Dipmeter displacement processing technique

Claims

What is claimed is:

1. A method of automatically determining with a machine and producing recorded representations of the relative position of formation features reflected on geophysical signals by processing displacements obtained between pairs of said signals by comparing the similarity of signal features in given intervals of said signals and in sequences of overlapping signal invtervals, said signals being derived from separate sources for said signals spaced at different positions around a borehole penetrating said formation; comprising:

(a) combining in each sequence a plurality of possibly corresponding pairs of displacements between said signals to obtain redundant indications of said formation feature positions in each sequence and in at least one sequence of overlapping signal intervals;

(b) classifying said indications for each sequence and overlapping sequence as a function of the relative position of said separate signal sources to obtain a multiplicity of classified redundant indications from at least two sequences;

(c) analyzing said classified indications to determine the position of classes of classified indications which have substantially similar classifications;

(d) comparing the position of the indications for one sequence with the position of said classes to determine those indications which contributed to one of said classes and the corresponding position of a formation feature for said one sequence; and

(e) producing recorded representations of the corresponding position of said formation feature.

2. A method of automatically processing with a machine displacements obtained between geophysical signals derived from sources spaced at different positions to determine and produce recorded representatives of the relative position of the features on said signals, comprising:

(a) producing displacements obtained between similar features in overlapping intervals of geophysical signals derived from said spaced sources, each of said intervals overlapping adjacent intervals on a signal derived from a given source;

(b) combining possibly corresponding displacement between said signals;

(c) classifying each combination as a function of the relative position of said sources;

(d) analyzing said classified combinations to determine the position of the dominant class and the corresponding relative position of features on said signals; and

(e) producing recorded representations of the corresponding relative position of features on said signals.

3. The method of claim 2 wherein said features on said signals correspond to variations in characteristics of the subsurface earth formations.

4. The method of claim 3 wherein said geophysical signals are derived from separate sources located at different positions in a borehole penetrating subsurface earth formations.

5. The method of claim 4 wherein said sources are signal sources located in different positions in the borehole to investigate the subsurface earth formations penetrated by the borehole.

6. The method of claim 5 wherein a plurality of possibly corresponding displacements are combined in pairs to produce a plurality of combinations for one sequence of signal intervals and for at least one additional sequence with overlapping signal intervals.

7. The method of claim 6 wherein said classification of combinations is referenced to the different positions of the signal sources in the borehole.

8. The method of claim 7 wherein said possibly corresponding displacements include displacements obtained between pairs of geophysical signals having at least one signal in common for each pair.

9. The method of claim 8 wherein said possibly corresponding displacements correspond to displacements within zones bounded by sequences of substantially stable displacements occurring in an essentially contiguous sequence for at least two pairs of geophysical signals having one signal in common between pairs.

10. The method of claim 9 wherein said step of combining possible corresponding displacements includes generating for each possible combination a function representing the relative position of said pairs of geophysical signals.

11. The method of claim 10 wherein said function representing the relative position may be represented as a vector and the step of classifying each combination includes the step of classifying each vector into one of a plurality of classifications representing possible relative positions of said pairs of geophysical signals.

12. The method of claim 11 wherein the step of analyzing the classified combinations includes forming groups of vectors which have substantially the same classification.

13. The method of claim 12 wherein the step of forming groups of vectors includes comparing the number of vectors in each class with the number of vectors in other classes and determining the class having the dominant mode.

14. The method of claim 13 wherein the step of analyzing said classified combinations to determine the corresponding relative position of said signals includes selecting vectors belonging to said dominant mode as corresponding to the relative position of said signals for at least some sequences of displacements.

15. The method of claim 14 wherein said displacements and corresponding combination of displacements are assigned a quality factor representing the degree of correlation between said pair of geophysical signals.

16. The method of claim 15 wherein said assigned quality factor is used to weight each vector generated in the step of combining possible corresponding displacements so that said weighted vectors with the high quality weights influence the determination of the dominant mode more than weighted vectors with low quality weights.

17. The method of claim 16 wherein vectors corresponding to the dominant mode and obtained from a limited sequence of displacements are combined into a single relationship representative of the relative position of a feature on said signals.

18. The method of claim 17 wherein said geophysical signals correspond to dipmeter signals and said displacements correspond to possible displacements between similar formation features and said relative position of a feature on said signals corresponds to the relative position of a formation characteristic at its intersection with the borehole wall.

