Note: Descriptions are shown in the official language in which they were submitted.
1~14~37
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to seismic surveys of bore-
holes, and is particularly concerned with the delineation of
oil and gas reservoirs using seismic sources at or near the
surface in conjunction with one or more geophones in the bore-
hole.
2. Description _ the Prior Ar
After an oil or gas discovery has been made, it is
common practice to delineate the extent of the reservoir by
drilling additional boreholes toward (and sometimes beyond)
its boundaries. This is very expensive, and there is a major
need for a technique to reduce the number of boreholes neces
sary.
~ 15 Occasionally it happens that, once the target is
- specified by the discovery well, the extent of the reservoir
can be seen, as a variation of seismic reflection character-
istics, after very careful processing of the normal seismic
sections through the well. More often, however, the resolu-
2~ tion afforded by the seismic reflection technique is inade-
quate for this purpose; an inade~uacy of vertical resolution
is imposed by the limited bandwidth of the reflection pulse
after two-way transmission through the earth, and an inadequacy
of horizontal resolution is imposed by the large area of the
,~ 25 reservoir reflectors which contributes to the reflection pulse.
!
The present method reduces these problems by plac-
ing the seismic geophone not at the surface,as in normal
reflection prospecting, but in the borehole, closer to the
reservoir.
2-
-
,
It is standard practice to use a surface source and
a borehole geophone in the technique known as check-shooting.
Historically, this technique is concerned only with obtaining
a measure of the seismic travel-time from the surface down to
depths of interest, and thus to calibrate the sonic log. -
The method normally uses a velocity-sensitive geophone in the
borehole and a mechanical seismic souree at or near the
surface.
SUMMARY O _ HE INVENTION
The present invention is concerned not only with
the direct downgoing signal from source to geophone, as in
the check-shooting techniques of the prior art, but also with
the upcoming signals reflected from reservoir boundaries below
the geophone. Specifically, it seeks to investigate the res-
ervoir material, and the lateral or vertical variations in its
properties, by means of the reflection characteristics of these
boundary interfaces, particularly as the reflection charae-
teristics are observed to change with angle of incidence and
with wave type. In order to study these variations and changes,
the method may provide one or more horizontal profiles of
source positions along the surface.
When such horizontal souree profiles are assoeiated
with each of several positions of the geophone in the borehole,
it becomes possible to gather groups of reflection signals
representing the same depth-point; such gathered groups may
be used both for analysis of incidence-angle effects and for
stacking.
The use of a borehole geophone, rather than a sur-
; face geophone, immediately improves the resolution of the
seismic refleetions from the reservoir; this is so because only
one path through the absorptive near-surfaee is involved, and
--3--
because the zone of insonification of the reflecting interfaces
is reduced in extent. An additional improvement is provided by
the present invention, in that the downgoing signal may be used
to deconvolve the reflected upcoming signal; by this means the
absorptive effects of the shallower materials above the geophone
are further compensated. The deconvolution is accomplished by
isolating the direct downgoing arrival, by calculating a decon-
volution operator to transform the pulse form of the downgoing
arrival into a more desirable pulse form, and by applying this
operator to the reflected arrivals.
In accordance with one aspect, there is provided, in
accordance with the invention, a method of processing seismic
signals from a borehole geophone, which signals include both a
direct downgoing arrival and one or more reflected upcoming
arrivals, comprising the following steps: a) isolating the
direct downgoing arrival, b) deriving a deconvolution operator
..
to transform the pulse form of the downgoing arrival into a more
desirable pulse form, and, c) applying this operator to selected
arrivals.
Accord:ing to a different aspect, there is provided,
in accordance with the invention, a method for displaying the
results obtained by recording the signals transmitted from a
seismic source at or near the surface and received at a geophone
~l disposed in a borehole, such display being formed by the follow-
''!
ing steps: a) isolating the direct downgoing signal by gating
and/or tapering, b) applying appropriate time shifts to a plural-
. . .
ity of observations of the downgoing signal obtained at a plural-
ity of geophone locations, compensating the amplitudes of these
observed signals for geometrical divergence and the local value
of acoustic impedance, and adding the observed signals to obtain
an improved downgoing arrival, c) calculating a deconvolution
operator to transform the said improved downgoing arrival into a
- 4 -
;37
more desirable pulse form, d) applying this operator to theoriginal signals from the geophone for each of a plurality of
geophone locations; and e) plotting the results in the form of
traces each representing the treated signal from one geophone
location.
