Translated and adapted from a document written by
A. Armienta, S De la Cruz-Reyna, P. Lesage, R. Ortiz, R. Van der Laat,
Mexico, March 2007

This document aims at groups with little experience in seismic monitoring of volcano. You can download a pdf version  here.

Click here for an example of seismograms recorded at Colima volcano observatory, Mexico during a vulcanian eruption (8 Dec. 2004)

  In order to investigate volcanic phenomena, it is recommended to have a large variety of sensors available to record seismic signals, deformations, geochemical composition, gravimetric or magnetic potentials, temperatures, … However, when only limited funding is available, suitable results can be obtained for the basic monitoring of volcanoes if the following minimal conditions are fulfilled:

1. An organization that includes permanent technical and scientific staff who allow the continuous operation of the monitoring system. The acquisition and analysis of telemetered data must be carried out continuously. Field measurements should be obtained with a sampling period regular and short enough to detect possible precursory signals.

2. To have reliable recording devices that allow the detection of changes with respect to previously observed reference levels.

3. To have trained staff and the procedures, software y methodologies that allow the recognition of volcanic unrest precursors.
4 To have sufficient logistical support (vehicules, radios, etc.).

  A precursor is a signal which reflects modifications in the internal state of a volcanic system that may precede an eruption. To detect precursory signals, a real-time, continuous and permanent system should be used. When possible, the signals should be telemetered from the sensors to the recording and analysis center. They must be processed and interpreted at the moment of their acquisition, at least in a preliminary form, by qualified staff. A temporal instrumental deployment cannot be considered as a monitoring system. Monitoring networks must be designed for sustained and permanent activities. They need full-time technical staff and a scientific comity that carry out more detailed interpretation and hazards evaluations.

  In order to detect the possible unrest of a dormant volcano, one short-period seismic station with telemetry and continuous recording may be sufficient. When an increase of the seismic activity of a volcano is detected, the monitoring system must be improved. In this case, it is recommended to install a basic network including at least four short-period stations, preferably with three components seismometers, and one broad-band station. Part of the stations should be installed close to the crater.

  The digital acquisition of data, their storage and processing require at least three personal computers. These must include software for the acquisition and real-time analysis of signals, such as Earthworm, and high level programming languages (C, SciLab, R, or equivalent). Some of them are freeware. The data processing must include at least the following tasks: hypocenter determination, spectral analysis of discrete events, continuous spectral analysis of the signal (spectrogram), including noise, calculation on a moving window of the mean amplitude of the signal (RSAM) or of its root mean square (rms). It could be useful to use different types of normalization of the spectrograms (per spectrum, per frequency, or global).

  In order to easily process large sets of recordings, it is important to save the data in long duration files containing one channel each. Usually, in systems dedicated to the detection and analysis of discrete events (earthquakes), data are stored in short multiplexed files (~1 mn) containing all the channels of the network. This is convenient for phase picking tasks but it makes difficult the processing of continuous recordings. A solution is to use two parallel recording systems: one with multiplexed format files for the detected tectonic or VT events, and another for the storage of continuous data in separated files of long duration (1 to 24 hours) for each component of each station.

  The classification of the volcanic seismic events is a fundamental task in monitoring systems. Several types of event are observed on volcanoes and they correspond to different physical processes. They provide information on the state and evolution of the magmatic or hydrothermal systems. Using previous observations, the types of events for a given volcano must be defined according to the main features of its seismic activity. Several classifications of events can be used. The most common classes are: volcano-tectonic earthquakes (VT events), long-period (LP) events, volcanic tremor, explosion quakes, collapses, lahars, as well as regional and teleseismic tectonic events. A systematic counting of the events of each class must be carried out. For this purpose, the same station and the same detection and classification criteria must be always used. The appearance of a new class of event, the evolution of the features of existing families, or the modification of the rate of occurrence of a given type of event may indicate changes in the volcanic system. Furthermore, the classification of the observed events can help carrying out signal processing adapted to each type of event.