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Claims  |
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What is claimed is:
1. A method for determining the relative position of formation features
along a borehole by processing three signals representative of a
characteristic of said formation along said borehole and derived at spaced
apart locations around said borehole; comprising the steps of:
(a) for each of two respective pairs of signals and at each given depth
level in a succession of adjacent depth levels, finding a
cross-correlation function between respective correlation intervals of the
signals of said each pair of signals, when the correlation intervals are
depth displaced relative to each other at selected steps;
(b) identifying, from a comparison of the cross-correlation functions of
said succession of adjacent depth levels for said two respective pairs of
signals, intervals of depth for which respectively the associated
cross-correlation functions emphasize, for said each pair of signals, a
consistent relative depth displacement of the respective signals of said
each pair of signals; and
(c) deriving, from said cross-correlation functions which are within said
identified intervals of depth, an estimate of the relative position of
said formation features.
2. The method of claim 1 wherein said deriving step comprises the following
steps:
providing a function which defines the relative position of said features
in terms of a weighted sum of said cross-correlation functions; and
finding an amplitude and an azimuth value relative to the borehole
relationship between said features which maximizes said provided function
for a selected depth.
3. A method for determining the relative position of formation features
along a borehole by processing three signals representative of a
characteristic of said formation along said borehole and derived at spaced
apart locations around said borehole; comprising the steps of:
finding the degree of cross-correlation between the respective signals of
each of two respective pairs of signals at a first selected depth level in
the formation when respective portions of the signals of said each pair of
signals are depth displaced relative to each other over a respective first
depth interval at selected steps;
finding the degree of cross-correlation between the respective signals of
said each pair of signals at a second selected depth level, adjacent to
said first depth level, when respective portions of of the signals of said
each pair of signals are depth displaced relative to each other over a
respective second depth interval at selected steps;
producing respectively first traces showing the degree of cross-correlation
found at said first depth level and respective second traces showing the
degree of cross-correlation found at said second depth level for said each
pair of signals;
respectively forming the first and second traces for said each pair of
signals on a side-by-side single record display such that said record
presents the respective first and second traces for said each pair of
signals adjacent to each other and arranged in an order related to depth
level;
identifying, from a comparison of the first and second traces for said each
pair of signals formed on said side-by-side single record display, an
interval of depth for which the first and second traces for said each pair
of signals emphasize, for said each pair of signals, a consistent relative
depth displacement of the respective signals of said each pair of signals;
and
deriving, from the first and second traces for said each pair of signals
which are within said identified interval of depth, an estimate of the
relative position of said formation features.
4. The method of claim 2 in which the first depth interval and the second
depth interval substantially overlap.
5. The method of claim 2 including selecting further successively adjacent
depth levels for the respective pairs of signals, finding the respective
degree of cross-correlation therebetween respectively at each respective
one of said successive depth levels when respective portions of the
signals are depth displaced relative to each other over respective depth
intervals at selected steps, and producing for each respective pair of
signals a respective one of said traces at further depth levels and
forming the last recited traces for each respective pair of signals on
said side-by-side display such that each record presents the respective
last recited traces adjacent to each other and arranged in an order
related to depth level.
6. A method for determining the relative position of formation features
along a borehole by processing three signals representative of a
characteristic of said formation along said borehole and derived at spaced
apart locations around said borehole; comprising the steps of:
at each given one of a succession of adjacent depth levels, finding the
degree of cross-correlation between respective portions of the signals,
which are over a respective depth interval above and below the given depth
level, when the respective portions of the signals are depth displaced
relative to each other at selected steps;
for each given one of said successive depth levels, producing respective
traces of the degree of cross-correlation found at said each given depth
level; and forming the traces on a single record, successively adjacent to
each other
whereby each of said single records obtained for respective pairs of
signals are presented in a side-by-side display such that the traces
obtained for any given depth level are adjacent;
identifying, from a comparison of the traces presented in said side-by-side
display, intervals of depth of which the traces emphasize, for each of
said respective pairs of signals, a consistent relative depth displacement
of the respective signals of said each pair of signals; and
deriving, from the traces which are within said identified intervals of
depth, an estimate of the relative position of said formation features.
