Seismic Monitoring

A Seismometer is an instrument which can record disturbances in the motion of the ground. These instruments can be installed on the ground, underground or under water. A Seismometer can also be installed on a major structure such as a dam wall to record disturbances in the structural integrity of that large structure.

A complete instrument package that records seismic signals is called a seismograph and there are many seismographs continuously recording ground motions around the world to facilitate the monitoring of earthquakes and the like.There are different seismic monitoring instruments for different applications as detailed on the next page, some being event recording instruments and some being continuous recording instruments and some are a combination of both.

Application Details

CONTINUOUS RECORDING OR EVENT RECORDING

Continuous recording is where the accelerometer / seismometer is monitored at a high speed sampling rate, perhaps 2000Hz, and the data is transmitted in real time to a remote site then processed. Continuous recording and sending that data to a remote site for analysis by a high speed telecommunications link is by far the best solution, as all the data is available for complex mathematical analysis by a large scale computer system. The problem is that for remote areas, a dedicated high speed link may require the laying of very expensive optical fibre cables or the installation of a high speed satellite link which is expensive to install and very expensive to use on an ongoing basis.

Event recording is where the accelerometer/ seismometer is packaged with “intelligent instrument” electronics with a fast processor. Such systems allow for both the routine recording of the high speed sampling, perhaps storing the raw data onto a high capacity SD RAM module, as well as doing local analysis of the data. Such systems can also issue event data messages, for any seismic disturbances via an RS232 interface, which can then be sent via a telemetry link back to a central computer to provide a record of all the events and alarm threshold alerts. This event based operation makes it practical to have instruments in the field reporting back from very remote installations, via cell phone or Satellite, without incurring very high satellite airtime charges.

With an “intelligent instrument”, the raw data can continue to be stored on a small SD ram module, allowing a person to visit the site and then unplug the SD ram module and take that module back to the central location for more detailed analysis of any seismic disturbances. As a guide, perhaps an 8GB SD Ram module could record one year’s data at a 100Hz sampling rate. It also may be possible to set the system such that the data on the SD RAM chip could be retrieved across the satellite link, however that process is very expensive. It is most likely that satellite airtime charges for this process may be prohibitive, much more expensive than having a person retrieve the on-site data on a periodic basis. There are several high quality Seismometer Instrument manufacturers, and two examples of these are listed here.

Strong Motion Accelerograph Instrument example for Structural Monitoring
The Kelunji EchoPro SMA (strong motion accelerograph) is a system appropriate for monitoring structures such as Dam walls. This instrument is a continuous recorder and if it is combined with 100GB storage module it can store more than 5 years of continuous triaxial data sampled at 100Hz. This instrument is ideally suited to structural monitoring. Its sensors are not as sensitive as those used by seismologists for research into earthquakes; however they are more than sensitive enough to detect signals that are well below the level where you can expect damage to structures.

Short Period Seismometers
One of Guralp’s range of short period seismometers (we call these “intelligent instruments” ) could be used for monitoring earthquakes in your local region, typically covering a region within about 500km of the sensor.
The closer you are to an earthquake, the more high frequency signals you can see, so with a range of 1-100Hz, the CMG-6TD-1 is the ideal sensor for this application. The collected data (perhaps a 100Hz sampling rate) can be recorded for analysis onto the SD ram memory. At the same time local processing on the data is done within complex filters within the instrument and short event messages are prepared and sent out of the instrument via a serial interface. These messages are appropriate to be sent across a telemetry link to be displayed on a central computer system.

An approach using Neon
Neon Remote Loggers, either cell phone based or satellite based are ideal for this application, where an “intelligent instrument” is to be used. A local program or scheme within the Neon Remote Logger can be set up to collect the event messages and transmit them routinely to a central Neon Server, to be checked for event content, to be recorded in a time series database and to raise alerts via alarm activation, via sms or email in the event of an earthquake or other seismic event. If the authority managing the earthquake activity wishes, a person can be sent to the location of the instrument to unplug the SD ram card to retrieve the raw data for further analysis. This approach is especially appropriate for remote areas, where there is little or no infrastructure as the whole system can be set up to be powered by a small solar power system.