MEASUREMENT OF GROUND MOTION

 

MEASUREMENT OF GROUND MOTION

 

Basically, two types of instruments are used to record earthquake motions. The instruments used by seismologists are generally very sensitive and record even the weakest motion of the earth. These records called seismograms are useful in inter preting earthquake occurrences and help in determination of epicenter, focal depth and other parameters of the event. These are, however, not suitable for strong motion which is relevant to design engineers. For this, strong motion instruments called accelerographs are employed. These instruments generally operate when the ground motion exceeds a threshold value (y. a ground acceleration of 0.01-0.02g) and are expected to recend the strongest ground motion. Both types are complementary to each other and provide useful data in earthquake engineering.

 

Fig 1 :Velocities Through the Interior of Earth

These instruments make use of the simple electromagnetic principle that when a coil moves in a magnetic field, a voltage is generated as an output which is proportional to the relative velocity between the magnet and the coil. By a suitable mechanical design of the coil-magnet mounting, the relative velocity may be made directly proportional to the ground velocity. In such a case, the unit is termed as a velocity pickup. The velocity pickup may be connected to a galvanometer, whose mirror would rotate in proportion to the voltage input. The rotation of the mirror can be recorded by photographic recording process employing a light source. (See Fig. 2).

Such a method which did not require electronic circuitry was used in the beginning (since 19th century) by seismologists and the same principle is used even now. The mechanical proper ties, namely, the natural frequency and damping of the seismometer and galvanometer would decide which frequency range would be magnified by a particular set of equipment and they are some times classified as short-period or long-period seismographs. Because of the sensitivity requirement of the seismographs to record weak motions, they would be thrown off the scale if strong motion occurs in the vicinity. These instruments record continuously and require an accurate time base in order to identify  arrival times of various phases of ground motion. The instruments are located on foundations, usually resting on rock and isolated such that they are insulated against spurious ground noise.

Fig 2: A Typical Seismograph Systems