7. The method of claims 3 or 6 in which the respective depth intervals for
the respective adjacent depth levels substantially overlap.
8. The method of claims 3 or 6 including normalizing the amplitude of each
of said traces prior to forming the trace on the record.
9. The method of claims 3 or 6 in which the step of forming the traces on
the single record includes forming the traces for each respective pair of
signals in a two-dimensional coordinate system in which one axis is depth
level, wherein the respective traces for a given depth level for the
respective pair of signals are presented side-by-side.
10. The method of claims 3 or 6 in which each step of finding the degree of
cross-correlation between the respective pair of signals at a given depth
level includes combining the respective portions of the signals in
accordance with a normal correlation function to find a normalized
correlogram indicative of the respective degree of cross-correlation
between the respective pairs of signals at the given depth level.
11. The method of claim 10 in which the step of combining log portions to
find a normalized correlogram for a given depth level includes combining
respective signals portions which extend a given distance up and down in
depth level from said given depth level.
12. The method of claims 3 or 6 in which each step of forming a trace on a
single record comprises forming a trace in which amplitude changes are
represented by intensity or density modulation of an otherwise straight
trace.
13. The method of claims 3 or 6 in which each step of forming a trace on a
single record comprises forming a trace in which amplitude changes are
indicated by excursions of the trace along the depth level axis direction.
14. A method for determining the magnitude, azimuth and depth of a dip
plane which intersects a borehole from correlations between related pairs
of signals obtained along portions of the length of and at spaced apart
points around the borehole, comprising the steps of:
producing overlapping correlations of the respective signals of related
pairs of signals, each of said overlapping correlations overlapping
adjacent correlations on the same related pair of geophysical signals;
comparing successive overlapping correlations of a first pair of signals to
determine a sequence that emphasizes consistent relative depth
displacements between common features of said first pair;
comparing successive overlapping correlations of a second pair of signals
to determine a sequence that emphasizes consistent relative depth
displacements between common features of said second pair;
selecting as a zone of valid determinations an interval of depth where both
said first and second pairs of signals exhibit said respective sequences;
and
deriving, from said overlapping correlations which are within said selected
zones, an estimate of the magnitude, azimuth and depth of a dip plane.
15. The method of claim 14 wherein said deriving step comprises the
following steps:
providing a function which defines the relative position of said features
in terms of a weighted sum of said cross-correlation functions; and
finding an amplitude and an azimuth value relative to the borehole
relationship between said features which maximizes said function for a
selected depth.
16. The method of claim 14 wherein each of said comparing steps includes
the following step:
providing respective traces showing the degree of overlapping correlations
found at successive depth levels, and forming the traces on a single
record, successively adjacent to each other.
17. The method of claim 16 further comprising the step of: presenting each
of said single records obtained for each respective pair of signals in a
side-by-side display such that the traces obtained for any given depth
level are adjacent.
18. Apparatus for determining the relative position of formation features
along a borehole by processing three signals representative of a
characteristic of said formation along said borehole and derived at spaced
apart locations around said borehole; comprising:
for each of two respective pairs of signals and at each given depth level
in a succession of adjacent depth levels, means for finding a
cross-correlation function between respective correlation intervals of the
signals of said each pair of signals when the correlation intervals are
depth displaced relative to each other at selected steps;
means for identifying, from a comparison of the cross-correlation functions
of said succession of adjacent depth levels for said two respective pairs
of signals, intervals of depth for which respectively the associated
cross-correlation functions emphasize, for said each pair of signals, a
consistent relative depth displacement of the respective signals of said
each pair of signals; and
means for deriving, from said cross-correlation functions which are within
said identified intervals of depth, an estimate of the relative position
of said formation features.
