What is Wrong with Tadpole Plots?

James D. Morse, C. A. Bengtson

Millions of dollars are spent annually on dipmeter surveys, but much of this money is wasted because conventional tadpole plots are not suited for three-dimensional statistical analysis of dip data. The first step in structural-dip interpretation is to estimate true structural dip at a sufficient number of points along the wellbore from calculated dips, which show considerable scatter due to measurement errors, correlation errors, and depositional effects. If dip were a scalar quantity, we could find true structural dip at any depth by fitting a trend line to the data and using the trend-line value at that depth. Dip, however, is a vector quantity and therefore cannot be analyzed statistically by using just a single display. For example, the vectorial average of 5°N ip 5°S dip is not 5° (as the tadpole plot would suggest) but 0°. To find true structural dip, we must fit trend lines to depth plots of apparent dip in two directions, preferably in mutually perpendicular transverse and longitudinal directions that can be identified by characteristic patterns on Mercator-type dip vs. azimuth and polar tangent plots used in the SCAT approach to structural-dip interpretation. Interpretation is further facilitated by fitting separate trend lines to plots of azimuth vs. depth and dip-angle vs. depth. Together, these latter plots contain the same calculated-dip information as the tadpole plot.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.