...record the vibrations of the earth. There is a weighted object on a spring which remains still as the rest of the unit moves with the vibrations - this means it can record the magnitude and frrequency of the virbrations in the earth's crust...
The vibration from volcano-tectonic activity earthquakes have a suden start and last fro a short time - they have high frrequency and usually occur in swarms that can last hours or days. They are caused by the fracutring of rock by the rising magma as it forces its way to the surface. They are common and highly frequent at the beginning of an eruption and slow as the pressure within the magma chamber of the volcano is released
Remoste sensing spectrometers
...record the chemical composition of escaping gases. The spectrromneter records the concentration of gases 2-5 times a week - dips or spikes in concentration of certain gases could then indicate an eruption is imminent.
Before an eruption, the concentration and composition of gases changes as the build up of magmo increase the temperautre of hot springs and water reserviors. For example, the ratio of hydrogen chloride and sulphur dioxide gas often increase
A Tilt meter/GPS imaging
...measures the slope gradient as it swells and subsides before and after an eruption. Within the tiltmeter, is a pot of water and as the ground deforms, the water shifts to maintain a perfectly horizontal surface. This is accurate to tiny increments. After mutliple eruptions, a particular angle of slope that is achieved immediately before an eruption may become apparent, making eruption prediction simpler.
...The most senstive types will detect minute chages in an area's topography. Satellite imaging often highlights grrouund deformation not visble from ground level or in areas not monitored by tiltmeters.
Before an eruption, the grouund changes shape - it usually bulges up as magma builds up in the magma chamber. This can be just a matter of millimeters.
Ground and Water Temperature
...Borehole Monitors detect the changes in the underground temperatures of the ground and hot springs. A deep borehole is drilled into the side slope of a volcano, often above the known location of a hot spring or underground reservoir. A sensor is then fed down into the hole until it can get accurate readings. These readings may then make a critical temperature that is reached just before an eruption.
As magma builds up, it raises the temperaure of the ground and water surrounding it. The magma is heated to really high temperatures in the earth's core and this heat conducts and convects into the ground and water nearby. The more magma, the more obvious the temperature change will be
Volcanic and Geological Observations
Recording eruption data and geological changes this causes can detect any patterns or cycles in a volcanoes' eruption history. It takes the magmo chamber time to refill upto the pressure required for another eruption which means there has to be a minimum period of time between eruptions
Observatories record the data and research done after each eruption of a volcano in living memory as well as investigation the eruption history of a volcano through its geology. It allows patterns in an area's volcanic history to become apparent so that volcanologists can predict the rough period during which an eruption should be anticipated and allows close observation and preparation to be made