19. Apparatus for determining the relative position of formation features
along a borehole by processing three signals representative of a
characteristic of said formation along said borehole and derived at spaced
apart locations around said borehole; comprising:
means for finding the degree of cross-correlation between the respective
signals of each of two respective pairs of signals at a first selected
depth level in the formation when respective portions of the signals of
said each pair of signals are depth displaced relative to each other over
a respective first depth interval at selected steps;
means for finding the degree of cross-correlation between the respective
signals of said each pair of signals at a second selected depth level,
adjacent to said first depth level, when respective portions of of the
signals of said each pair of signals are depth displaced relative to each
other over a respective second depth interval at selected steps;
means for producing respectively first traces showing the degree of
cross-correlation found at said first depth level and respective second
traces showing the degree of cross-correlation found at said second depth
level for said each pair of signals;
means for respectively forming the first and second traces for said each
pair of signals on a side-by-side single record display such that said
record presents the respective first and second traces for said each pair
of signals adjacent to each other and arranged in an order related to
depth level;
means for identifying, from a comparison of the first and second traces for
said each pair of signals formed on said side-by-side single record
display, an interval of depth for which the first and second traces for
said each pair of signals emphasize, for said each pair of signals, a
consistent relative depth displacement of the respective signals of said
each pair of signals; and
means for deriving, from the first and second traces for said each pair of
signals which are within said identified interval of depth, an estimate of
the relative position of said formation features.
20. Apparatus for determining the relative position of formation features
along a borehole by processing three signals representative of a
characteristic of said formation along said borehole and derived at spaced
apart locations around said borehole; comprising:
at each given one of a succession of adjacent depth levels, means for
finding the degree of cross-correlation between respective portions of the
signals, which are over a respective depth interval above and below the
given depth level, when the respective portions of the signals are depth
displaced relative to each other at selected steps;
for each given one of said successive depth levels, means for producing
respective traces of the degree of cross-correlation found at said each
given depth level; and means for forming the traces on a single record,
successively adjacent to each other
whereby each of said single records obtained for respective pairs of
signals are presented in a side-by-side display such that the traces
obtained for any given depth level are adjacent;
means for identifying, from a comparison of the traces presented in said
side-by-side display, intervals of depth for which the traces emphasize,
for each of said respective pairs of signals, a consistent relative depth
displacement of the respective signals of said each pair of signals; and
means for deriving, from the traces which are within said identified
intervals of depth, an estimate of the relative position of said formation
features.
21. The apparatus of claims 19 or 20 further including means for
normalizing the amplitude of each of said traces prior to forming the
trace on said record.
22. The apparatus of claim 21 in which the normalizing means includes means
combining respective signal portions which extend a given distance up and
down in depth level from a given depth level.
23. The apparatus of claims 19 or 20 in which the single record forming
means includes means for forming the traces for each respective pair of
signals in a two-dimensional coordinate system in which one axis in depth
level, wherein the respective traces for a given depth level for the
respective pair of signals are presented side-by-side.
24. A well logging method for producing a single record containing
side-by-side displays, each display being of respectively two or more
adjacent traces showing the respective degrees of correlation, at two or
more respective adjacent depth levels for the same depth interval in an
earth formation, of respective pairs of a plurality of well logs taken at
different positions around a borehole in the formation, each respective
trace showing the degree of cross-correlation of the logs of each
respective pair of logs at the respective depth level when respective
portions of the logs are depth displaced relative to each other over the
depth interval at selected steps, comprising the following steps:
deriving a first, second and third well logs taken at different positions
around a borehole in the earth formation;
finding the degree of cross-correlation of the respective logs of each of
two different pairs of the well logs at a first selected depth level in
the formation when respective portions of the respective logs of the pairs
of logs are depth displaced relative to each other over a respective first
depth interval at selected steps;
finding the degree of cross-correlation of the respective logs of each of
the respective pairs of logs at a second selected depth level, adjacent to
the first selected depth level, when respective portions of the respective
logs of the pairs of logs are depth displaced relative to each other over
a respective second depth interval at selected steps;
producing respectively first traces showing the degree of cross-correlation
found at the first depth level and respective second traces showing the
degree of cross-correlation found at the second depth level for each of
the respective pair of logs, and
respectively forming the first and the second traces for each respective
pair of logs on a side-by-side single record display such that said record
presents the respective first and second traces for each pair of logs
adjacent to each other and arranged in an order related to depth level
with the respectively first and respectively second traces from each of
said respective pair of logs arranged adjacent to each other.
