Seismic Instruments

An earthquake is the result of sudden release of energy in the earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. At the earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity. In its most generic sense, the word earthquake is used to describe any seismic event whether a natural phenomenon or an event caused by humans that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.

As earthquakes result in huge loss of life and property, it is necessary to monitor earthquakes and analyze it so that in future proper precautions can be taken.

Earthquake monitoring equipments can be broadly classified into the following categories. Strong motion recorder is generally used to record earthquakes of magnitude 3 and above. This type of equipment usually records acceleration using a force balance accelerometer (FBA). The maximum boundary of this instrument is about 100km. Strong motion equipments are used in dams, bridges, buildings etc to record the vibrations.

Broad band Sensor is the one which is based on the principle of a velocity sensor. It is capable of recording earthquake events over a large boundary.

 

Broad Band Sensor

 

Broad band sensor is one of the most precise earthquake monitoring equipment which is based on the principle of a velocity sensor. It consists of a sensor, recorder, a hard disk and a GPS.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Triaxial seismometer (Sensor)

 

 

 

 

 

 

 

 

 

 

 

 

Recorder

 

 

 

 

 

 

 

 

 

 

 

 

 

20 GB Hard disk

The recorder consists of 6 channels and is connected to both hard disk and the sensor.

 

Installation of Broad Band Sensor

 

The broad band sensor purchased by the Civil Engineering Department, IISc is installed at the IISc site.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Site of installation

The equipment is transported to the site. Site consists of a small building which is resting on hard bed rock. While transporting, utmost care should be taken to see that the sensor is in locked position. The sensor should be placed on a flat surface.

 

After placing the sensor, it should be aligned. The design requirement is that the sensor should always be parallel to east direction. If the sensor is not properly aligned then it may be impossible to co-relate the data obtained from all the channels during the event of an earthquake.

 

The seismometer consists of a bubble tube which can be used for leveling. Locking jam rings are provided for leveling operation. Once the bubble is at its central position, by rotating the steel legs either inwards or outwards, tighten the jam rings.

 

The sensor is placed in its position and recorder is also installed. In the recorder, connectors are provided for external GPS, laptop, power, GPS antenna, hard disk, and for connecting to satellites.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Completed setup

 

Strong Motion Accelerographs

 

The Department of Civil Engineering, IISc has procured 8 strong motion accelerographs and installed at various sites. These include 6 strong motion and 2 shallow borehole triaxial force balance accelerometer with 40m cable.

 

 

 

 

 

 

 

 

 

 

 

Strong Motion Accelerograph

The key features of this accelerograph are:

Timing accuracy to 0.5 milliseconds due to synchronized sampling with optional GPS timing system.

18 bits of resolution with 108 dB dynamic range.

Remote alerting capability for system event or auto-diagnostic failure

Triaxial EpiSensor force balance accelerometer, orthogonally oriented, internal

Full scale range: +/- 2g

Trigger bandwidth:0.1 Hz-12.5 Hz

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Location of installation of SMA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Completed Set Up

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bore Hole Sensor

Department of Civil Engineering

Indian Institute of Science, Bangalore

Webpage is maintained by

Dr P Anbazhagan

Lecturer

Department of Civil Engg

Indian Institute of Science

Bangalore, India 560012

Suggestions and queries may be directed to

Dr P Anbazhagan

Copyright  All Rights Reserved.

This page was last updated on August 25, 2011