19. The method of claim 18 wherein said determined relative position of the signal sources is employed to determine the dip and azimuth of the formation characteristic.

20. The method of claim 18 wherein said possibly corresponding displacements must satisfy certain requirements in order to be considered as corresponding.

21. The method of claim 20 wherein one of said requirements is that displacements obtained between signal sources located at different positions in the borehole must be obtained from signal sources determined to be at a known position relative to the borehole wall in order to be considered as corresponding displacements.

22. The method of claim 21 wherein said known position relative to the borehole wall is substantially in contact with the borehole wall.

23. The method of claim 22 wherein the determination of the position relative to the borehole wall includes determining a function of the departure from a plane of the displacements obtained for a given sequence and regarding as not corresponding those displacements departing substantially from said plane.

24. The method of claim 23 wherein said displacements departing substantially from said plane are considered as corresponding displacements unless said displacements were obtained between at least one signal derived from a signal source indicated to be substantially displaced from the top side of the borehole wall.

25. The method of claim 21 wherein the determination of the position relative to the borehole wall includes determining a function of the relative nature of the signals and regarding as not corresponding those displacements obtained from signal sources having a nature corresponding to relatively less variation in the characteristics of said subsurface earth formation.

26. A method of automatically determining with a machine and producing recorded representations of the relative position of formation features reflected on geophysical signals by processing displacements obtained between pairs of said signals by comparing the similarity of signal features of given intervals of said signals and in sequences of overlapping signal intervals, said signals being derived from separate sources for said signals spaced at different positions around a horehole penetrating said formation, comprising:

(a) comparing displacements obtained from successive overlapping correlations of one pair of geophysical signals to determine a sequence of substantially stable displacements between said pair;

(b) comparing displacements produced from successive overlapping correlations of a related pair of geophysical signals to determine another sequence of substantially stable displacements between said related pair;

(c) determining as a zone of stable displacement combinations those displacements bounded by at least two sequences of said substantially stable sequences which contain pairs of stable displacements related by possible correspondence;

(d) combining in each sequence a plurality of possibly corresponding pairs of displacements between said signals to obtain redundant indications of said formation feature positions in each sequence and in at least one sequence of overlapping signal intervals, said sequences being included in a zone bounded by the boundaries of said zone of stable displacement combinations;

(e) classifying said indications for each seuqence and overlapping sequence as a function of the relative position of said separate signal sources to obtain an multiplicity of classifed redundant indications from at least two sequences;

(f) analyzing said classified indications to determine the position of classes of classified indications which have substantially similar classifications;

(g) comparing the position of the indications for one sequence with the position of said classes to determine those indications which contributed to one of said classes and the corresponding position of a formation feature for said one sequence; and

(h) producing recorded representations of the corresponding position of a formation for said one sequence.

27. A method of automatically determining with a machine and producing recorded representations of the relative position of formation features reflected on geophysical signals by processing displacements obtained between pairs of said signals by comparing the similarity of signal features in given intervals of said signals and in sequences of overlapping signal intervals, said signals being derived from separate sources for said signals spaced at different positions around a borehole penetrating said formation, comprising:

(a) summing said displacements to determine if said displacements are substantially devoid of closure error and thereby correspond to the same formation feature;

(b) summing combinations of said displacements which are substantially free of closure errors to determine those combinations of displacements which indicate a planarity error;

(c) locating the signal source most likely not to be in the proper position in the borehole and determining if those displacements common to said source are exaggerated as compared to other displacements common to other sources;

(d) nullifying from further processing as possibly corresponding displacements those displacements common to said source if those displacements are exaggerated;

(e) combining in each sequence a plurality of the remaining possibly corresponding displacements between said signals to obtain redundant indications of said formation feature positions in each sequence and in at least one sequence of overlapping signal intervals;

(f) classifying said indications for each sequence and overlapping sequence as a function of relative position of said separate signal sources to obtain a multiplicity of classified redundant indications from at least two sequences;

(g) analyzing said classified indications to determine the position of classes of classified indications which have substantially similar classifications;

(h) comparing the position of the indications for one sequence with the position of said classes to determine those indications which contributed to one of said classes and the corresponding position of a formation feature for said one sequence; and

(i) producing recorded representations of the corresponding position of a formation for said one sequence.