25. The well logging method of claim 24 in which the first depth interval
and the second depth interval substantially overlap.
26. The well logging method of claim 24 including selecting further
successively adjacent depth levels for the respective pairs of logs,
finding the respective degree of cross-correlation therebetween
respectively at each respective one of said successive depth levels when
respective portions of the logs are depth displaced relative to each other
over respective depth intervals at selected steps, and producing for each
respective pair of logs a respective one of said traces at further depth
levels and forming the last recited traces for each respective pair of
logs on said side-by-side single display such that each record presents
the respective last recited traces adjacent to each other and arranged in
an order related to depth level.
27. The well logging method of claim 26 in which the respective depth
intervals for the respective adjacent depth levels substantially overlap.
28. The well logging method of claim 27 including normalizing the amplitude
of each of said traces prior to forming the trace on the record.
29. The well logging method of claim 28 in which the step of forming the
traces on the single record includes forming the traces for each
respective pair of logs in a two-dimensional coordinate system in which
one axis is depth level, wherein the respective traces for a given depth
level for the respective pair of logs are presented side-by-side.
30. The well logging method of claim 29 in which each step of finding the
degree of cross-correlation between the respective pair of logs at a given
depth level includes combining the respective portions of the logs in
accordance with a normal correlation function to find a normalized
correlogram indicative of the respective degree of cross-correlation
between the respective pairs of logs at the given depth level.
31. The well logging method of claim 30 in which the step of combining log
portions to find a normalized correlogram for a given depth level includes
combining respective log portions which extend a given distance up and
down in depth level from said given depth level.
32. The well logging method of claim 31 in which each step of forming a
trace on a single record comprises forming a trace in which amplitude
changes are represented by intensity or density modulation of an otherwise
straight trace.
33. The well logging method of claim 31 in which each step of forming a
trace on a single record comprises forming a trace in which amplitude
changes are indicated by excursions of the trace along the depth level
axis direction.
34. A well logging method of producing a single record containing a
multiplicity of successively adjacent traces showing the respective
degrees of cross-correlation at respective successively adjacent depth
levels in an earth formation of a plurality of well logs taken along
different positions in a borehole in the formation, each given trace
showing the degree of cross-correlation at a given depth level of portions
of a respective pair of logs which are within a selected depth interval
above and below the given depth level, comprising the following steps for
each of two respective pairs of logs:
at each given one of a succession of adjacent depth levels, finding the
degree of cross-correlation between respective portions of the logs, which
are over a respective depth interval above and below the given depth
level, when the respective log portions are depth displaced relative to
each other at selected steps;
for each given one of said successive depth levels, producing respective
traces of the degree of cross-correlation found at said each given depth
level; and forming the traces on a single record, successively adjacent to
each other; and
further comprising the steps of:
presenting each of said single records obtained for each respective pair of
logs in a side-by-side display such that the traces obtained for any given
depth level are adjacent.
35. The well logging method of claim 34 in which the finding step includes
finding the mutual degree of cross-correlation at successive depth levels
for respective successive depth intervals which overlap with each other.
36. The well logging method of claim 35 in which the step of forming the
traces on a single record comprises forming the traces in an orthogonal
coordinate system in which one axis is relative depth displacement between
logs and the other is depth level in the earth formation, each trace
extending generally along the depth displacement axis and the traces for
adjacent depth levels being adjacent to but spaced from each other along
the depth level axis.