In accordance with a still further aspect, there is
provided, in accordance with the invention, a method for displaying
the results obtained by recording the signals received at one or
more borehole geophones from a plurality of seismic source po-
sitions spaced apart laterally, such display being formed by
the following steps: a) applying normal-moveout time corrections
to the reflected signals, such time corrections being such as to
correct the reflection time for the increased path length as the
source and geophone are separated laterally; and, b) displaying
the corrected reflection arrivals as traces whose separation :
represents the lateral separation of source positions or reflec- ~ ~;
tion zones. ~:~
From a still further aspect, there is provided, in
accordance with the invention, a method using a horizontal pro-
file of source positions and a borehole geophone to map the
thickness or other physical property of a reservoir, the improve-
ment of including the following steps: a) isolating a particular
direct downgoing arrival; b) deriving a deconvolution operator
..... .
;; to transform the pulse form of the downgoing arrival into a more
desirable pulse form, c) applying this operator to the particular
direct downgoing arrival and to selected upcoming arrivals; and,
d) using the treated arrivals for the determination of thickness
or other property.
From a still further aspect, there is provided, in
accordance with the invention, a method of obtaining a common
depth point group of traces derived from the seismic reflection
at the boundary of a reservoir, using the following steps: a)
5 -
, ., , , . , . . . - .
~14C~7
making records of the arrivals obtained at a borehole geophone
for each source position along one or more extended source pro-
files, for each of a plurality of geophone locations, and b)
selecting a group of records which, having regard to the geometry
of the ray-paths, the dip of the strata and the velocity distri-
bution, represent a common reflection zone on the said boundary.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the present practice of seismic
check-shooting,
FIG. 2 shows the signals typically obtained by such
practice, before and after the application of the deconvolution
disclosed in the specification,
FIG. 3 illustrates a vertical profile of geophone
positions above a reservoir,
1 FIG. 4 illustrates the concept of down-stacking the
.1 outputs from a vertical seismic profile,
FIG. 5 depicts a horizontal profile of seismic sources
whose signals are recorded at a borehole geophone by both the
, direct path and by reflection from a hydrocarbon reservoir,
~"~
FIG. 6a illustrates the seismic signals obtained by
the arrangement of FIG. 5, after deconvolution using the
: direct downgoing signal observed at the geophone, and FIG. 6b
illustrates a similar display oriented to appear in cross-
sectional form,
FIG. 7a shows the surface-to-surface ray-path geo-
metry for conventional common-depth-point stacking, and
~,
- 6
~'
~ ' ,
1~4S~37
FIG. 7b shows the surface-to-borehole ray-path geometry for
j common-depth-point stacking according to the invention, and
FIG. 7c illustrates a basic technique for selecting those
source and geophone positions which yield a common depth-
point.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a standard arrangement, known in the
prior art, for the operation of check-shooting a borehole 1.
A borehole geophone 2, usually adapted to lock into the
wall of the hole by the arm 3, is lowered down the hole by
means of an armored conducting cable 6. A seismic source
7 at or near the wellhead, typically an air gun in the
mudpit 8, or in the sea, initiates a seismic signal in the
earth. The output produced by the borehole geophone in
response to this signal is recorded by the recording instru-
ments 9. The outgoing signal is monitored by a geophone or
: hydrophone 10 close to the seismic source 7, and thismoni~or output is also recorded by the instruments 9.
The object of the conventional check-shooting
operation is to obtain a measure of the time taken by the
seismic pulse to travel from the source 7 to the geophone
2, as a function of the depth of the geophone in the
borehole. This time-depth relation is then used to cali-
brate the acoustic log observed in the borehole, and
further used for the conversion of seismic reflection times
to depth.
FIG. 2 shows the type of recording obtained con-
ventionally. The signal 11 displayed on trace 12 is the
record of the outgoing pulse as obtained from the hydro-
phone 10, close to the source. The signal 13 displayed on
trace 14 is the record of the "direct" arrival as obtained
, ~:
. . .
37
from the borehole geophone 2, deep in the earth. The con-
ventional ob~ective is then achieved by measuring the time
between some suitable part (normally the first trcugh) of
pulse 11 and the "corresponding" part of pulse 13, for
different depths of the geophone 2.
The present invention reguires an understanding
- of the factors influencing the form of the borehole trace 14.
