Journal of the Japanese Association for Petroleum Technology

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Vibrations of bit weight and torque during drilling by roller cone bits are converted into power spectra, and prominent frequencies are estimated. Three kinds of new bits and two kinds of worn bits with diameters from 43/4 to 45/8 in. are tested with a bit test equipment at room conditions. Vibrations of bit weight and torque are measured by load cells under the rock specimen drilled by the bit.
Power spectra are calculated from normalized data with zero average value and unit standard deviation. The frequencies are normalized to eliminate the effect of bit rotational speed. The dimensionless spectra of experimental results comprise normalized spectra and frequencies.
Theoretical dimensionless frequencies of bit motion are calculated under an assumption of no-slipped rotation of cones, and apparent power spectra of bit motion are made from the calculated values. A displacement factor is introduced as a measure of shift of the experimental spectra from the apparent ones. The displacement factor as well as the shape of the ower spectra are usefull to evaluate bit characteristics and status of bit. If bit cones slip, the displacement factor has a lower value than 1. Without slip, the factor may be equal to 1. As a result of experiments, the soft formation bit has the lower displacement factor than the hard formation bit. The shapes of dimensionless spectra obtained by worn bits differ irregularly from the shape of apparent spectra. The shape of spectra offers a means to detect a worn-out bit qualitatively.

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Rivers characterized by suspended load and bed load tend to build fine-grained deltas and coarse-grained deltas, respectively. Gilbert-type deltas consisting of topset, foreset, and bottomset beds are recognized in marine environments as well as in lakes. Coarse-grained deltas tend to be Gilbert-type deltas. However, fine-grained deltas built in deep-water rivermouths and in wave-dominated rivermouths are mostly Gilbert-type deltas.
Bird foot, lobate, cuspate, and estuarine deltas are recognized in marine environments. In contrast, estuarine deltas do not occur in lakes, since lakes are less subjected to tidal influences than seas.

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