A new method for identifying type of material ahead of an optic fiber sensor is proposed. The sensor is made of two plastic optic fibers that are aligned side by side and glued together. The tips of these fibers are cut so that outward normal vectors of the cut surfaces are 90 degrees apart and at the same time each of them is at 45 degrees from fiber axes. As light is sent into the first fiber and reaches its tip, part of the light gets refracted and goes out of the fiber into the material ahead. The other part gets reflected, goes out of the first fiber through its side face and gets into the second fiber. This light reaches the cut surface of the second fiber from within and then part of it goes out into the material outside. The remaining light gets reflected and travels through the second fiber back to a photo sensor to be digitally recorded. The intensity of the recorded light is a function of differences between refractive indices of the plastic optic fiber and the material ahead of the fiber tip. Analysis of this intensity leads to understanding of if the fiber tip is in air or in water in geo-material, which is of great interest in geotechnical engineering projects.

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