Glossary

Mirror image dip

Mirror image dips have a dip equal to the hole inclination and are directed opposite to the hole azimuth for a given depth. Mirror image dips usually arise from automatic processing of dipmeter data when individual curves are affected at the same time by an excursion that swamps the resistivity or acoustic signal from the formation. Stick and pull is the commonest cause of this type of artefact. As each pad is affected at the same time it generates an event on each trace that defines a plane perpendicular to the well bore. All processed dip data should be checked against hole orientation for the presence this feature. The use of angular filters during processing can help minimise this problem, although when holes are drilled almost perpendicular to bedding this problem it can be difficult to untangle mirror image dips from genuine dips. Visual examination and manual picking of dipmeter curves allows a further avenue of analysis when a succession is heavily affected by mirror image dips.