Dictionary:Blind zone

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FIG. H-5. Hidden layers. (a) A layer whose velocity (V3) is lower than that of an overlying layer (velocity inversion) does not produce a head wave. (b) The head wave from a thin layer (V2) without sufficient velocity contrast or too thin may not produce a first arrival and not be observable as a distinct event.

1. A layer with lower velocity than overlying layers so that it cannot be detected by refraction methods.

2. A layer that does not give a refraction first break. Also called hidden layer or shadow zone. See Figure H-5. The blind zone (a) may have a velocity lower than that of a shallower refractor, in which case it may lead to overestimating the refractor depth, or (b) it may have a velocity intermediate between those of layers above and below but not have sufficient thickness or velocity difference to produce first arrivals; in which case it is apt to cause underestimating the refractor depth.

3. A zone from which reflections do not occur.

4. A zone from which no drill cuttings are returned to the surface.

5. A portion of a formation in which logging tool response is too low. A blind zone can occur because of the finite size or configuration of the logging tool. For a lateral log, a blind zone (abnormally low reading) is recorded when a bed that is highly resistive compared to the overlying and underlying formations is present between current and measuring electrodes.

6. A layer that cannot be detected by electrical methods because its resistivity is not sufficiently different from that of other layers or because it is too thin.

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