Changes

==Dipmeter data acquisition==

==Dipmeter data acquisition==

The determination of dip angle and direction of a planar surface requires the elevation and geographical position of at least three points. Dipmeter tools achieve this result by measuring some sensitive formation parameter by means of three or more identical sensors mounted on caliper arms so as to scan in detail different sides of the borehole wall. A bedding plane crossing the borehole at an angle would generate anomalies at each sensor, and these anomalies would be recorded at slightly different depths on the surface recording. The relative displacements and the radial and azimuthal positions of each sensor are then used to compute dip relative to the tool. Microresistivity has been the traditional formation parameter logged. Modern dipmeter tools usually carry more than three sensor arms, the latest version being a device with six arms. More measure points provide the advantage of systematic redundancy, which allows the application of statistical error minimization techniques.

The determination of dip angle and direction of a planar surface requires the elevation and geographical position of at least three points. Dipmeter tools achieve this result by measuring some sensitive formation parameter by means of three or more identical sensors mounted on caliper arms so as to scan in detail different sides of the borehole wall. A bedding plane crossing the borehole at an angle would generate anomalies at each sensor, and these anomalies would be recorded at slightly different depths on the surface recording. The relative displacements and the radial and azimuthal positions of each sensor are then used to compute dip relative to the tool. Microresistivity has been the traditional formation parameter logged. Modern dipmeter tools usually carry more than three sensor arms, the latest version being a device with six arms. More measure points provide the advantage of systematic redundancy, which allows the application of statistical error minimization techniques.

For the results to be geographically significant, it is necessary to define the orientation of the tool in space. This involves continuous measurements of the orientation of the electrode array relative to north, its rotation relative to the high side of the hole, and the inclination of the tool axis from vertical. Such navigation data are produced from the output of an assembly of three orthogonally mounted magnetometers and a similar array of accelerometers. Figure 1 is a sketch of a four-arm tool illustrating the orientation measurements. Figure 2 is an expanded scale recording of the raw dipmeter data showing the orientation curves, calipers, gamma ray, and correlation curves from a six-arm tool. Note that the curves are responding to apparent bedding features less than [[length::1 in.]] thick.

For the results to be geographically significant, it is necessary to define the orientation of the tool in space. This involves continuous measurements of the orientation of the electrode array relative to north, its rotation relative to the high side of the hole, and the inclination of the tool axis from vertical. Such navigation data are produced from the output of an assembly of three orthogonally mounted magnetometers and a similar array of accelerometers. [[:file:dipmeters_fig1.png|Figure 1]] is a sketch of a four-arm tool illustrating the orientation measurements. [[:file:dipmeters_fig2.png|Figure 2]] is an expanded scale recording of the raw dipmeter data showing the orientation curves, calipers, gamma ray, and correlation curves from a six-arm tool. Note that the curves are responding to apparent bedding features less than [[length::1 in.]] thick.

[[file:dipmeters_fig1.png|thumb|{{figure number|1}}Sketch of a four-arm dipmeter tool illustrating pertinent orientation measurements.]]

[[file:dipmeters_fig3.png|thumb|left|{{figure number|3}}Example of thin bed laminated sandstone-shale resolution by means of a dipmeter correlation curve.]]

 

[[file:dipmeters_fig2.png|thumb|{{figure number|2}}Expanded scale recording of raw dipmeter data from a six-arm tool.]]

==Thin bed resolution==

==Thin bed resolution==

* Flasering and load structures

* Flasering and load structures

In Figure 3, a raw dipmeter curve sharply distinguishes between sandstone and shale layers thinner than [[length::1 in.]] By establishing a cutoff line as shown, everything to the right can be identified as sand and everything to the left as shale. A reliable measurement of net sand thickness is thus provided, as well as the bulk volume fraction of shale in laminated form for input into laminated shaly sand saturation equations.

In [[:file:dipmeters_fig3.png|Figure 3]], a raw dipmeter curve sharply distinguishes between sandstone and shale layers thinner than [[length::1 in.]] By establishing a cutoff line as shown, everything to the right can be identified as sand and everything to the left as shale. A reliable measurement of net sand thickness is thus provided, as well as the bulk volume fraction of shale in laminated form for input into laminated shaly sand saturation equations.

==Dip computation==

==Dip computation==

In the dip determination phase, one has a choice between a geometric solution and a stochastic approach. While a geometric solution works well in a uniquely determined (three-point) situation, it becomes cumbersome in an overdetermined condition such as that found with four-arm and six-arm tools. In these cases, a stochastic or global mapping approach is more effective in that it uses the redundancy to advantage in minimizing errors. Actually, this approach acts like another stage of stacking filtering. It has the added advantage of neatly solving for orientation of the tool in space.

In the dip determination phase, one has a choice between a geometric solution and a stochastic approach. While a geometric solution works well in a uniquely determined (three-point) situation, it becomes cumbersome in an overdetermined condition such as that found with four-arm and six-arm tools. In these cases, a stochastic or global mapping approach is more effective in that it uses the redundancy to advantage in minimizing errors. Actually, this approach acts like another stage of stacking filtering. It has the added advantage of neatly solving for orientation of the tool in space.

==Dipmeter presentations==

==Dipmeter presentations==

The most common presentation of dipmeter data is the ''arrow'' or ''tadpole plot'', which is a clever two-dimensional representation of a three-dimensional quantity. In this plot, the base of the arrow is positioned at the depth of the midpoint of the correlation interval, and the distance from the left-hand margin to the base of the arrow is proportional to the true dip angle as calibrated by the scale shown on the heading. The shaft of the arrow points in the downdip direction with true north being straight up the page. Figure 4 is a standard arrow plot that also carries a correlation gamma ray curve and maximum and minimum caliper values. On the right-hand side is a representation of the inclination angle and direction of the tool, which will usually be similar to the deviation of the borehole. A number of other computer generated presentations have been introduced. Many are useful in special situations, but none is a replacement for the arrow plot.

The most common presentation of dipmeter data is the ''arrow'' or ''tadpole plot'', which is a clever two-dimensional representation of a three-dimensional quantity. In this plot, the base of the arrow is positioned at the depth of the midpoint of the correlation interval, and the distance from the left-hand margin to the base of the arrow is proportional to the true dip angle as calibrated by the scale shown on the heading. The shaft of the arrow points in the downdip direction with true north being straight up the page. [[:file:dipmeters_fig4.png|Figure 4]] is a standard arrow plot that also carries a correlation gamma ray curve and maximum and minimum caliper values. On the right-hand side is a representation of the inclination angle and direction of the tool, which will usually be similar to the deviation of the borehole. A number of other computer generated presentations have been introduced. Many are useful in special situations, but none is a replacement for the arrow plot.

 

 

==Applications of dipmeters==

==Applications of dipmeters==