These logs have gained popularity for detecting fractures. They are presented commercially as a recording of depth vs. the time after the initiation of an acoustic pulse at the transmitter (t). Amplitude changes are indicated by a succession of varying shades of gray across the film track. The darkest areas correspond to the largest positive amplitudes, while the lightest areas correspond to the largest negative amplitudes.

Figure 3 5 shows an example of a variable intensity log (or variable density log or microseismogram) in a limestone cored well. A 1-ft vertical fracture is present in the core at depth indicated by an arrow. This corresponds to drastic band breaks in the log. When no fractures are present, the log gives the impression of banding.

Figure 3 5: Micro-seismogram/Fracture Finder (Courtesy of Welex)

Since the reflected/refracted waves arrive later than the compressional wave, they might create interference patterns that are usually referred to as Chevron patterns.

The Stoneley (Tube) wave has a low frequency ( 2 Khz), high amplitude which varies with hole size, and a velocity slower than the fluid. These properties are related to the Shear Modulus of the formation. Generally, Stonely waves occur after the fluid wave arrival. In small boreholes and fast formations, however, they can arrive together with the fluid wave.

While moving along the borehole the Stoneley wave is able to exchange energy with the formation through acoustic flow. This represents relative mobility of flows. Consequently, the Stoneley wave attenuation is correlatable with formation...