Long Period Seismometer
A long period seismometer is an instrument in which the resonant frequency is very low. The lag between the seismometer and the ground motion becomes zero and the amplitude of the seismometer displacement becomes equal to the amplified ground displacement. It is sometimes called a displacement meter.
It is usually designed to record seismic signals with frequencies in the range 0.01 Hz to 0.1 Hz which is ‘periods’ of 100 seconds down to 10 seconds.
In a channel designation these instruments normally begin with the letter L
Short Period Seismometer
The short period seismometer is constructed to have a very short natural period and a correspondingly high resonant frequency which is higher than most frequencies in a seismic wave. The displacement of a short period seismometer is proportional o the acceleration of the ground, and this instrument of accordingly called an accelerometer. It is usually designed to respond to frequencies of 1 Hz – 10 Hz (periods in the range 1 second down to 0.1 seconds)
This type of instrument is particularly suitable for measuring strong motion where the amplitude of the ground motion would swamp a normal displacement seismometer.
In a channel designation these instruments normally begin with the letter S, or E for Extremely Short Period.
The broadband seismometer has an inertial pendulum design with a force feedback mechanism. The amount of feedback force applied is determined by using an electrical transducer to convert the motion of the mass into an electrical signal.
Seismic signals in the range 0.1 Hz to 1 Hz (periods of 10 seconds down to 1 second) can be hindered by the presence of a natural background form of noise called a microseism. Some of these may be caused by local effects such as traffic, rainfall or wind action on trees, but an important source is the action of storm waves and deep swells at sea. The drumming of surf on a shoreline or the interference of waves over deep water are the principle source of this low amplitude seismic noise. This signal may be as strong as the signal from a distant earthquake which cannot be selectively amplified without also amplifying the noise.
Short period instruments give records dominated by high frequencies and long period instruments smooth things out giving only a low frequency content record. The range between the strongest and the weakest signals that can be recorded by an instrument without distortion is called the dynamic range.
Broadband seismometers have high sensitivity over a very wide dynamic range. They can be utilised for registering a very wide range of signals and the dynamic range extends from ground noise up to string acceleration such as would come from a very major earthquake, and the periods that can be recorded range from high frequency body waves to the very long period oscillations associated with Earth Tides