Ground deformation is the change in shape that can occur prior to, during or after an eruption. Such ground movement can occur in response to the influx or withdrawal of molten material (magma) and hydrothermal or magmatic fluids in the volcano. Increases in ground deformation may signal the start of a new eruptive episode. There are numerous ways to measure such deformation, like levelling, triangulation and more recently using continuous Global Positioning System (cGPS) measurements. It is also possible to use lakes as large tiltmeters.
Geodetic levelling is used to measure elevation differences between benchmarks and, by repeating surveys, elevation changes (vertical displacements) with time can be recorded. Regular surveys are made of the crater floor at White Island for example.
Triangulation and Distance Measurements
A theodolite and EDM (electronic distance measurements) can be used to measure the change in shape of an area by making repeat measurements. This type of surveying has been replaced by GPS (see below).
A lake can be used as a large natural tiltmeter by measuring changes in the water-surface elevation relative to nearby stable bench marks. A lake located within an area of active deformation presents a unique monitoring opportunity. Lakes are often present about volcanoes. In New Zealand we use lakes Tarawera and Taupo, and Blue Lake on Raoul Island in the Kermadecs.
In order to make accurate cGPS measurements a stable monument must first be constructed that is well anchored to the bedrock at a depth of 5 - 10m. Attached to the monument is a geodetic-grade antenna and GPS receiver combination. These instruments are capable of metre accuracy in standalone mode. However when the data are processed with those of nearby stations, accuracy of a few millimetres can be achieved. The necessity for an extremely stable monument then becomes obvious.
Most of our volcano cGPS stations record their position every 30 seconds which is averaged to produce hourly and daily positions. Some stations transmit their location once a second which allows them to be used as base stations by surveyors for cadastral surveying. In this mode millimetre accuracy coordinates are available on a continuous real-time basis.