37. The well logging method of claim 34 in which each finding step
comprises combining the respective portions of the logs in accordance with
a normalized correlation function in order to produce a correlogram for
the respective depth level based on the respective depth interval and the
selected steps.
38. The well logging method of claim 34 in which the step of forming the
traces on a single record comprises representing amplitude changes in the
trace by density modulation or intensity modulation of a trace which
generally extends along a straight line.
39. The well logging method of claim 34 in which the step of forming the
traces on a single record comprises forming each trace as a line which
extends generally in one direction but is amplitude modulated to make
excersions in an orthogonal direction. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates generally to techniques used in geophysical well
logging, and more particularly to new techniques for automatically
processing dipmeter signals to produce more accurate dip and azimuth
representations of subsurface formations.
A common method of measuring the dip angle and direction or azimuth of
subsurface formations employs a dipmeter tool passed through a borehole
drilled into the subsurface formations. This tool may apply any of
numerous means to obtain geophysical signals representative of variations
of a particular formation characteristic, such as resistivity. One such
tool is described in the paper: "The High Resolution Dipmeter Tool", by L.
A. Allaud and J. Ringot, published in the May-June 1969 issue of The Log
Analyst.
Dip and azimuth measurements representing the inclination of a formation
characteristic or feature may be determined from dipmeter signals
containing information representing the intersection of such a feature at
three or more radially spaced points on the borehole surface. A two-step
decision process is traditionally employed whereby in a first step the
displacement between two points intersecting a common feature may be
determined, under favorable circumstances, by correlating pairs of
dipmeter signals, each having a similar response to the common feature.
Thereafter, in a second step the displacements between at least three
different points are examined to determine the position of a plane. The
position of such a plane is conveniently expressed by its dip, an angle
measured from a reference (usually horizontal) plane, and its azimuth, an
angle measured from a reference direction (usually true North). Typically,
the dipmeter signals are recorded as a function of depth on computer
compatible magnetic tape at the well site for later processing. The
measured signals can be processed either at the well site or off the well
site using any of several techniques such as manual, semi-automatic and
fully automatic processing which may be aided by either analog or digital
computers.
A computer program to perform the digital processing operations is
described in a paper entitled "Automatic Computation of Dipmeter Logs
Digitally Recorded on Magnetic Tape" by J. H. Moran, et al, and published
in the July, 1962 issue of the Journal of Petroleum Technology. An
additional computer program is described in the paper, "Computer Methods
of Dip Log Correlation" by L. G. Schoonover et al, pages 31-38, published
in the February 1973 issue of Society of Petroleum Engineers Journal.
Furthermore, programs to process digitally-taped dipmeter data are
available from digital computer manufacturers, such as IBM.
Results from the processing of the measured signals are normally presented
in tubular listings as dip and azimuth measurements versus borehole depth.
When desired, the individual displacements found between the correlation
curve pairs which led to the dip and azimuth values may also be presented.
At each step or depth level, one sequence of displacements between various
pairs of signal combinations may be obtained. A typical sequence includes
at least two displacements but may include a round of up to six
displacements in each sequence when four separate signals are employed.
When a round or more than two displacements in one sequence is obtained,
the displacements may be combined into many more possibly different
combinations, each combination corresponding to perhaps a different dip
and azimuth measurement. Since only two related displacements are
required, it is presently a common practice to utilize only what appears
to be the two best qualified displacements. All other displacements are
discarded without further consideration, thereby producing only one result
per sequence of displacements. Further, little information is retained
regarding the position of the sources or of the measured signals of the
dipmeter pads corresponding to the utilized displacements other than
perhaps a display of a caliper measurement.
When large numbers of measurements result, as from recent high resolution
dipmeter techniques, tabular listings are usually augmented by graphic
presentations of dip and azimuth representations. The graphic displays
vary with the interpretation objective, depending upon whether the purpose
is for stratigraphic or structural studies. Accordingly, relationships
between the corresponding dip and azimuth measurements and their
continuity with depth are considered in different manners.
