| Hawaiian Volcano Observatory (HVO) scientist using laser-ranging instrument (left) to make electronic- distance measurement (EDM). (Photograph by Cristina Heliker.) The laser beam is reflected back to the EDM instrument by a cluster of retro-reflectors (right), and a precise determination of the horizontal measurement is maade by a small computer within the instrument. (Photograph by Robin T. Holcomb.) |
  HVO scientists using an optical-level instrument to determine ground tilt, calculated from readings to three or more stadia rods (one seen at right). Use of umbrella improves readings by eliminating disruptive air-temperature fluctuations caused by passage of clouds. (Photograph by J.D. Griggs). |
 Changes in the shape of the volcano during inflation and deflation are determined by ground-deformation measurements. Tilt changes can be measured continuously and extremely precisely by use of instruments called tiltmeters, which can detect a change in angle of less than 1 microradian (about 0.00006 degree). A 1-microradian increase in tilt would be equivalent to steepening the slope of a 1-mile-long board by placing a nickel under one end. Tilt changes and associated relative vertical displacements also can be detected by periodic remeasurement of arrays of benchmarks by leveling, a high-precision field surveying method. Changes in horizontal distances between benchmarks can be monitored in the field by using portable electronic distance measurement (EDM) instruments that utilize laser or infra-red beams. Collectively, these commonly used ground-deformation monitoring techniques have a measurement precision of a few parts per million or less. The notion of one part per million can be visualized in terms of a very dry martini -- 1 drop of vermouth in 16 gallons of gin! |
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