The dipmeter tool records microresistivity measurements which are used to determine the dip and strike of resistive features in the subsurface. The modern tool is a four-arm device, in which the arms are arranged radially around a central mandrel and are pressed against the borehole wall by a spring mechanism. Small electrodes are embedded in a rubber pad at the end of each arm to record microresistivities of the formation in the borehole wall. As the tool is pulled upwards, the vertical deviation of the hole causes it to tilt while the cable winding and borehole rugosity make it rotate. A compass and weighted pendulum in the tool housing continuously monitor the geographic orientation of the arms and the deviation of the tool from the vertical.
The raw results of a dipmeter run are four microresistivity traces, together with the azimuth of one of the arms used as a reference and the borehole deviation angle. The fine vertical resolution allows features as thin as one to two centimeters to be resolved. Because lithological changes are relatively minor when traced across the width of the borehole, the overall form of the resistivity traces tends to be similar. Major differences between them is expressed in a relative depth shift between correlative features, which is a function of their strike and dip.
The results of computer processing are most frequently presented as a vector plot which is more commonly known as a "tadpole plot" (see Figure 8). The vertical axis records depth in the same convention as other logs and the horizontal axis is scaled for magnitude of dip. At various depths, "tadpoles" are marked where successful correlations of microresistivity segments were made by computer processing. The quality of the correlation is usually indicated by the symbol used for the tadpole "head". The orientation of the "tail" of the plotted tadpole shows the azimuth of the dip as related to a conventional compass circle.
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.