Tiltmeters and Tilt Measurements |
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.
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" |
Return to:
[Monitoring Techniques Menu]...