Volcano Watch — The evolution of the tiltmeter at the Hawaiian Volcano Observatory

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The basic strategy in monitoring a volcano has not changed much since 1912 when Thomas A. Jaggar founded HVO. He recognized the importance of measuring ground deformation, earthquakes, and gases. These three programs remain the essence of HVO's monitoring efforts - only the tools have changed. In today's column we focus on the evolution of methods to monitor slope changes or tilt.

Tilt was first measured by the movement of a suspended plumb bob. As the slope of the ground changes, the frame holding the suspended bob moves with the ground, and the bob points to a new center of gravity away from the original point. The movement of the bob is recorded as the tip scratches soot off a smoked watch glass placed beneath the bob. Tilt was also measured by the offset of a scribing pen of a seismograph from a fixed line on the recording drum. The precision of these measurements depended on the length of the pendulum. Tilt changes fewer than a few microradians were difficult to detect.

Tilt measurements were limited to areas where an underground vault was available. In addition to the Whitney vault at the old Observatory, three vaults were established on the caldera floor around Halema`uma`u, and another was built at the end of the Mauna Loa Strip Road. This was the only method of measuring tilt until 1956, when the watertube tiltmeter was developed.

The watertube tiltmeter works because water seeks its own level. If water containers are placed on two points and connected with a tube, relative changes in elevations of the two points are reflected by changes in water level of the two containers. Calibrated water pots to measure the water level changes were built at HVO from spent artillery shells. Eleven long-base stations were established on Kīlauea, each station consisting of three permanent hubs placed at the apices of an equilateral 50-m (160-foot)-triangle. The watertube tiltmeter is able to detect changes of less than a microradian (one part per million).

As good as the watertube tiltmeter was, it had limitations. Measurements could only be made at night when the air temperature was nearly constant. Stations had to be on extremely flat land, because the measuring range of the tilt pots was only 13 mm. With the advent of very precise optical levels and invar leveling rods, the watertube system was gradually replaced by the spirit-level tilt system, starting in 1968.

Station configuration for the spirit-level system is nearly identical to the watertube system, except that sides of the triangle were reduced to 40 m (130 ft). Measurements could be taken at any time of day, and station hubs could have elevation differences up to nearly 3 m (10 ft). The spirit-level system is not as precise as the watertube, however. Changes of a few microradians can be detected.

The watertube and spirit-level tilt systems lacked a key element of the pendulum system - time. Whereas tilt changes were recorded in real-time by the pendulum system, actual measurements had to be made before changes could be detected by the watertube or spirit-level systems. This was solved with the development of the electronic tiltmeter.

The first electronic tiltmeter, manufactured by Ideal-Aerosmith Co., was introduced to HVO in 1965. It is based on the watertube principle, constantly measuring fluid levels at the pots by detecting changes in capacitance caused by a change in the airspace gap between a plate and the surface of an electrolytic fluid. The change in capacitance caused by the fluid level movement is converted to a voltage. The system is calibrated by tilting the instrument a known amount and recording the output voltage change. This first electronic tiltmeter is capable of detecting changes down to a tenth of a microradian and is still in use today.

A limitation of the capacitance tiltmeter is that it requires a temperature-controlled vault, and it can measure slope changes in only one direction. To preclude the need for a vault, borehole tiltmeters were developed. These tiltmeters have sensors similar to those found on a carpenter's level, except that they have electrodes placed in them. The sensors are attached to the bottom of a meter-long, stainless steel pipe.

The change in position of the bubble in the sensor causes a change in resistance between electrodes. The change in resistance is converted to a voltage by a simple Wheatstone bridge circuit. The advantage of this tiltmeter is that it can be placed anywhere a hole could be drilled, and it provides slope changes in two orthogonal directions. Presently, 13 borehole tiltmeters on Mauna Loa and Kīlauea volcanoes provide continuous data in near real-time to HVO.

The tiltmeter, one of the basic instruments utilized by the founder of HVO, has been in continuous use in one form or another since 1912.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Three surface flows were observed descending Pulama pali on May 17. One flow passed within 600 m (2,000 ft) of the bed and breakfast facility in Royal Gardens. Flows are seen throughout the coastal flats with the most active area beyond the eastern boundary of Hawai`i Volcanoes National Park near Kapa`ahu. Lava is entering the ocean east of Kupapa`u along a 670 m (2,200 ft) span of coastline. The widest bench extends about 60 m (200 ft) from shore.

One earthquake was reported felt during the week ending on May 17. Residents of Hawai`i Volcanoes National Park, Volcano, and the Volcano Golf Course subdivision felt an earthquake at 11:01 p.m. on May 13. The magnitude-3.5 earthquake was located 2 km (1.2 mi) east of the summit of Kīlauea Volcano at a depth of 1.4 km (0.8 mi).