The first such factor is obviously the form of
the pulse 11 generated by the source 7. To this basic
pulse shape is added the "ghost" reflection from the free
surface 15 of the mudpit 8 (or the sea, in offshore opera-
tions), and possibly a significant train of multiple
reflections generated in the water. There is, therefore,
a basic source-pulse complex associated with the source
itself and its near-surface environment. This is regarded
as the downgoing "input" to the earth.
Thereafter several agencies produce further
shaping and modification of the downgoing pulse waveform.
One of these is absorption, which produces a progressive
loss of the high frequencies (particularly within rock
formations characterized by frictional grain contacts and
poor sorting). The high-cut effect on the pulse spectrum
is believed to be smooth, so that the pulse broadens
progressively but simply.
Another such agency is the transmission loss at
interfaces. This is known to be offset at low frequencies
-I by short-path multiple reflections in rock sequences
, characterized by cyclic stratification, and thus appears
in practice as a high-cut effect analogous to absorption.
However, the process operates by continuous removal of
energy from the truly direct transmitted signal, and
restoration of much of this energy in the form of a tail
- 8 -
:: ,
$~
to the pulse. This makes the pulse broader, but it is not
necessarily nor generally true that the broadening effect
is simple; in general, the tail added to the pulse is com-
plicated in form, depending as it does on the reflection
statistics of the interfaces traversed. This effect is
discussed in detail in the article "Reflections on Ampli-
tudes", by O'Doherty and ~nstey, in Geophysical Prospect-
ing, volume 19, no. 3, p. 430.
Further agencies which may affect the amplitude
and/or form of the signal observed at the deep geophone
include scattering from geological inhomogeneities, and
the focusing and cusp-forming consequencies of refraction.
Both these agencies can add a complicated tail to the down-
going pulse, as strictly localized effects associated with
particular scatterers or particular acoustic-lens systems.
Finally, the amplitude of the downgoing seismic
pulse as it is detected by the borehole geophone depends
on the acoustic impedance of the geological formation into
which the geophone is coupled.
Those of the above agencies which have the
effect of removing energy from the truly direct arrival at
: the borehole, and of adding back some of this energy at
later times, introduce a difficulty in defining what part
.
of the pulse 13 should be selected for timing purposes as
"corresponding" to a particular part of pulse 11. This
difficulty is real, since the magnitude of the transmission-
loss effect in real geology is almost always so great that,
for any realistic source energy, the direct transmitted
signal is undetectable at depths of common concern.
The downgoing signal at the borehole geophone,
therefore, has a stable component dependent on the source
and the source environment, a depth-dependent but smooth
_ 9 _
:, - . . - -- , - . , , . ~
4633~7
component dependent on absorption in the geologic sequence,
a depth-dependent but complicated component dependent on
the nature of contrasty stratification in the geologic
sequence, a location-dependent component dependent on local
inhomogeneities and lenses, and a final amplitude dependent
on the acoustic impedance at the geophone depth.
The signal actually observed from the geophone is
; the sum of this so-called "direct" downgoing arrival with
numerous upcoming reflections from below the geophone.
Each of these reflections has a~form at least as compli-
cated as the downgoing arrival,and the final interference
pattern on trace 14 is therefore very complicated indeed.
Since the present invention is concerned with
turning particular reflections to good account, it becomes
important to reduce the complications.
The first technique, known in the prior art, is
to use several or many versions of the borehole record,
~i obtained at different depths in the borehole, and to add
such signals, after the application of appropriate static
.,,
,l 20 time shifts, to enhance selectively either the downgoing or
upcoming components in the signals. This technique is
evident from Galperin's book "Vertical Seismic Profiling",
'~ published by SEG in 1974, and from the paper "Well Geophone
Signals as an Aid to Hydrocarbon Indication", by Xennett
and Ireson, read to the Society of Exploration Geophysi-
cists (SEG) in Mexico City, November 1973. The technique
for enhancing the downgoing signal will be referred to
hereinafter as down-stacking,and that for enhancing the
reflected signal as up-stacking.
- 30 Even after the best that can be done with down-
, stacking, the derived signal is still very complex in
nature. The complications associated with "bubbling" in
.
- 10 -
~ 37
the source, and the free-surface and near-surface reflec-
tion systems as described above, can be substantially
removed by the process of deterministic or source-signature
deconvolution; in this the knowledge of the outgoing source
signal 12, and of the depth of the source and the water, is
used to deconvolve the source and near-surface characteris-
tics out of the downhole signal.