28. A method of automatically determining with a machine and producing recorded representations of the most valid combination of displacements from a multiplicity of displacements and combinations thereof which may be obtained from correlations between related pairs of geophysical signals comprising:

(a) producing displacements obtained from overlapping correlations of related pairs of geophysical signals, each of said overlapping correlations overlapping adjacent correlations on the same related pair of geophysical signals;

(b) combining possibly corresponding displacements from the produced displacements to generate for each possible combination a function representing an angular relationship between said related pairs of signals, said relationship being relative to the position of the sources of said signals;

(c) analyzing said generated angular relationships for said combinations and determining the most probable angular relationship;

(d) retrieving, as the most valid displacement combinations, those displacement combinations belonging to the most probable angular relationship; and

(e) producing recorded representations of said most valid displacement combinations.

29. The method of claim 28 wherein said related pairs of signals each have one signal in common in each pair of signals.

30. The method of claim 29 wherein said angular relationship between each signal and each related pair of signals is known in one plane.

31. The method of claim 30 wherein said possibly corresponding displacements correspond to zones of displacements whose limits are defined by instabilities in an essentially contiguous sequences of substantially stable displacements for at least two pairs of signals.

32. The method of claim 31 wherein the function representing the angular relationship between pairs of geophysical signals may be represented as a vector.

33. The method of claim 32 wherein the step of analyzing the generated angular relationships includes locating the highest concentration of represented vectors for said zone, said location determining the most probable angular relationship and most valid displacement combination.

34. The method of claim 33 wherein the step of analyzing said generated angular relationships includes the step of classifying each vector into one of a plurality of classifications representing possible angular relationships between said signals.

35. The method of claim 34 wherein the step of locating the highest concentration of vectors includes the step of counting the number of said classified vectors in each of said plurality of classifications.

36. The method of claim 35 wherein the step of locating the highest concentration of vectors includes locating clusters of vectors in adjacent classifications.

37. The method of claim 36 wherein the step of locating the highest concentration of said vectors includes ranking said located clusters in accordance with the relative concentrations of vectors in each cluster.

38. The method of claim 37 wherein vectors located in the highest ranking cluster are selected as representing the most valid combinations of displacements for at least some sequences of displacements in said zone.

39. The method of claim 38 wherein said limits for zones of possibly corresponding displacements are determined from sequences of displacements obtained from overlapping correlations between pairs of geophysical signals wherein each of said at least two pairs of signals have one signal in common for the correlation.

40. The method of claim 39 wherein said displacements are assigned a quality factor representing the degree of correlation between said pair of geophysical signals.

41. The method of claim 40 wherein said assigned quality factor is used to weight the vectors and said step of locating the highest concentration of vectors includes locating the highest concentration of said weighted vectors so that vectors with high quality weights increase said concentration substantially more than vectors with low quality weights.

42. The method of claim 41 wherein vectors located in the highest concentration of vectors which are obtained from a contiguous sequence of displacements within said zone are pooled into a single representation of the combination of displacements by combining the components of said located vectors.

43. The method of claim 42 wherein each of said geophysical signals is derived from a different one of a plurality of sources on a borehole dipmeter, with each of said sources separated by a given angular relationship in one plane.

44. The method of claim 43 wherein said displacements obtained from correlating said pairs of dipmeter signals correspond to displacements between signals measured at sources around the wall of a borehole drilled through subsurface formations, said signals responsive to the intersection of a common formation feature with the borehole wall.

45. The method of claim 44 wherein said most valid combination of displacements determine the position of a planar surface representative of a formation feature intersecting the borehole wall at points displaced by an amount corresponding to said most valid combination of displacement and wherein said most valid combination of displacements are converted to expressions of the dip and azimuth of said planar surface.

46. An apparatus for automatically processing displacements obtained between pairs of geophysical signals derived from sources spaced at different positions to determine and produce recorded representations of the position of features on said signals, comprising:

(a) means for producing displacements obtained between similar features in overlapping intervals of said signals, each of said intervals overlapping adjacent intervals on a signal derived from a given source;

(b) means for combining possible corresponding displacements produced from said displacement producing means;

(c) means for classifying displacement combinations combined in said combining means, said classification being related to the relative position of said signal sources;

(d) means coupled to said means for classifying displacement combinations for analyzing said classified combinations to determine the location of a formation feature on the dominant class and thereby the corresponding position of said signals; and

(e) means for producing recorded representations of the corresponding position of a feature on said signals.

