Renewal of volcanic activity is often associated with deformation (swelling) of the ground caused by the rising magma “pushing” the surrounding rocks to reach the surface. Such deformation is usually very small (1’s mm – 1’s cm) and, although it usually cannot be felt by people living in the area, it can be detected by geodetic techniques.
The SRC has a network of benchmarks or reference points -usually brass-pins cemented int
o the ground- in all the volcanic islands we monitor. The main technique used to measure their position (i.e., determine the geographical coordinates) is Global Positioning System (GPS). The overall concept is to determine the precise position of the benchmarks by using data obtained from satellites orbiting the earth (whose positions we know precisely). A radio signal is sent from the satellites to receivers set up at the benchmarks and, by accurately measuring the time it takes for the signal to reach the receiver, we can determine the precise distance between the satellite and receiver. By using signals obtained from at least 4 satellites, one can determine the precise position (i.e., coordinates) of the GPS receiver in three dimensions (e.g. longitude, latitude & height). By comparing the GPS coordinates of the benchmarks through time, we can determine whether they have moved and determine the direction of this movement. Such movement could indicate ground deformation due to a change in volcanic activity.
At the SRC we also use other techniques to monitor ground deformation. These include: a) Electronic Distance Measurements or EDM which enables us to determine the precise distance between various benchmarks; b) precision levelling used for measuring small changes in height and c) tilt meters used to measure changes in three dimensions over a very small area.
Since 2006, in collaboration with a number of other regional and international agencies, the SRC established a number of Continuously Operating Reference Stations (CORS) in several islands in the Eastern Caribbean (including Grenada, St. Vincent, Dominica, Saint Lucia, Antigua and St. Kitts). Data from these sites are streamed to SRC in Trinidad via the Internet. Additional CORS stations are planned over the next few years.
Volcano monitoring using Geodetic techniques can essentially be divided into 3 steps:
1. Data acquisition: campaign-style field surveys are conducted regularly in all of the volcanic islands. The frequency of the visits depends on the level of activity at the volcano being monitored.
2. Data processing: this involves determination of the precise location of each benchmark relative to a predetermined reference point. This is a two-staged process with the preliminary location computed using the ‘estimated’ position of the orbiting satellites while the second location is used using the precise position of the satellites (called the “precise ephemeris”).
3. Deformation modelling: this is used to assess the nature of the pressure source underground (e.g. magma intrusion, pressurization of the hydrothermal system) and to determine its characteristics (e.g. geometry, values for overpressure).
The SRC uses a mixture of Leica (GPS, EDM) and Trimble (GPS) instruments. The continuous GPS sites use Trimble NetRS receivers while the campaign-style surveys mainly use Leica instruments. The benchmarks consist of 4 inch brass-pins cemented into the ground (need for a tripod) as well as antenna-mounts (antenna screwed straight to the ground). A minimum of three GPS receivers must be operated simultaneously during the field campaign to achieve good triangulation. Since all measurements are made relative to a reference point (i.e. a GPS point with no deformation associated with the volcano), the reference points are occupied for longer periods of time (e.g. days compared to hours for the other stations). This allows us to process the reference station GPS data with continuous GPS stations in other islands of the Lesser Antilles for a better constraint on every network. GPS processing at SRC is performed using the GAMIT/GLOBK software developed at MIT. Data acquisition and processing for the continuous GPS stations is fully automated.
Eastern Caribbean cGPS Network