However, there still remains in this signal (even
after down-stacking to suppress the upcoming reflections,
and source-signature deconvolution to suppress the charac-
teristics of the input pulse) the complicating effects of
the short-path multiple phenomenon, and of absorption,
scattering and refraction. These complicating effects, as
noted hereinbefore, introduce fundamental and inescapable
difficulties in the interpretation of arrival times and
velocities. And all of these complications, plus those due
to the particular reflecting sequence below the borehole -~
geophone, are also present in an upstack made to enhance
the indications of this reflecting sequence.
For highly-detailed analysis of the said reflect-
ing sequence, it is desirable to remove from the upcoming
reflection train the complexities of form and spectrum
present in the downgoing signal. Indeed the resolution of
events representing the top and bottom of a typical hydro-
carbon reservoir can scarcely be contemplated without the
removal of such complexities, because the latter ordinarily
~: have a form much like that to be expected from such closely-
^ spaced reservoir reflections.
Accordingly, the present invention relates to
using the downgoing arrival (which may be enhanced by down- -
stacking) to deconvolve the upcoming reflection signal.
As is well known, the process of deconvolution
uses knowledge of a known pulse to compute an operator
--11-- .
,. , , . . : ~ .
~ 37
.
which will convert the known pulse into a desired pulse.
The process is limited, in all but its deterministic appli-
cations, by the fact that the "known" pulse is usually
known only statistically (that is, from the autocorrelation
function of a series of pulses assumed stationary, and to
the limits of the minimum-phase assumption). In the
present application, however, the "known" pulse is known
very well; whether observed at a single location or obtained
by down-stacking, it represents the downgoing pulse complex
as it passes the geophone location. Therefore, the operator
can be derived with good assurance, and may thereafter be
applied to the raw geophone signal (without either down-
stacking or up-stacking), or to a selectively up-stacked
signal; in both cases the objective is to transform to a
; 15 spike, or to some other simple pulse form, the component of
the received signal which is due to the complexities of the
down-going path from source to geophone.
Figure 3 illustrates an appropriate field tech-
nique. The borehole is assumed to traverse a hydrocarbon
accumulation 18 having an upper boundary 19 and a lower
boundary 20. The first delineation problem is to separate
and identify the seismic reflections from the boundaries 19
and 20; these two reflections ordinarily defy such separa-
tion and identification.
In the simplest implementation of this aspect of
the invention, which implementation does not employ down-
stacking, a geophone position such as 21 is occupied; the
depth of the accumulation 18 below the surface 27 might
typically be 3000 m(10,000 ft), and the geophone position
21 might typically be 300 m (1000 ft) above the said accum-
ulation. Under these conditions, the geophone output is
~ikely to appear broadly as in Figure 2, in which the
.: .
- 12 -
937
complex 16 represents the complicated downgoing signal, and
the reflections from interfaces 19 and 20 interfere to
yield an even more complicated arrival (but a substantially
separated one) at 28. Then the "known" component for the
deconvolution process is taken as the complex 16, suitably
gated (and possibly tapered towards longer times) to ensure
that the effective signal taken as "known" is concentrated
on the downgoing pulse. After such gating and tapering,
the signal taken as the known downgoing complex is likely
to appear as at 29. A deconvolution operator is then cal-
culated, by techniques well known in the art, to convert
- this signal into a spike. When the operator is applied to
trace 14, the trace emerges as at 30. The ascribed arrival
time for the downgoing signal is now represented by the
approximate spike 31, while the reflections from interfaces
19 and 20 are now clearly separated and identifiable at 32
and 33. This vastly improved resolution from the reservoir
allows amplitude ratio measurements and calculation of the
acoustic properties of the reservoir material, according to
, . .. .
principles well known in the art.
The degree to which the deconvolved arrival 31
represents a spike is a convenient measure of the success
of the deconvolution operation. The degree to which the ~ ~ ~
shape of the reflections 32 and 33 differ from that of the '! ~ '
direct arrival 31 is a measure of the frequency-selective
processes occurring in the interval between the geophone
and the reservoir interfaces, and/or of any complex reflec-
tion character present at the reservoir level. Such
appraisals are highly important for study of the reservoir
characteristics.
If local geologic features dictate that the geo-
phone position 21 should be closer to the reservoir 18 than
suggested above, the gating and/or tapering function may
- 13 -
~ 4~37
need to be more severe. However, no change of principle
results; the reflections 32 and 33 may be seen riding on
other oscillations, and followed by other oscillations,
but recognition of and numerical computations on these
reflections are generally still feasible.