47. The apparatus of claim 46 wherein said displacement producing means produces more than two possibly corresponding displacements for one sequence of signal intervals and for at least one additional sequence with overlapping signal intervals, and wherein said combining means combines a plurality of different corresponding displacement combinations for at least one of said sequences, thereby providing redundant combinations to said classification means.

48. The apparatus of claim 47 and including means for outputting representations of said classified displacement combinations to provide indications of distribution modes and their locations, and thereby the positions of formation features on said signals for at least some of said sequences.

49. The apparatus of claim 47 and including means for comparing displacement combinations from at least one sequence with the combinations located within at least one class to determine if at least one combination from said sequence contributed to one of said classes, and further including means for outputting an indication of said contribution for said sequence.

50. A method of automatically processing with a machine displacements produced by correlating geophysical signals derived from a borehole apparatus to determine and record more accurate dip and azimuth values for a subsurface formation, comprising:

(a) producing displacements obtained from correlating substantially overlapping correlation intervals of pairs of said geophysical signals responsive to changes in formation characteristics near the borehole wall;

(b) combining displacements corresponding to a common interval to produce representatives of possible apparent dip and azimuth values for a subsurface formation, said apparent values being relative to the position of the borehole apparatus, said representatives each corresponding to one combination of two related displacements;

(c) classifying said representatives to form groups of representatives which have common apparent dip and azimuth values;

(d) comparing said groups of representatives to select the most reliable group;

(e) retrieving the representatives of a given interval which contributed to forming said selected group and determining from said retrieved representatives the corresponding dip and azimuth values as the more accurate values for said formation; and

(f) recording the corresponding dip and azimuth values for application outside said machine.

51. The method of claim 50 wherein said interval corresponds to a depth interval.

52. The method of claim 51 wherein said displacements corresponds to depth displacements.

53. The method of claim 52 wherein said similar geophysical signals are obtained from a dipmeter apparatus passed through a borehole penetrating said subsurface formation.

54. The method of claim 53 wherein said representatives of apparent dip and azimuth values are vectors.

55. The method of claim 54 wherein the step of classifying said vectors to form groups includes forming as a group vectors having substantially the same classifications.

56. The method of claim 55 wherein the step of forming groups includes comparing the number of vectors in each classification with other neighboring classifications and determining the classification having the dominant group of vectors.

57. The method of claim 56 and further including the step of selecting at least one of said vectors in said dominant group of vectors as more accurately representing one depth in said interval.

58. The method of claim 57 and further including the step of determining a vector representative of a given number of vectors within said group of vectors, said determined vector more accurately representing at least a portion of said common interval.

59. A method of automatically processing with a machine displacements produced by correlating overlapping intervals of geophysical signals to determine and record the dip of a subsurface formation, each of said intervals overlapping adjacent intervals on a signal derived from a given source; comprising:

(a) producing a plurality of possible displacements between a first and second geophysical signal over a specified interval;

(b) producing additional possible displacements between one of said first and second signals and an additional geophysical signal over said interval;

(c) combining certain of said displacements according to known physical relationships between the sources of said signals to produce vectors functionally related to possible apparent dips of a subsurface formation;

(d) forming groups of vectors which have common apparent dips;

(e) comparing said groups of vectors to select the most reliable group of rectors;

(f) retrieving a vector representative of said interval from the vectors in said selected group and determining the corresponding actual dip of the subsurface formation in the specified interval; and

(g) recording the corresponding actual dip of the subsurface formation in relation to said specified interval.

60. The method of claim 59 wherein said interval corresponds to a depth interval.

61. The method of claim 60 wherein said displacements correspond to depth displacements.

62. The method of claim 61 wherein said similar geophysical signals are obtained from a dipmeter apparatus passed through a borehole penetrating said subsurface formation.

63. The method of claim 62 wherein said displacements have an associated quality factor representing the quality of the correlation between said signals which produced said displacements.

64. The method of claim 63 wherein said vectors are weighted in accordance with said quality factors so that combinations of high quality displacements form better comparing groups of vectors than combinations of low quality displacements.

65. The method of claim 64 wherein said step of combining certain said displacements includes combining only displacements meeting certain quality criteria.

66. The method of claim 65 wherein one of said criteria is that said certain displacements must have a quality factor of at least a given quality.

