A three dimentional vector method as an aid to continuous-dipmeter interpretation

Abstract

Means of dipmeter readings derived by a three dimensional vector method have proven to be useful in deriving structural dip and azimuth from a collection of widely scattered dips. A computer program to calculate 3-D vector means and related statistics has been developed. Dip distributions are comparable to the spherical normal distribution so that for practical purposes a measure of reliability of the vector means can be calculated. The 95% confidence cone around the vector mean is chosen as the most convenient yard-stick. Confidence angles for the mean dip angle and mean azimuth are derived separately from this confidence cone. Preferably, study and correlation of well logs should precede the averaging procedure. This enables the selection of intervals, which contain readings mote-or-less conforming to the structural dip and azimuth. The calculated average dips can be displayed on a tadpole plot, or other plots against depth. The average dips are much easier to interpret than the original data, especially when using a condensed depth scale. The difference between successive vector means is a measure of the rate of change of dip with depth and can reveal the presence of discontinuities, such as faults and unconformities which might otherwise remain undetected in the "raw" data. The approach is applicable to both 3-arm dipmeter logs and the mote modern 4-arm dipmeter logs for which the digitized curves are correlated by the computer.