The reduction of the distance from geophone to
reservoir may be reyuired because the interval above the
reservoir abounds with reflectors. Where conditions are
such as to preclude the selection of a gate which is satis-
factorily dominated by the downgoing signal, it may be
desirable to enhance this downgoing signal, relative to
unwanted reflections from the interval between geophone
and reservoir, by down-stacking. In this case, the geo-
phone is caused to occupy a succession of positions as
suggested at 21-26 (typically over a depth range of 150 m
or 500 ft), and the geophone signal is recorded for each
position. Preferably the source is such that its output
remains sensibly constant for each of these geophone
positions; if not, it may be desirable to apply a scaling
factor or a source-signature deconvolution derived from
the source pulse 12 obtained from the near-source hydro-
phone 10. Better still, a "spread" of borehole geophones
is deployed on the cable 6, over the depth range suggested,
and the borehole signals associated with a plurality of
depth locations are obtained from a single source excita-
tion. In either case, the plurality of geophone signals,
thus obtained,appear as in Figure 4. The improved estimate
- of the downgoing signal is then obtained by applying static
time shifts to these traces 34, in order to represent the
delay associated with the extra depth from one position to -
another, and by adding the traces to obtain the new trace
35. In this new trace 35, the downward-propagating align-
ment 36 is enhanced and the upward-propagating (reflection)
- 14 -
~$~4937
alignment 37 is suppressed. The down-stacked trace 35 is
then used for deriving the operator, as described above,
and the operator is then applied to a selected one or a
plurality of the traces 34. The traces may also be up-
stacked at this stage, for greater clarity of the reflec-
tion indications.
Since the geophone is likely to be coupled into
formations of different acoustic impedance at the differ-
ent levels 21-26, the amplitudes of the traces 34 (even
for a constant source output, and after proper compensa-
tion for geometrical divergence effects) may not be equal.
There may be merit in correcting these amplitude variations
(from the formation velocity and density values known from
the borehole logs), or in bringing their average or peak
amplitude to be constant by some suitable normalization
process.
It is clear from the foregoing that the present
!
invention, in using the downgoing signal to deconvolve the
upcoming signal, effects a major clarification of the form
and interpretability of the upcoming reflections. Since
the majority of the effects complicating the downgoing
signal occur either near the source, or in the near-
surface,or in the shallow unconsolidated sediments, or in
sequences of highly contrasty stratification at shallow or
medium depth, it is further apparent that any changes of
pulse shape between the detected downgoing and upcoming
~-~ signals are either small or else of interpretational sig-
nificance in the context of reservoir conditions. And the
resolution of the reservoir reflections is vastly better
than on a surface-to-surface seismic section - not only in
terms of pulse bandwidth (which defines the vertical reso-
lution) but also in terms of the insonified area of the
- 15 -
.; ' .
L14g37
reservoir (which defines the horizontal resolution of
reservoir irregularities).
A further aspect of the present invention, which
is applicable to any type of source, becomes of particular
interest in that it raises the possibility of replacing an
impulsive-type source 7 by a controllable vibrator, and of
designing the excitation signal for this vibrator to
include at least part of the desired deconvolution. Thus,
the component of the downgoing signal form which is conse-
quent on the short-path multiple phenomenon can be computed
from the velocity and density logs in the borehole, and an
operator inverse to this effect can be applied to the down-
hole signal derived from an impulsive source, or to the
coded signal used to drive the vibrator. This latter
possibility means that part of the transition from the
complex downgoing form 16 to the simple downgoing form 31
occurs in the process of transmission through the earth.
As noted above, the improved resolution of the
top-reservoir and bottom-reservoir reflections, which
allows their separation and measurement, yields the
acoustic specification of the reservoir where it is tra-
versed by the borehole. If this approach is to be useful
for a real delineation of the reservoir, however, it is
necessary to modify the technique to obtain reflection
indications away from the borehole, and towards the limits
of the reservoir. -
Figure 5 illustrates a method for achieving this.
Instead of a single source position at or near the well-
head, as used in conventional check-shooting practice, one
or more profiles of source positions are used. In general,
these profiles are disposed to pass through the wellhead,
though the approach may be varied for particular purposes
- 16 -
:
,' :: :'. ' '
37
(for example, in the case of deviated wells). The wellhead
is shown at 38, the borehole at 39, the reservoir at 40,
and a typical geophone position at 41. TwO representative
source positions along the source profile 42 are shown at
43 and 44. In offshore operations, the mobile source is
conveniently a conventional seismic survey vessel. On land
it may be a vehicle-mounted vibrator or impulser, or the
sources may involve the use of explosives or air-guns (in a
plurality of drilled shotholes, or at or near the surface).