67. The method of claim 66 wherein said step of combining certain of said displacements includes combining only displacements from zones of displacements bounded by instabilities in sequences of displacements indicated to be stable over at least a specified number of displacements.

68. The method of claim 67 wherein said step of combining certain of said displacements include combining only pairs of displacements indicating substantially the same displacement for at least a given number of displacements over a common interval.

69. The method of claim 68 wherein said step of combining certain of said displacements includes combining only displacements produced by correlating geophysical signals obtained from sources on said dipmeter apparatus which are indicated to be in a known physical position relative to one another and the walls of said borehole penetrating said subsurface formation.

70. A method of automatically determining with a machine and producing recorded representations of the most valid combination of displacements from a multiplicity of displacements and combinations thereof which may be obtained from correlations between related pairs of geophysical signals comprising:

(a) producing displacements from overlapping correlations of related pairs of geophysical signals, each of said overlapping correlations overlapping adjacent correlations on the same related pair of geophysical signals;

(b) comparing displacements produced from successive overlapping correlations of one pair of geophysical signals to determine a sequence of substantially stable displacements between said pair;

(c) comparing displacements produced from successive overlapping correlations of a related pair of geophysical signals to determine another sequence of substantially stable displacements between said related pair;

(d) determining as a zone of valid displacement combinations those displacements in said substantially stable sequences which ae related by possible correspondence; and

(e) recording representations of these displacements in said substantially stable sequences.

cl 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 or displacement measurements obtained between these 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 its 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. The displacement between two points intersecting a common feature may be determined, under favorable circumstances, by correlating pairs of the dipmeter signals, each having a similar response to the common feature. Two displacements between three different points determine the position of a plane. The position of the plane is conveniently expressed by its dip .theta., an angle measured from a reference (usually horizontal) plane and its azimuth .PHI., an angle measured from a reference direction (usually true North). Typically, the dipmeter signals are recorded on computer compatible magnetic tape at the well site for later processing. The recorded signals are processed using any of several techniques. Manual, semi-automatic and fully automatic processing may be used with the automatic processing being performed with either analog or digital computers. When digital computers are used, a computer program is also required.

A computer program to perform the digital processing operations is described in a paper, "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 Diplog Correlation" by L. G. Schoonover, et al, pages 31-38, published in the February 1973 issue of Society of Petroleum Engineers Journal. Further, programs to process digitally-taped dipmeter data may be obtained from digital computer manufacturers, such as IBM.

Results from digital processing are normally presented in tabular listings as dip and azimuth measurements versus borehole depth. When desired, the individual displacements found between the correlated curve pairs which led to the dip and azimuth values may be also presented. Further, most such programs will provide the ability to vary both the length of the correlation interval and the step used to move this interval between each correlation sequence. For the next sequence, the same correlation length is used, but the actual interval correlated is moved by one correlation step length.

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, for example. When a round of 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 common practice to utilize only what appears to be the two best qualified displacements. All others are discarded without further consideration, thereby producing only one result per sequence. Further, little is retained as to the position of the sources or dipmeter pads corresponding to the utilized displacements.

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 purposes, trends of adjacent dip measurements with depth are usually used to classify the measurements. For example, measurements representing a trend of rapidly increasing dip with depth will be considered separately from measurements representing a trend of rapidly decreasing dip with depth.

In the stratigraphic analysis, it is important that the azimuth of these dips must remain substantially constant and thereby represent the general direction of sediment transport or perhaps the probable direction of down dip thickening. Also, dipmeter results are combined in a given analysis from intervals corresponding to a given depositional or stratigraphic unit.

Graphic displays used for stratigraphic analysis often ignore the actual depths once the above dip versus depth trend for a given azimuth range qualifies a group of measurements. Further, since in many cases the actual dip angle is not important and only the dip azimuth is significant, the dip angle may be completely ignored in the graphic display. Such displays are designed to statistically determine the azimuth corresponding to a primary and perhaps a secondary direction of transport or deposition.

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. Note that in their use, the dip and azimuth value of 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 the paper "Stratigraphic Applications of Dipmeter Data in Mid-Continent" by R. L. Campbell, Jr., published September 1968 in the American Association of Petroleum Geologists Bulletin.

