Tiltmeter - AbsoluteAstronomy.com

A tiltmeter is an instrument designed to measure very small changes from the horizontal level, either on the ground or in structures. A similar term, in less common usage, is the inclinometer

Inclinometer

An inclinometer or clinometer is an instrument for measuring angles of slope , elevation or depression of an object with respect to gravity...

. Tiltmeters are used extensively for monitoring volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

s, the response of dams to filling, the small movements of potential landslide

Landslide

A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

s, the orientation and volume of hydraulic fractures, and the response of structures to various influences such as loading and foundation settlement. Tiltmeters may be purely mechanical or incorporate vibrating-wire or electrolytic sensors for electronic measurement. A sensitive instrument can detect changes of as little as one arc second

Minute of arc

A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

.

Tiltmeters have a long history, somewhat parallel to the history of the seismometer

Seismometer

Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...

. The very first tiltmeter was a long-length stationary pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

. These were used in the very first large concrete dams, and are still in use today, augmented with newer technology such as laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

reflectors. Although they had been used for other applications such as volcano monitoring, they have distinct disadvantages, such as their huge length and sensitivity to air currents. Even in dams, they are slowly being replaced by the modern electronic tiltmeter.

Volcano and earth-movement monitoring then used the water-tube, long baseline tiltmeter. This was a simple arrangement of two water pots, connected by a long water-filled tube. Any change in tilt would be registered by a difference in fill-mark of one pot compared to the other. Although extensively used throughout the world for earth-science research, they have proven to be quite difficult to operate. For example, due to their high sensitivity to temperature differentials, these always have to be read in the middle of the night.

The modern electronic tiltmeter, which is slowly replacing all other forms of tiltmeter, uses a simple bubble level principle, as used in the common carpenter level. As shown in the figure, an arrangement of electrodes senses the exact position of the bubble in the electrolytic solution, to a high degree of precision. Any small changes in the level are recorded using a standard datalogger. This arrangement is quite insensitive to temperature, and can be fully compensated, using built-in thermal electronics. A newer technology uses Microelectromechanical systems

Microelectromechanical systems

Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...

electronics, but it is not clear whether this can eventually displace the common bubble.

The most dramatic application of tiltmeters is in the area of volcanic eruption prediction. As shown in this figure from the USGS, the main volcano in Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

(Kilauea

Kilauea

Kīlauea is a volcano in the Hawaiian Islands, and one of five shield volcanoes that together form the island of Hawaii. Kīlauea means "spewing" or "much spreading" in the Hawaiian language, referring to its frequent outpouring of lava. The Puu Ōō cone has been continuously erupting in the eastern...

) had a habit of filling the main chamber with magma, and then discharging to a side vent. The graph shows this repeated action, with a pattern of swelling of the main chamber (recorded by the tiltmeter), draining of that chamber, and then an eruption of the adjoining vent. Each number at the peak of tilt, on the graph, is a recorded eruption.

See also