The dimensions and areal configuration of the
source profiles are selected, with regard to the depths of
the reservoir and the geophone, to provide suitable reflec-
tion paths from source to geophone via the expected outer-
most limits of the reservoir. -
This concept of a profile of sources is then com-
; bined with the previous proposal of using the downgoing
signal to deconvolve the upcoming signal. Thus, if the
source is constant and accurately repeatable along the
profile (as in the marine case), the do~lgoing signal from
a source position above the geophone(s) may be used, after
selection and enhancement as described hereinbefore, to
deconvolve all the borehole signals recorded from the other
source positions.
The result of this operation is exemplified in
~5 Figure 6a, for several of the many source positions
actually occupied. Trace 45 is the output obtained when
the source is vertically above the borehole geophone(s);
the direct arrival 31 and the two reservoir reflections 32
I and 33 are the same as in Figure 2. Trace 46 is the output
obtained when the source is at position 43 in Figure 5; the
direct arrival 47 follows the dashed path 48, and the top-
reservoir reflection 49 follows the path 50. Trace 51 is a
similar trace obtained when the source is on the other side
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~ ~ ~14~37
of the well 38. Trace 52 is the output obtained from
source position 44; again, the direct arrival 53 follows
the path 54 and the top-reservoir reflection 55 follows
the path 56. Trace 57 is the counter-part of trace 52 on
the other side of the well.
; The direct arrivals typified by 31, 47 and 53
have an approximate hyperbolic alignment. They may also
S]lOW some broadening of form away from the apex trace 45,
relative to arrival 31; in the case under discussion -
(constant source excitation), this broadening is due to
the obliquity now associated with the short-path multiple
phenomenon. If the degree of broadening is negligible, the
! interpreter can proceed to the analysis of the reservoir
reflections. If not, it may be desirable to deconvolve
each trace individually, using the separated direct arrival
twhich may also be specifically down-stacked, taking
account of the time changes introduced by the obliquity) to
..
deconvolve the upcoming reflections on the same trace. As
is evident from the geometry of Figure 5, the obliquity
associated with the reflection path is not quite the same
as that associated with the direct path; however, for the
circumstances of each case, it is easy to make a judgment
as to whether the upcoming signals should be deconvolved
; from the downgoing signal on the same trace, or the
vertical-incidence trace, or some other trace having a
preferred degree of obliquity.
When the source signal is not precisely repeat-
able (in particular, for operations on land), the above
options are not usually available; it becomes desirable to
use each downgoing signal to deconvolve its own upcoming
.
signal, and no other, in order that the variations of
source signal shall be compensated.
- 18 -
~4~3~
When the interpreter is satisfied on these points,
he proceeds to study the reservoir reflections. In the
illustration of Figure 6a, the top of the reservoir (which,
for a velocity-sensitive geophone, is clearly a negative
reflector of computable reflection coefficient) can be
traced across the arrivals 32, 49, and 55 in one direction
and across the arrivals 58 and 59 in the other direction;
likewise the bottom (positive) reflection 33 can also be -~
traced across the reservoir. The characteristic signature
of the reservoir complex lS seen to disappear by thinning
between traces 52 and 60, and by faulting between traces
57 and 61. This is basically the delineation information
which the interpreter is seeking.
The reservoir delineation can be made more pic-
torial by manipulating the display of Figure 6a into that
of Figure 6b. In this illustration (which shows traces
I additional to those shown in Figure 6a), the time axis is
; made vertical to simulate depth in the earth; further, the
reflected arrivals are time-shifted to compensate their
generally hyperbolic configuration, using normal-moveout
corrections derived from the known velocities and horizon-
tal distances. Thus, trace 45, representing the vertical
paths, is identical to the same-numbered trace on Figure
6a; the direct arrival 31 and the reflected arrivals 32
and 33 (from the top and bottom of the reservoir) are as
before. After normal-moveout corrections, however, the
;~l other traces show the time configuration of the reservoir,
pinched-out at 73 to the right and faulted at 74 to the
left, in a true pictorial representation. On these other
traces, the direct arrivals corresponding to 31 have been
suppressed for clarity.
