2020 - 67
American Oil and Gas Reporter - July 2020 - 68
American Oil and Gas Reporter - July 2020 - 69
American Oil and Gas Reporter - July 2020 - 70
American Oil and Gas Reporter - July 2020 - 71
American Oil and Gas Reporter - July 2020 - 72
American Oil and Gas Reporter - July 2020 - 73
American Oil and Gas Reporter - July 2020 - 74
American Oil and Gas Reporter - July 2020 - 75
American Oil and Gas Reporter - July 2020 - 76
American Oil and Gas Reporter - July 2020 - 77
American Oil and Gas Reporter - July 2020 - 78
American Oil and Gas Reporter - July 2020 - 79
American Oil and Gas Reporter - July 2020 - 80
American Oil and Gas Reporter - July 2020 - 81
American Oil and Gas Reporter - July 2020 - 82
American Oil and Gas Reporter - July 2020 - 83
American Oil and Gas Reporter - July 2020 - 84
https://www.nxtbook.com/nxtbooks/aogr/202103
https://www.nxtbook.com/nxtbooks/aogr/202102
https://www.nxtbook.com/nxtbooks/aogr/202101
https://www.nxtbook.com/nxtbooks/aogr/202012
https://www.nxtbook.com/nxtbooks/aogr/202011
https://www.nxtbook.com/nxtbooks/aogr/202010
https://www.nxtbook.com/nxtbooks/aogr/202009
https://www.nxtbook.com/nxtbooks/aogr/202008
https://www.nxtbook.com/nxtbooks/aogr/202007
https://www.nxtbook.com/nxtbooks/aogr/202006
https://www.nxtbook.com/nxtbooks/aogr/202005
https://www.nxtbook.com/nxtbooks/aogr/202004
https://www.nxtbook.com/nxtbooks/aogr/202003
https://www.nxtbook.com/nxtbooks/aogr/202002
https://www.nxtbook.com/nxtbooks/aogr/202001
https://www.nxtbook.com/nxtbooks/aogr/201912
https://www.nxtbook.com/nxtbooks/aogr/201911
https://www.nxtbook.com/nxtbooks/aogr/201910
https://www.nxtbook.com/nxtbooks/aogr/201909
https://www.nxtbook.com/nxtbooks/aogr/201908
https://www.nxtbook.com/nxtbooks/aogr/201907
https://www.nxtbook.com/nxtbooks/aogr/201906
https://www.nxtbook.com/nxtbooks/aogr/201905
https://www.nxtbook.com/nxtbooks/aogr/201904
https://www.nxtbook.com/nxtbooks/aogr/201903
https://www.nxtbook.com/nxtbooks/aogr/201902
https://www.nxtbook.com/nxtbooks/aogr/201901
https://www.nxtbook.com/nxtbooks/aogr/201812
https://www.nxtbook.com/nxtbooks/aogr/201811
https://www.nxtbook.com/nxtbooks/aogr/201810
https://www.nxtbook.com/nxtbooks/aogr/pbios_201810
https://www.nxtbook.com/nxtbooks/aogr/201809
https://www.nxtbook.com/nxtbooks/aogr/201808
https://www.nxtbook.com/nxtbooks/aogr/201807
https://www.nxtbook.com/nxtbooks/aogr/201806
https://www.nxtbook.com/nxtbooks/aogr/201805
https://www.nxtbook.com/nxtbooks/aogr/201804
https://www.nxtbook.com/nxtbooks/aogr/201803
https://www.nxtbook.com/nxtbooks/aogr/201802
https://www.nxtbook.com/nxtbooks/aogr/201801
https://www.nxtbook.com/nxtbooks/aogr/201712
https://www.nxtbook.com/nxtbooks/aogr/201711
https://www.nxtbook.com/nxtbooks/aogr/201710
https://www.nxtbook.com/nxtbooks/aogr/201709
https://www.nxtbook.com/nxtbooks/aogr/201708
https://www.nxtbook.com/nxtbooks/aogr/201707
https://www.nxtbook.com/nxtbooks/aogr/201706
https://www.nxtbook.com/nxtbooks/aogr/201705
https://www.nxtbook.com/nxtbooks/aogr/201704
https://www.nxtbook.com/nxtbooks/aogr/201703
