ABSTRACT:

Fiber optic based strain sensors have been developed that measure earth pressure loads. This paper describes the utilization of a borehole and cable sensor to monitor loads and vehicle type passing over a roadway. These sensors are shown to possess both high sensitivity and large dynamic range. These attributes make these sensors extremely useful for monitoring a variety of stress/strain situations.

INTRODUCTION

The motivation to pursue the application of fiber optics technology to the development of new geophysical sensors at Los Alamos began in late 1983. The many benefits and advantages of the newly emerging optical fiber technology and the availability at reasonable cost of fiber optic components indicated that new and improved geophysical sensors should be developed. A borehole sensor was developed and installed near Los Alamos in early 1986 to pursue these ideas, and has remained operational since then. In late 1986 DOE/Office of Arms Control began funding further sensor development for test ban verification applications under its Technology Development program area. Since then, several improved versions of these sensors have been built and installed at several locations.

§ Fiber optics was developed and has had its greatest use in the area of long distance telecommunications where its special properties are exploited to transmit large quantities of information at speeds previously unobtainable by the use of electrical conductors. These special properties of fiber optics also can be used in the development of fiber optics based sensors for many applications. Fiber optics based sensors possess many advantages compared to conventional types of sensors. Some of their attributes are: High sensitivity,

§ Large dynamic range,

§ Wide frequency bandwidth,

§ New sensor configurations or geometries,

§ Near in situ operation, and

§ Reliable long lived operation.

There are two basic types of fiber optic sensors depending upon how the optical fiber is used to measure the quantity of interest. In the first type, called Amplitude Based Sensors, the quantity being measured produces changes in the intensity of the light within the optical fiber. These sensors often only employ fiber optics to transmithe light into and out of the optical device that is making the desired measurement. Problems such as limited dynamic range, calibration, reproducibility, and drift usually limit the successful application of this type of sensor. These sensors have been used successfully in temperature, high pressure, high voltage, and chemical process measurements. Generally, the performance of this type of sensor is no better than existing conventional sensors, but sensor simplification and improved reliability may result due to the use of fiber optics. The second type of fiber optic sensors are those called

Phase Based Sensors in which the quantity being measured produces changes in the phase of the light traveling within the fiber. This type of sensor, although more complex than the intensity type, can provide substantial improvements in sensor pedormance compared to conventional sensors. These sensors offer the important advantages of high sensitivity, a very large dynamic range, and self calibration of the fiber optics. Thus they can offer substantial improvements in the sensor signal to noise ratio and adapt well to digital signal processing techniques. The greater complexity of hardware in these sensors is usually offset by the improved sensor pedormance they provide. Successful sensors of this type measure rotation (fiber optic gyroscope), strain, undenNater acoustics (hydrophone), and pressure.

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