Further useful information on the reservoir
characteristics is available from a detailed study of the
- 19 --
C~37
amplitude and form of the reflections. For this, the
interpreter must understand the factors influencing these
reflections, which include:
(a) the directional nature of the geophone
(if of velocity-sensitive type),
(b) the curvature (if any) of the reflector(s),
(c) the variation of reflection coefficient
with angle of incidence,
(d) if the reflections interfere, the
sensitivity of that interference to the
time between reflections, and, hence,
to the angle of incidence,
(d) refraction effects within the reservoir.
Thus, the interpreter, having made approximate
corrections for the first two of these factors, then pro-
ceeds to explore the effect of the others. In this, he
searches for the increase of a positive reflection coeffi-
cient near the critical angle, for the transition from
reflection to refraction, and for events which may be
recognized as conversions from compressional waves to
vertically-polarized shear waves. The locations at which
any or all of these phenomena appear are then interpreted
in terms of reservoir characteristics, according to Snell's
law and the equations for reflection coefficient.
The most obvious example of this type of analysis
occurs when a highly porous gas saturated reservoir yields
a very strong negative reflection from its upper boundary.
Then, the present technique allows areal mapping of the
strength of this reflection, after correction for the
factors listed above, from several or many profiles through
the well. Of course, the most marked reservoir-induced
anomalies of this type are evident on conventional seismic
through the well; the compelling advantage of the present
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14~37
method is the vastly improved resolution and accuracy of
measurement which become possible when the effects of the
near-surface and shallow section are substantially removed.
Another significant advantage arises in the
mapping of the fluid contact. This is always a positive
reflection, and so can be studied advantageously at wide
angle, and by refraction, if the source profile 42 is
extended to sufficient distance.
It is apparent from the geometry of Figure 5
that one geophone location 41 may be sufficient to explore
the limits of a reservoir in all directions if the well is
approximately central in the reservoir and if the geophone
is placed a suitable distance above the reservoir. The
larger the areal extent of the reservoir, the further the
geophone is ordinarily spaced above the reservoir~ If the
well is not believed to be central, it may be appropriate
to occupy a plurality of geophone positions, in order to
ensure appropriate reflection paths from the near limits
and the far limits of the reservoir. Of course, it is
inevitable that the deconvolution process can remove the
effects of less of the overburden if the limit of the
reservoir is distant from the well (and the geophone is
therefore at shallow depth).
The possibility of occupying a plurality of
geophone positions for each horizontal profile of source
positions is an interesting one. First, it allows the
partial separation of angle-of-incidence effects from
simple reflection-coefficient effects, in that it becomes
possible to select reflection paths representing a con-
stant angle of incidence, and to map the derived reflection
strengths. Further, and more important, it allows a
further component of the present invention - common-depth-
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37
point stacking of borehole signals. This is illustrated
in Figure 7.
Figure 7a depicts three ray-paths, from sources
62 to geophones 63, for a conventional surface-to-surface
common-depth-point gather. As it is widely used in the
prior art, common-depth-point stacking of the plurality of
paths 64 to a common-depth-point 65 on a reflector 66 is
accomplished by first estimating a velocity to be ascribed
to the over-burden 67; this velocity is that which allows
all ray-paths 64 to be corrected to that time value which
would apply if source and receiver were coincident. Then,
after these normal-moveout corrections, the several compon-
ents in the common-depth-point gather are added to yield a
single stacked trace of improved signal-to-noise ratio and
improved primary-to-multiple ratio.
The concept of common depth-point stacking of
borehole signals is illustrated in Figure 7b. The bore-
hole is shown at 70, and the target reflector at 71. Then
:1 .
' the combinations of sources 68 and geophone positions 69
~, 20 are chosen to provide a common depth-point 72 on the
, reflector 71. The objective of the operation is similar
to that of the surface-to-surface stacking in Figure 7a,
but there is a fundamental difference. In surface-to-
`! surface stacking the common nature of the depth-point
i 25 follows from the geometry of the situation, and the essen-
l tial determination is that of the stacking velocity; in
surface-to-borehole stacking the velocity is known rather
precisely from the basic check-shooting operation (parti~
cularly as improved by the deconvolution technique dis-
closed hereinbefore) and the essential determination is
, that of selecting the combinations of source and geophone
position which yield a common-depth-point.
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~,4~37
The field work for borehole cdp-stacking must
include the shooting of a profile of source positions into
each one of a plurality of borehole-geophone positions.
