Auxiliary Dipmeter Presentations

Dipmeter computation data are displayed graphically and in tabular form in many different formats, to facilitate interpretation. The standard output consists of a raw data plot, arrow plot, and numerical listings, many of which have been shown earlier in the discussion of tool and program theory. The balance are optional at extra cost. They are usually run only after evaluation of the standard output.

Stick Diagrams
The cross section plot or stick diagram, is a two dimensional cross section representing the dipping bedding planes at a pre-selected azimuth. It shows the apparent dip of each bedding plane as it would cross the borehole at the specified cross section azimuth. A common use is to establish the dip expected between a well with computed dipmeter information and a projected well close to the original well, or between two wells.


Stick diagram in steep regional dip - gamma ray (not shown) was used to aid correlation

This allows the person using the plot to estimate the depth to particular horizons in the new well. Another use is in correlating formations from one well to another when both have dipmeter data. The ability to compute a stick diagram with apparent dip along any defined azimuth makes it easy to project formation tops from one well to another. The direction of the stick plot can also be presented parallel and/or perpendicular to a seismic line and the apparent dips compared with the dips observed on the seismic line.

Cylindrical plot in complex cross bedding

Cylindrical Plots
The cylindrical plot is a two-dimensional presentation that has the appearance of the borehole split along the south axis. When placed in a transparent cylinder, the bedding planes appear as they would in an oriented core.

The cylindrical plot is especially useful for locating the position of faults or major unconformities where these are reflected by a change in dip direction or magnitude. The STRATIM and DIPVUE images described earlier offer the same advantages.

Schmidt Plots
The modified Schmidt diagram is used to determine structural dip when it is hard to find from the arrow plot. The paper is polar with North at the top. Dip magnitudes are represented by concentric circles. The plot is divided into cells at 1 degree magnitude and 10 degree azimuth; the dots are plotted for all dips computed. In some cells there may be no dots; in others, one dot; in still others, two or more dots. The plot can be generated by hand or by computer.

The dots will fall into distinctive groupings or patterns, which can be outlined by contour lines. Structural dip is an elongated pattern hugging the outer rim of the plot, possibly extending over a wide range of azimuths. The remaining dips (slope and current patterns) will plot in rough triangles with their apexes pointing toward the center of the plot.


Schmidt plot separates regional from stratigraphic dips

Azimuth Frequency Plots
Azimuth frequency plots, often called rose diagrams, are plotted on polar coordinate paper with north at the top and 10 degree azimuth increments. The length of each 10 degree segment is proportional to the number of dips in the interval having that azimuth range, with zero frequency at the center. The result will be little fans originating at the center which may be composed of structural dip and current patterns, often at right angles to each other.

There is no information in the azimuth frequency plot concerning the magnitude of dip. This information must come from other plots. Azimuth frequency diagrams are excellent tools for delineating bars, reefs, channels, and troughs. An illustrative example is shown below, along with a schematic diagram of the channel represented by the frequency plots.


Azimuth frequency plots (rose diagrams) show preferential sedimentation directions

This plot is is called a pattern azimuth frequency plot, because dips belonging to red and blue patterns (to be described later) are preserved and plotted separately. Blue patterns show direction of transport and red patterns show direction to the thicker sand. If plotted in black and white, as is the normal case, the lobes of the diagram are often still identifiable.


Rose diagrams on FMS Image Examiner

The arrow plot presented to the customer contains azimuth frequency plots generated for each 100 ft. interval or other regular interval as designated by the analyst. These plots are used for general information concerning the direction of dip for each interval of the computed analysis. Additional computer generated azimuth frequency plots can be run over specific zones which have a particular geologic significance.

These zones can be the upper and lower boundaries of a formation, the zone between two faults, the zone between a fault and an unconformity or any other breakdown which is indicated by knowledge of the local geology or interpretation of the dipmeter data itself. With the advent of interactive computer programs, decisions on what to plot can be made as processing and analysis take place. using the FMS Image Examiner program.

Regional Dip Removal Plots
If structural dip is greater than three or four degrees, it should be vectorally subtracted from the dips by the computer, leaving the absolute current and slope pattern dips. This provides better definition of stratigraphic dips. The effect can be quite dramatic, and some events may appear after dip subtraction that were not noticed before.


Regional dip removal changes the dip patterns, making sedimentary interpretation easier

All the above plots are available in a hands-on mode when using Schlumberger's Dipmeter Advisor, and most are available on the Atlas Wireline DIPVUE program.
 

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