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.