For stratigraphic analysis purposes, trends of adjacent dip measurements,
for example, measurements representing a trend of rapidly increasing dip
with depth, are considered separately from measurements representing a
trend of rapidly decreasing dip with depth. It is important that the
azimuth of these dips remain substantially constant and thereby represent
the general direction of sediment transport or perhaps the probable
direction of down dip thickening. Dipmeter results may be further combined
in a given analysis from intervals corresponding to a given depositional
or stratigraphic unit.
Graphic displays used in stratigraphic analysis are typically the azimuth
frequency plot (no dip or depth representation) and the Schmidt net and
the Stereonet (azimuth versus dip but still no depth representation).
These nets and several variations thereof have known statistical
characteristics in that they may enhance either low or high dip
measurement point groupings. In their use, the dip and azimuth value for
each measurement is combined and represented by a point in these nets. A
description of some of these displays and their application is given in a
paper entitled "Stratigraphic Applications of Dipmeter Data in
Mid-Continent" by R. L. Campbell, Jr., published September 1968 in the
American Association of Petroleum Geologists Bulletin.
Structural analysis is distinguished from stratigraphic analysis in the
type of information needed. While in stratigraphic analysis, the measured
signals hopefully represent bedding planes within the boundaries of a
given geological unit, these bedding planes have little, if any, regional
extent. Structural analysis, in contrast, requires a deliberate attempt to
mask out such sedimentary features in favor of enhancing the boundaries of
the individual strata.
Conventionally, short lengths (1 to 2 feet) of dipmeter signals along a
borehole are correlated to obtain stratigraphic information while long
lengths (10 to 20 feet) are correlated to obtain structural information.
While use of long correlation lengths to obtain structural dip has been
standard practice for some time, there are certain disadvantages
associated with the use of long correlation lengths. One such disadvantage
is that the use of long correlation lengths masks dip patterns needed for
stratigraphic analysis, thus additional computatons must be made using a
shorter length to obtain stratigraphic information. Another disadvantage
is that most techniques employing long correlation lengths are influenced
by frequently occurring stratigraphic features having a common dip and
direction, even though each such feature is less pronounced than the
structural feature. Thus, the use of long correlation lengths does not
assure that accurate structural dip information has been obtained. Yet
another disadvantage is that current correlation techniques tend to ignore
the possibly objectionable effects of rotation of the dipmeter tool within
the long correlation interval. While it would be more desirable to obtain
the detailed information availiable only from short correlation intervals
and then apply previously mentioned trend analysis to separate the
stratigraphic and structural dips, it will be appreciated that as the
correlation interval is shortened, the probability of obtaining a
completely erroneous displacement increases substantially. The wrong peak
on the correlation function produced in the correlation process may be
used to determine the displacement. Such invalid displacements may be
combined with valid displacements to produce erroneous dip information
which add scatter and confuse valid trends or, when the invalid
displacements are systematically erroneous, may even appear as false
trends. As a compromise, it has been the practice to employ correlation
intervals having a length which is greater than the length actually
desired so as to reduce this scatter to an acceptable level such that any
valid trend which may be present might be found. As a result, the
occurrence of dip estimates has no relationship to the occurrence of bed
boundaries, deposition boundaries between regions of different geological
activity or the degree of geological activity.
SUMMARY OF THE INVENTION
In accordance with principles of the present invention shortcomings of
aforementioned prior art methods of dipmeter signal processing, involving
the computation of sets of displacements followed by the fitting of dip
planes to these displacements, are overcome by optimizing a single
criterion in going from resistivity signals to dip estimate.