https://www.nxtbook.com/nxtbooks/aogr/201702
https://www.nxtbook.com/nxtbooks/aogr/201701
https://www.nxtbook.com/nxtbooks/aogr/201612
https://www.nxtbook.com/nxtbooks/aogr/201611
https://www.nxtbook.com/nxtbooks/aogr/201610
https://www.nxtbook.com/nxtbooks/aogr/pbios2016_programguide
https://www.nxtbook.com/nxtbooks/aogr/201609
https://www.nxtbook.com/nxtbooks/aogr/201608
https://www.nxtbook.com/nxtbooks/aogr/201607
https://www.nxtbook.com/nxtbooks/aogr/201606
https://www.nxtbook.com/nxtbooks/aogr/201605
https://www.nxtbook.com/nxtbooks/aogr/201604
https://www.nxtbook.com/nxtbooks/aogr/201603
https://www.nxtbook.com/nxtbooks/aogr/201602
https://www.nxtbook.com/nxtbooks/aogr/201601
https://www.nxtbook.com/nxtbooks/aogr/201512
https://www.nxtbook.com/nxtbooks/aogr/201511
https://www.nxtbook.com/nxtbooks/aogr/201510
https://www.nxtbook.com/nxtbooks/aogr/201509
https://www.nxtbook.com/nxtbooks/aogr/201508
https://www.nxtbook.com/nxtbooks/aogr/201507
https://www.nxtbook.com/nxtbooks/aogr/201506
https://www.nxtbook.com/nxtbooks/aogr/201505
https://www.nxtbook.com/nxtbooks/aogr/201504
https://www.nxtbook.com/nxtbooks/aogr/201503
https://www.nxtbook.com/nxtbooks/aogr/201502
https://www.nxtbook.com/nxtbooks/aogr/201501
https://www.nxtbook.com/nxtbooks/aogr/201412
https://www.nxtbook.com/nxtbooks/aogr/201411
https://www.nxtbook.com/nxtbooks/aogr/201410
https://www.nxtbook.com/nxtbooks/aogr/201409
https://www.nxtbook.com/nxtbooks/aogr/pbios2014_programguide
https://www.nxtbook.com/nxtbooks/aogr/201408
https://www.nxtbook.com/nxtbooks/aogr/201407
https://www.nxtbook.com/nxtbooks/aogr/201406
https://www.nxtbook.com/nxtbooks/aogr/201405
https://www.nxtbook.com/nxtbooks/aogr/201404
https://www.nxtbook.com/nxtbooks/aogr/201403
https://www.nxtbook.com/nxtbooks/aogr/201402
https://www.nxtbook.com/nxtbooks/aogr/201401
https://www.nxtbook.com/nxtbooks/aogr/201312
https://www.nxtbook.com/nxtbooks/aogr/201311
https://www.nxtbook.com/nxtbooks/aogr/201310
https://www.nxtbook.com/nxtbooks/aogr/201309
https://www.nxtbook.com/nxtbooks/aogr/201308
https://www.nxtbook.com/nxtbooks/aogr/201307
https://www.nxtbook.com/nxtbooks/aogr/201306
https://www.nxtbook.com/nxtbooks/aogr/201305
https://www.nxtbook.com/nxtbooks/aogr/201304
https://www.nxtbook.com/nxtbooks/aogr/201303
https://www.nxtbook.com/nxtbooks/aogr/201302
https://www.nxtbook.com/nxtbooks/aogr/201301
https://www.nxtbook.com/nxtbooks/aogr/201212
https://www.nxtbook.com/nxtbooks/aogr/201211
https://www.nxtbook.com/nxtbooks/aogr/201210
https://www.nxtbook.com/nxtbooks/aogr/201209
https://www.nxtbook.com/nxtbooks/aogr/2012_pbios
https://www.nxtbook.com/nxtbooks/aogr/201208
https://www.nxtbook.com/nxtbooks/aogr/201207
https://www.nxtbook.com/nxtbooks/aogr/201206
https://www.nxtbook.com/nxtbooks/aogr/201205
https://www.nxtbook.com/nxtbooks/aogr/201204
https://www.nxtbook.com/nxtbooks/aogr/201203
https://www.nxtbook.com/nxtbooks/aogr/201202
https://www.nxtbook.com/nxtbooks/aogr/201201
https://www.nxtbook.com/nxtbooks/demo/aogr_clone
https://www.nxtbook.com/nxtbooks/aogr/201112
https://www.nxtbookmedia.com