The Tadpole Plot
Learning objectives
- Read a tadpole, dot at the dip magnitude and depth, tail toward the azimuth
- Recognize the green motif, constant dip and azimuth, as structural dip
- Recognize the red motif, dip increasing downward, as faults, channels, and drape
- Recognize the blue motif, dip decreasing downward, as foresets and bars
One Symbol for Two Numbers
Every plane the last section read off a sinusoid becomes a single symbol, the tadpole. Its dot sits at the dip magnitude across the track, flat at the left and steep to the right, at the depth where the plane was picked. Its tail points in the dip azimuth as a compass bearing, north up and east right. Thousands of these stacked down a well are the dipmeter log, and an interpreter reads them by their shape.
The Three Motifs
Three patterns carry most of the meaning, and they have colors by long convention. A green motif is constant dip and azimuth over an interval, the structural or regional dip, the simple tilt of undisturbed bedding. A red motif is dip increasing downward; bedding steepens with depth, the signature of drag against a fault, the base of a channel, or drape over a buried high. A blue motif is dip decreasing downward; bedding flattens with depth, seen in the foreset beds of a delta or bar and just beneath an unconformity.
Reading the Departures
The skill is to fix the structural dip first, the green baseline, and then read the reds and blues as departures from it. A red motif sitting below a blue one often brackets a fault; a blue motif alone can map the downdip direction of a sand body. The colors are not the geology, they are the grammar that lets you find it.
References
- Schlumberger (1986). Dipmeter Interpretation Fundamentals. Schlumberger Educational Services.
- Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.