Sensing the Earth

Using super sense technology, changes in the Earth’s structure can be measured without having to actually be there:

• Motion sensors called seismographs record tremors or small earthquakes that often precede eruptions.

• Some volcanoes, such as Ruapehu and White Island, have crater lakes. Just before an eruption, changes in the magma chamber may cause the level of the water in these lakes to rise or fall. Water level changes in crater lakes are monitored by sensors and shown using satellite imagery.

• Ground changes, land movements and changes in sea level are measured using satellites, radar and GPS stations.

• Volcanic eruptions can release vast amounts of gas, and some of this gas can combine with water, making it highly acidic. By measuring the presence of certain gases in the atmosphere associated with an eruption, we can predict if an eruption is likely. We can also measure the acidity of crater lakes to see if any of these gases are being released into the water.

• Pressure and speed sensors in the oceans measure changes in the amount, speed and depth of water in the sea and can detect tsunamis.

Sensing a tsunami

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These sensors are attached to buoys anchored in the oceans. Data is gathered from sensors and passed via satellite to ground stations that then send the information to tsunami warning centres. This buoy can detect tsunamis as small as 1cm. DART stands for Deep-ocean Assessment and Reporting of Tsunamis.

Acknowledgement: National Oceanic and Atmospheric Administration/Department of Commerce

Rights: NOAA

The sensors are part of electrical circuits that detect changes in electric current. Data from these circuits is then transmitted back to research centres using satellites, cell phones or the internet to be analysed.

Infrared

Infrared sensors detect the amount of infrared radiation and therefore the amount of heat given off from different objects on the ground. The warmer the object, the more infrared radiation is given off. This is useful when mapping living things, such as plants and animals, which generate heat, so infrared sensing is commonly used when exploring changes in the Earth, studying crops and sustainability.

NASA infrared image of Mount St Helens

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Infrared images taken on 1 October 2004 in southern Washington revealed signs of heat below the mountain's surface. The blue part shows snow cover, orange and yellow indicate the rock type and lava age, and the red reveals heat.

Acknowledgement: NASA/courtesy of nasaimages.org.

Rights: NASA

Microwave

By scanning the Earth using microwave sensors, geographers get a unique type of map that is very useful in showing the characteristics that exist beneath the surface of the Earth. For example, the thickness of the Earth's crust can indicate where oil or gas deposits are more likely to be found.

Radar

Radar sensing works by sending out radio signals or microwaves and waiting for them to bounce off the ground and measuring the amount of time it takes for the signals to return. This produces a very accurate topographic map that shows the height and size of geological features. An important advantage is that radar can penetrate thick clouds and moisture, which allows us to accurately map difficult landforms and areas such as rain forests.

Mapping landslides using radar satellite

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A series of images comparing satellite images of landslips in the Whanganui hill country in 1998 and 2004. Landslips show up as brighter areas.

Acknowledgement: Radarsat R-Data © Canadian Space Agency 1998, 2004. All rights reserved

Rights: Landcare Research

Sonar

Sonar sensing sends out sound waves and is often used in water. By measuring the time it takes for these sound waves to travel towards an object, bounce off of it and then return, it is possible to calculate distances.

This allows scientists to accurately map the two-thirds of the Earth that is under water.

Satellites

These are used to gather data from remote sensing equipment located on the Earth. The presence of many satellites has also led to the development of the global positioning system (GPS), which helps measure the position of a receiver anywhere on the Earth's surface relative to several satellites.

GPS allows scientists to map very subtle changes in the Earth surface, as well as allows data collection in the field to be recorded on a very fine scale.

Xerra uses Earth observation satellites to monitor and protect New Zealand’s coastal waters by detecting illegal fishing and uncovering dark vessels. It also helps prevent biosecurity issues.

Early warnings

Geological processes such as earthquakes and volcanoes can be dangerous to people and property. It is important to monitor these processes to predict when a potentially hazardous event is likely to occur.

GeoNet, developed by GNS Science, is a New Zealand hazard monitoring system that uses a variety of sensors, such as pressure sensors, strain gauges, gas and fluid analysers and motion recorders to monitor earthquakes, volcanoes, tsunamis and landslides:

• The New Zealand seismograph network is a series of 46 seismic stations spread throughout New Zealand. Each station has a seismometer and a seismograph. Ground movements throughout New Zealand are monitored in real time, and this information can be used to support an emergency response to an earthquake.

• If a tsunami occurs, tsunami sensors (or gauges) around the New Zealand coast and on offshore islands sense the height of water in the sea and will monitor its progress. Unfortunately, warnings will usually come too late for people who live on the affected coast.

• GeoNet also monitors volcanic activity using visual observations, seismic monitoring, chemical analysis and ground deformation. New Zealand’s volcanoes are classified using a volcanic alert level (on a 1–5 scale), and volcanic alert bulletins are issued whenever there is a significant change in volcanic activity in New Zealand.

Learn more about determining risk in Determining Auckland's volcanic risk. Watch the video DEVORA Project – DEtermining VOlcanic Risk in Auckland and learn about how scientists are working to determine what, when and how the next volcanic eruption might occur around the Auckland Volcanic Field.

Explore tsumanis further by look at these researchers and their work:

• Dr Rob Bell is NIWA researcher, specialising in ocean waves, including storm surges and tsunamis, with a particular interest in sea-level changes.

• Dr Willem de Lange is an Earth sciences university lecturer involved in numerical modelling, coastal processes and climatic hazards and tsunami research.