Dictionary:Blind zone

{{#category_index:B|blind zone}}

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.

External links

find literature about Blind zone