Advantageously, this optimization may involve maximizing a weighted sum of
cross-correlation functions between the different pairs of resistivity
signals. Through the employment of such single criterion the limitation
inherent in the prior art processing, due to the dependency of the dip
determination on the selection process of significant displacements from
the resistivity curves which selection process by necessity ignores other
possible displacements, is overcome since all possible displacements
between resistivity curves are considered in the determination of dip
planes. Additionally, the different weighting of the cross-correlation
functions in the proposed signal processing scheme provides an assurance
that the resulting dip estimate is derived with due consideration of the
relative merit of factors such as the quality of measured resistivity
signals and the presence of isolated dominant features in the resistivity
curves. Therefore, by taking account of the totality of information
provided by the resistivity curves and by the further consideration of the
relative merit of such information, more accurate and geologically
consistent dip and azimuth values for subsurface formations may be
determined.
In further accordance with principles of the present invention the
cross-correlation functions, derived from respective pairs of resistivity
signals, are determined for a selected first depth level from the
cross-correlation of a sample (i.e., a one foot interval) of one of the
curves with respect to a larger interval comprising several samples of the
other curve which encompass the selected depth level. A second sample
adjacent to the first sample is then selected and values for the
cross-correlation function are again determined. This process is repeated
for any interval of depth and the normalized values of the
cross-correlation functions are displayed as a function of depth. This
display, for dip analysis purposes, affords considerable insight into the
nature of the information provided by pairs of resistivity curves and aids
in the interpretation of dip planes since trends and discontinuities in
the cross-correlation functions, as a function of depth, between pairs of
curves are rendered clearly visible. Additionally, the degree and
sharpness of cross-correlation over different portions of the curves are
also rendered clearly visible and may therefore be employed to provide a
measure of the quality of the resistivity curves obtained from the
dipmeter tool and therefore affords the opportunity of adaptively
adjusting the respective weights given to each cross-correlation function
in the process of fitting a dip plane.
Yet in further accordance with principles of the present invention, the
display of the cross-correlation functions as a function of depth affords
the further opportunity of avoiding the use of fairly long correlation
lengths, dictated by the need in the prior art practices to resolve
ambiguities in displacements, in favor of shorter correlation lengths
since the ambiguities in displacements can be resolved by detecting the
aforementioned discontinuities in the cross-correlation functions between
pairs of resistivity curves. Therefore, according to the novel practices
of the present invention, both the resolution afforded by short
correlation lengths, which preserve rapid changes in the curves, and the
smoothing required to resolve ambiguities can be attained.
Yet in further accordance with principles of the present invention, the
display of the cross-correlation functions as a function of depth affords
the added opportunity of zoning the data along the depth dimension. This
zoning of the data provides for a greater accuracy and efficiency in the
processing of the dipmeter data since dip estimates are not produced at
fixed intervals of depth but are rather produced in intervals of depth
which are chosen from segments of data where the undesirable effects of
discontinuities in the cross-correlation functions between pairs of curves
are greatly reduced. Therefore a positive relationship is established
between dip estimates and the occurence of bed boundaries, deposition
boundaries or degree of geological activity.
In accordance with one embodiment of the present invention, methods and
apparatus, for processing signals derived along portions of the length of
a borehole to determine the relative position of geological function
characteristics, effect the correlation of pairs of signals using
substantially overlapping correlation intervals to obtain
cross-correlation functions for each pair of signals. These
cross-correlation functions are then side-by-side displayed as a function
of borehole depth for each pair of curves such that discontinuities in the
cross-correlation functions are rendered obvious. From such
discontinuities, zoning of the data is enabled through determination of
the position of significant continuous intervals. Sequences of
cross-correlation functions which belong to one of these intervals are
then used to directly determine, through the employment, for example, of a
function maximizing criterion, dip and azimuth values for subsurface
formations.
