Dictionary:Drift

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1. A gradual and unintentional change in the reference value with respect to which measurements are made. If drift is slow and fairly uniform in time, the difference produced by drift can be determined by subsequently rereading the value of the quantity being measured and prorating the difference over other readings made in between. Gravity-meter drift may be caused by gradual heating up of the meter as the day progresses, ‘‘creep’’ in the spring, elastic aging, hysteresis, lunar tide, etc. Drift is different from tare, which is a sharp, sudden change in reference value. 2. A layer of glacial deposits. Glacial drift may vary with position and hence may require a variable correction on seismic records, the effect being similar to that of a weathering layer. Drift often requires a double-layer weathering correction (part for the entire drift layer and part for the lower-velocity layer of the top part of the drift). 3. The attitude of a borehole. The drift angle or hole deviation is the angle between the borehole axis and the vertical; the drift azimuth is the angle between a vertical plane through the borehole and north. 4. A shoran measurement of location with respect to one fixed point. Measurement with respect to a second fixed point is called rate or range. Both drift and rate values are necessary to establish a fix. 5. A horizontal opening driven from a shaft to an ore body, generally along strike. Crosscuts are driven from it. 6. In geostatistical analysis, a trend in data.