Six curve pair correlations can be attempted between four curves. The adjacent curve pair displacements are designated respectively as h12, h23, h34, and h41, and the diagonal displacements as h13 and h24. These six displacements can in turn be paired in thirteen different ways to provide thirteen dip evaluations for the same level. For the six arm dipmeter, 15 pairs are possible, leading to additional redundancy. The result from each combination is referred to as a dip determination. In recent practice, however, only four or five correlations are made, leading to a maximum of eight possible dip determinations per level. This reduces computer time.
arm closure error (Ec) is given by the algebraic sum of the four
adjacent curve displacements:
For perfect closure, Ec = 0.
Three arm closure error can also be computed on a four arm or six arm dipmeter. In this case, closure error is given by the algebraic sum of two adjacent curve displacements and their associated diagonal displacement. This error is distributed around the displacements in proportion to the amount of each displacement.
When four or six arm closure exists, or has been created by distributing
the error, another error, the planarity error can be measured
among the four adjacent curve displacements. Because opposite
pairs of pads in the four pad array form a parallelogram, the
displacement observed between curves 1 and 2 should be the same
as that between curves 4 and 3, and the displacement between curves
2 and 3 should be equal to that between curves 1 and 4. Thus,
for perfect planarity:
four arm closure error is zero, planarity error (Ep) is defined
For perfect planarity, Ep = 0. Similar equations exist for the six arm dipmeter.
closure error is zero and planarity is not zero, then several
things may be possible. One is that the bedding may not be planar,
such as in the case of festoon current bedding or aeolian dune
surfaces. Other possibilities are lack of pad contact with the
hole wall and possible miscorrelations. The latter are,
The flow chart below shows the complex logic involved in Schlumberger's high resolution dipmeter program. It handles the closure and planarity problems in numerous ways, based on the number and quality of correlations found.
The output listing from this program is shown below. Notice that some of the logic choices are coded on the listing and others on the arrow plot by use of alternate symbols.
Dips can also be coded and presented in such a way as to indicate the fact that they are non-planar. This would help an analyst interpret the bedding, as shown in the example below, which was processed using Gearhart's OMNIDIP program.
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