BRIEF DESCRIPTION OF THE DRAWINGS
In the Drawings:
FIG. 1 is a view of an investigating device in a borehole along with
apparatus at the surface of the earth for controlling the investigating
apparatus and recording the measurments derived therefrom in accordance
with an embodiment of the present invention;
FIG. 2 is an example of logs produced from laterally spaced apart sensors
of the device of FIG. 1, when the device passes through a borehole;
FIG. 3 represents the relationship between the sampling intervals of
respective curves employed in the derivation of cross-correlation
functions in accordance with the present invention;
FIG. 4 is a graphical representation of a cross-correlation function, or
correlogram, for one depth level;
FIGS. 5a, 5b and 5c are respective displays of a number of correlation
functions for respective pairs of resistivity signals provided by the
apparatus of FIG. 1, displayed as function of borehole depth; and
FIG. 6 is a block diagram of steps useful in the practice of the present
invention in accordance with one embodiment thereof.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there is illustrated a method of acquiring and
processing signals obtained from a borehole investigating device commonly
known as a dipmeter. This device is described in one form in U.S. Pat. No.
3,521,154 issued July 21, 1970 to J. J. Maricelli. The purpose of the
dipmeter device is to obtain signals from three or more radially spaced
sources usually supported on pads which contact the borehole wall. Signals
obtained from such sources reflect formation features at their
intersection with the borehole wall and are useful in determining the
orientation of the formations penetrated by the borehole.
Typical earth formations are represented by the shale formations 13 and 14
shown in FIG. 1, and intervening sand formation 15. Typical formation
features are boundaries 16 and 17 shown between these formations.
As shown in FIG. 1, the borehole apparatus 18 is lowered on a cable 38 into
a borehole 10 for investigating the earth's formations. The downhole
investigating device 18 is adapted for movement through the borehole 18
and as illustrated, includes four pads designated 19, 20, 21 and 22 (the
front pad 19 obscures the view of back pad member 21 which is not shown).
The pad members 19 through 22 are adapted to derive measurements at the
wall of the borehole. Each pad includes a survey electrode shown as Ao.
One of the pads, herein designated as pad 19, may contain an additional
survey electrode A1 useful in determining the speed of the tool. Each
survey electrode is surrounded by an insulating material 48. The
insulating material in turn is surrounded by a main metal portion 45 of
the pad. The metal portion 45 of each pad, along with certain other parts
of the apparatus, comprise a focussing system for confining a survey
current emitted from each of the different survey electrodes into a
desired focussed pattern. Survey signals representative of changes in the
formation opposite each pad are obtained from circuits comprising Ao
electrodes, focussing elements, and a current return electrode B.
The upper end of the borehole tool 18 is connected by means of the armored
multiconductor cable 30 to a suitable apparatus at the surface for raising
and lowering the downhole investigating device through the borehole 10.
Mechanical and electrical control of the downhole device may be
accomplished with the multiconductor cable which passes from the downhole
tool 18 through the borehole to a sheave wheel 31 at the surface and then
to a suitable drum and winch mechanism 32.
Electrical connections between various conductors of the multiconductor
cable, which are connected downhole to the previously described
electrodes, and various electrical circuits at the surface of the earth
are accomplished by means of a suitable multi-element slip-ring and brush
contact assembly 34. In this manner, the signals which originate from the
downhole investigating device are supplied to a control panel 39 which in
turn supplies signals to a processor 40 and a recorder 41. A suitable
signal generator (not shown) supplies current to the downhole tool and to
signal processing circuits located at the surface. More details of such
circuits are described in the aforementioned Maricelli patent.
Signals obtained from the downhole device may be recorded graphically by a
plotter 42 and displayed on a CRT 43. In addition, the signals may be
processed to obtain discrete samples which may then be recorded on digital
tape. A suitable digital tape recorder is described in U.S. Pat. No.
3,648,278 issued to G. K. Miller, et al on Mar. 7, 1972.
The signals may be sampled by driving sampling devices, such as those
described in the above-mentioned Miller, et al patent, by the cable motion
as measured at the surface. For example, the cable length measuring wheel
35 may be used in controlling the signal processing, sampling and
recording functions as indicated by signal line | | |
