DESCRIPTION:
Tiltmeters and Tilt Measurements

In addition to cracking and faulting, the crater floor (of Mount St. Helens) tilts before eruptions. Electronic tiltmeters, which are widely used in Hawaii, measure changes in slope or inclination of the ground surface. Tiltmeters specifically designed for use at Mount St. Helens have been installed on the crater floor tens to several hundred meters from the dome. Tilt data contributed to accurate predictions for eruptions during 1981 and 1982.

The tiltmeters, which employ two sensitive bubbles mounted at right angles on a 15-centimeter base plate, measure tilt of the crater floor in two directions. The direction of tilt is generally outward from the dome but is sometimes complicated by nearby cracks or faults. Amount of tilt is expressed in microradians, which is the angle turned by a 1-kilometer-long rod if one end is raised 1-millimeter. Although these tiltmeters are capable of measuring one-tenth of a microradian, precision is limited to 5 to 10 microradians in the crater because of surface thermal effects. Data from the crater tiltmeters are radioed directly to CVO in Vancouver and are stored in computer files. Tiltmeters provide the only realtime information about crater deformation and are, therefore, especially valuable when field work is impossible because of poor weather or hazardous volcanic activity.

Tilting of the crater floor began several weeks before each eruption in 1981 and 1982, accelerated a few days before, and, on several occasions, abruptly reversed direction minutes or hours before the eruption began; for example, tilt before the March 19, 1982, eruption at one station increased from about 14 microradians per day 3 weeks before the event to 360 microradians per day on March 19. The tilt reversed direction about 30 minutes before the eruption began.

Tiltmeters are available in assorted shapes, sizes, and price ranges that are suited to a wide variety of applications. They range from relatively simple designs used as security devices in executive briefcases to sophisticated instruments capable of recording the effects of solid Earth tides or providing inertial guidance for aircraft and missiles. For volcano applications, the most common types use a fluid to measure tilt changes along a baseline that ranges from a few centimeters to a few meters in length.

Water-tube tiltmeters are fluid-filled U-shaped tubes,typically a few meters long. As the tube tilts, the fluid (usually water) rises relative to one end of the tube and drops relative to the other end. The change in water level can be measured manually or sensed electronically and converted to an equivalent tilt change. A state-of-the-art, half-filled, water tube tiltmeter developed by R. Bilham features a 1-km baseling, extreme precision (resolution of one part in 10^10), and very low long-term drift rate (Agnew, 1986). It is relatively easy to install and has been used successfully to monitor uplift of the resurgent dome at Long Valley caldera in east-central California.

Bubble tiltmeters use the motion of a bubble in a fluid to accomplish the same task. Typically, a bubble a few millimeters in diameter floats in an electrolyte fluid sealed in a vial a few centimeters across. Motion of the bubble caused by tilting of the vial is measured electronically by electrodes that project into the fluid. Tilt along a single axis can be measured using a cylindrical vial, and tilt along two axes can be measured using a disk-shaped vial.

Mercury tiltmeters employ a pool of mercury as a capacitance plate in an LC resonant bridge circuit; tilting of the opposite plate relative to the surface of the mercury causes a capacitance change that can be measured electronically.

..."dry tilt" measurements, which are made by leveling around a 40 m triangle using a precise spirit level and Invar metal rods. Dry tilt has a precision of +/- 2-3 microradians. (One microradian equals 0.0000573 degrees and can be visualized in terms of a board 1 km long that is elevated by 1mm, the thickness of a dime, on one end.) This method is inexpensive and suited to all relatively flat areas where the elevation difference around the 40 m triangle does not exceed 3 m, the height of the rods. Dry tilt changes are displayed on a map as vectors that point in the downward direction and are proportional in length to the amount of change since the last occupation.

Single-setup leveling (SSL) (also called dry tilt or tilt-leveling) has routinely been used at the Hawaii Volcano Observatory (HVO) to measure and detect changes in ground tilt on Hawaiian volcanoes since the technique was developed in 1968. SSL has been used at Mount St. Helens; at various volcanoes in the Cascade Range, ... at La Soufriere volcano, Guadeloupe ... on various active volcanoes within the Taupo volcanic zone, New Zealand ... at Karkar volcano Papua New Guinea ... in Costa Rica ... on Arenal and Poas volcanoes, and in Colombia ... at Ruiz and Galeras volcanoes ... A complete description of the original spirit-level tilt technique can be found in Kinoshita and others (1974). Yamashita (1981) also presented a detailed description ...

The SSL technique requires the use of two or three long Invar rods and rod stays, a level, and a micrometer plate. The bench-mark array is typically an equilateral triangle, 40 m on a side. When 3-m rods are used, sites on which a 40-m equilateral triangle can be placed are limited to relatively level areas with no more than 2.5 m elevation differences between monuments at the triangle apices (1.5 m if 2-m rods are used). Sites such as these tend to be rare on the flanks of steep-sided volcanoes. In addition to site limitations, 3-m Invar rods are difficult to transport on small vehicles, pack animals, and aircraft. These factors combine to make SSL of limited utility on many composite volcanoes ...

To circumvent the difficulties of the wet tilt system, Jackson and Wright began development of a system of precise tilt leveling at HVO. This procedure involved precise leveling of a bench-mark array ... Because no fluids were involved, the term "dry tilt" was soon adopted.

"Dry tilt" is not an accurate term ... "single-setup leveling" (SSL) ... the new name used to denote this technique of monitoring for vertical displacement ...

The configuration of SSL tilt stations used by CVO is a nearly equilateral triangle with sides of 30-40 m ...

When the configuration is acceptable, a bench mark is installed at each vertex and the central instrument site is marked with a nail or bar. The vertices are labeled X, Y, and Z counterclockwise from the southernmost site.

A 40-m equilateral triangle is not a rigid requirement for a tilt configuration, and neither size nor shape is critical to the measurement of tilt. Linear, triangular, square, and hexagonal arrays have all been employed to advantage. Likewise, the station orientation and lettering scheme described above are standardized for the sake of convenience but can be modified as appropriate for specific installations. ...

If three rods are available, they are labeled X, Y, and Z and leveled with self-supporting braces on the corresponding bench mark. The instrument is then leveled over the center mark of the triangle. The purpose of this scheme is to measure the elevation difference between each of three bench-mark pairs.

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