Stratigraphic and structural analyses distinguish themselves in the type of information needed. In stratigraphic analysis, the dipmeter signals hopefully represent bedding planes within the boundaries of a given geological unit. These bedding planes have little, if any, regional extent. In structural analysis, a deliberate attempt may be made to mask out such sedimentary features in favor of enhancing the boundaries of the individual strata.

Short lengths (1 to 2 or 3 feet) of dipmeter signals are correlated to obtain stratigraphic information while long lengths (10 to 20 or 30 feet) of signals are often 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 this practice. One is that the use of long correlation lengths masks dip patterns needed for stratigraphic analysis, thus additional computations must be made using a short length to obtain stratigraphic information. Another is that most long correlation length techniques may be 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 obtaining accurate structural dip information. Yet another disadvantage is that current correlation techniques tend to ignore possibly objectionable effects of rotation of the dipmeter tool within the long correlation interval.

The preferred approach is to obtain the detailed information available only from short correlation intervals and then apply previously metioned trend analysis to separate the stratigraphic and structural dips. However, 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 and produce an erroneous dip which add scatter and confuse valid trends or when systematically erroneous, may even appear as false trends.

As a compromise, longer correlation intervals than are actually desired are employed to artificially reduce this scatter to an acceptable level so that any valid trend which may be present might be found.

It is therefore an object of this invention to provide a technique to reduce the scatter in dip and azimuth measurements determined from short correlation intervals.

One technique which is employed to reduce scatter and find dip and azimuth trends is to average long intervals of dip measurements obtained from much shorter intervals. Unfortunately, the valid trends present only as short intervals may be masked completely by such an averaging process. Further, the resolution and position of the correct peak obtained by correlating short intervals tends to vary considerably, consequently, the corresponding displacements lack accuracy. Certain combinations of such displacements may compound the variation and introduce unacceptable inaccuracies in the resulting dip and azimuth measurements.

It is therefore an additional object of the present invention to provide a technique to improve the accuracy and reduce the scatter of dip and azimuth measurements without necessitating long interval averaging.

Some of the averaging techniques include a preliminary process of sorting or discarding apparently stray dips before averaging to prevent their contributing to the average. This process adds both time delays and expense to a process which already produces too few dips for many purposes. Further, some of the apparent strays may actually be part of a valid trend which was unfortunately just sampled infrequently. Both the discarding and averaging processes suppress such valid dips.

It is therefore a further object of the present invention to provide an automatic technique to improve the accuracy of dip and azimuth determinations without reducing the number of valid dips or discarding dips because they do not comply with some long interval trend.

When such averaging techniques are employed, the intervals to be averaged are often chosen arbitrarily such as every 100 feet or the like. Yet such zoning or sample grouping is an important factor in most statistical analysis. In some techniques, independent geological information is examined (usually manually) to select specific zones to be averaged. This latter process requires considerable time as well as accurate coordination of the depths of the geological information and the dipmeter information. This depth coordination may be a problem in deviating holes where the dipmeter information might not correspond to true depths. It would therefore be advantageous to have the determination of zones be made from the dipmeter data itself.

It is therefore a further object of the present invention to provide a technique for automatically zoning dipmeter information by analyzing the dipmeter information itself.

As previously discussed, there are prior art techniques for statistically analyzing either the dip or azimuth information for long interval trends. These methods usually employ polar chart representations to classify the dip and/or azimuth measurements. In these plots, the dip varies with distance from either the center or the edge of the plots and the azimuth varies with the radial distribution from the center of the plot.

However, when one considers the type of errors likely to take place in the correlation processes, particularly in deviated holes, it is desirable that any analysis not separate the dip from the azimuth values for the purposes of the analysis. The analysis should be able to detect any interrelationship between the dip and azimuth for the individual measurements. More particularly, the analysis should respect the fact that erroneous displacements can be concealed when expressed only as the resulting dip and azimuth measurements.

It is therefore a further object of the present invention to provide a technique for analyzing displacements and combinations of displacements rather than computing and analyzing the resulting dip or azimuth measurements.

Prior art methods of dip and azimuth analysis largely ignore the direction of th borehole when deviated. Yet this may be an important control on the distribution of the measurements. Due to the type of problems associated at times with the borehole tool operation, the position of the tool and the signal sources relative to the borehole deviation and direction should be considered in the analysis in case they are also a factor in the distribution of the measurements.

Therefore, it is a still further object of the present invention to provide a system