remote SENSING
Fiber measures ground and building distortion
Paul Mortensen
A method for measuring changes in the shape of the ground or buildings has been developed by researchers at Nippon Telegraph and Telephone (NTT; Tokyo, Japan) who have deployed optical-fiber cables to act as passive sensors. The fiber-based method offers ease of installation, continuous measurement, and a significant reduction in cost compared to previous types of distortion gauges.
Fiber has the property of returning light reflections from distorted or damaged sections when 1.55-µm pulses of light are transmitted through it by a distortion/loss-integrating optical time-domain reflectometer (OTDR). The setup enables detection of the degree and position of distortion or loss over the entire length of the fiber based on the time it takes the reflected light to return.
This technique has been applied by NTT to check the condition of its optical-fiber cable networks and identify the causes of troubles. When a fault occurs, the problem fiber is easily identified because its OTDR peak from the dichroic reflector (located at the end of the fiber) is lower than the initially recorded value. This method also has been used in a survey of fiber damage after the Hanshin earthquake (1994) and fiber-cable frost damage at the Nissho-toge Pass in Hokkaido in northern Japan.
By analyzing the results of these actual site surveys, NTT concluded that the technology would be effective in measuring distortions in ground and buildings and so began to develop the method for a wider range of applications.
Toward greater accuracy
Accurate evaluation of the distortions that have accumulated in a building are vital for diagnosing its structural integrity. Distortion surveillance of ground conditions provides a way to detect the signs of landslides before they occur. In the past, such distortions were detected by scattering electrical distortion gauges; this technique, however, could not measure continuous distortion, and the results were affected by the random distribution of measuring units.
The NTT researchers began by improving the accuracy of distortion measurement. They found that the frequency of the Brillouin scattered light produced in optical fiber is proportional to the amount of fiber distortion, enabling the size and location of distortions to be pinpointed. They also succeeded in increasing the sensitivity of the measurement of scattered light, tripling the accuracy over 1 m (elongation [contraction]/ original length = ۭ ¥ 10-5 from a previous ۫ ¥ 10-4). Significantly, this sensitivity threshold enables the distortion in a representative structural material such as concrete to be detected before it fractures.
Practical tests
The researchers note that fiber offers several advantages in these applications. For example, it allows measurement over its length, and so, by precise installation, distortion of planes and surfaces can be measured. Fiber is a passive component and does not generate a signal or physically move, eliminating the need for any power supply. And fiber cable is easy to produce and install compared to other types of distortion gauges, thus greatly reducing costs.
In a practical test of the method, fiber cables have been set inside 10-m-long concrete beams in a road tunnel in Hokkaido (see figure). Because national highways in Hokkaido have many dangerous locations on coastal stretches where rockfalls are a constant threat, the Hokkaido government ex pects that fiber installation will warn of these dangers by allowing deformation changes in underpasses to be continuously monitored.
Having finished the basic experimentation and evaluation of this technology, the researchers are now studying effective installation techniques and the relationships between measured materials and observed distortions.