For each of a plurality of common depth-points on the
target reflector appropriate combinations of source posi-
tion and geophone depth are then selected, and a common-
depth-point gather is made.
For the simplest assumption of straight ray-
paths and horizontal reflectors, appropriate combinations
of source position and geophone are based on the equation
x/p = l+l/(l-d/z),
where
x = distance of source from wellhead
(or vertical through geophone);
p = distance of common depth-point
from borehole (or vertical through
geophone);
d = depth of borehole geophone;
z = depth of target reflector.
Alternatively, those source positions and geophone
positions which yield a common depth-point may be estab-
lished by the simple graphical construction of Figure 7c.
In this the image-points 6~' of the geophone positions 69
in the reflector 71 are located, and combinations of source
,,,,1
and geophone positions are selected to yield ray-paths
passing through a single point or small zone in the
xeflector 71.
For more complicated situations, the appropriate
` combinations of source position and geophone depth are found
using the concept of rms velocity, or by a ray-tracing exer-
....
cise which takes into account all the reflector dips and
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all the known velocities. For a first estimate of
reflector dips, it is an advantage to have a conventional
seismic reflection section on each source profile, and in
some cases, it makes good sense to associate the shooting
of the source profile into the borehole geophone with the
double function of shooting a conventional spread of
surface geophones.
After the appropriate combinations of source
positions and geophone depths have been determined, norm-
ally for each of a plurality of common depth-points, the
relevant traces are assembled into common-depth-point
gathers. Each trace is deconvolved according to one of
the techniques set out hereinbefore (the deconvolution
being advantageously arranged so that it also compensates
variations in trace amplitude associated with the acoustic
impedance of the material local to each geophone position),
and each trace is also compensated for the effects of
geometrical divergence by techniques well known in the art.
The first and major utility of the common-depth-
point gathers is, of course, that the traces can be cor-
rected for moveout and other time variations (using the
known velocity information, as discussed previously), and
stacked. This yields an improved version of the signal
reflected from each depth-point, and allows an improved
map of reflection strength over the reservoir area. Where
faults are recognized on the gathers, the stacking may be
done selectively, not to use paths which can be seen to
have passed through a faulted zone.
Various auxiliary measurements can also be made
on the common-depth-point gathers. One of these is the
variation of reflection coefficient with angle of inci-
dence, since the gathered paths represent the same reflec-
tion zone observed at different angles. Thus, critical-
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: -
3 ~
angle effects, and the transition into refraction at a
measurable velocity, can be observed very clearly, and
interpreted in terms of the contrast and anisotropy of both
velocity and acoustic impedance existing at the reflector.
These wide-angle effects are very much in evidence, because
with usual reservoir depths and extents, there is every
inducement to occupy source positions representing an x/p
ratio as high as 3 or more, thus incurring incidence
angles of 45 or more. Another auxiliary measurement is
; 10 therefore the amplitude, velocity and phase change asso-
ciated with the wave converted at oblique incidence from
compressional to vertically-polarized shear. Where shear
conversions can be identified from the top and bottom of
the reservoir, the shear velocity within the reservoir
can be obtained; this, and its ratio to the compressional
velocity,is also a quantity to be mapped over the extent
of the reservoir. The compressional (and shear) velocity
in the materials overlying the reservoir are also
measured, since it is observed that anomalous mineraliza-
tion sometimes introduces significant variations in
velocity in the rocks locally above hydrocarbons.
It is also true that many or all of the opera-
tions and measurements disclosed in this specification may
be employed with horizontally-polarized shear waves. This
requires specially adapted sources and an array of horizon-
tally-sensitive units in the borehole geophone.
A further vari2tion of interest becomes possible
if several boreholes exist in the reservoir area; then
there is merit in disposing borehole geophones in a plural-
ity of them simultaneously, and of recording the signals
derived from profiles of sources between the wellhead
locations, as well as generally radially from each
borehole.
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. . .
4~37
The techniques of this invention are also
generally applicable to the case where the seismic source
can also be disposed in a borehole, below the worst of
the near-surface frequency-selective agencies.
It will be understood that various other changes
in the details, materials, and arrangements of parts which
have been described and illustrated in order to explain
the nature of the invention will occur to and may be made
by those skilled in the art upon a reading of this disclo-
sure. Such changes are equivalents and are intended to be
included within the principle and scope of this invention
which is limited only by the Claims attached hereto.
This is a divisional application of application
serial number 287,178, filed September 21, 1977.
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