Types of seismometer

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.

Broadband Seismometers

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.

Earth Tide on day of Japan 8.9

In a channel designation these instruments normally begin with the letter B. You also find the letter H for High Broad Band.

For a full list of channel naming conventions see the Appendix A to the SEED Manual (PDF 4.23 Mb).

8 Responses to Types of seismometer

  1. dare kayode says:

    Please can someone give me an idea on broadband seismic concept and application. Thank you.

  2. Erol Kalkan says:

    I don’t agree with Nekos, in structural engineering, natural period is the term to describe oscillator’s or structure’s dynamic response.

  3. Oliver says:

    “The short period seismometer is constructed to have a very short natural frequency and a correspondingly high resonant frequency”
    – Should it say “a very short natural period” instead of frequency?

    • PuterMan says:

      Indeed it should. Many thanks for pointing out my error. 🙂

      • Nekos says:

        I disagree with Oliver and PuterMan for the use of “period” that were accepted by both parties: – Should it say “a very short natural period” instead of frequency?
        In fact the two were wrong, we are talking about standard, even in engineering or physics its Natural frequency. Usually books will not use Natural period or manufacturer’s specification. I have been in seismic industry in oil and gas exploration applications for more close to 2 decades, we use “Natural frequency” in recorder, station unit, geophone, hydrophone, even in string vibration for gravity data acquisition they use Natural Frequency.
        I may agree that, another word could be “a very short Natural Period” but for PuterMan to accept and acknowledge as error is in fact wrong. There is no error here my brother.

        • PuterMan says:

          Thank you for your response. Interesting. I shall look into the terminology further and acknowledge your experienced contribution.

        • PuterMan says:

          I have cone to the conclusion that the word period refers to the range of frequencies and thus a seismometer, say a short period seismometer, indicates that the instrument covers a band of short frequencies. In effect short period is the group word for the band of frequencies whereas a frequency is a unique value.

          From PASSCAL Short-period sensors:

          Short-period sensors are rugged three-component seismometers that cover the frequency band from 1 Hz to 100 Hz. These sensors are used in both passive and active-source experiments. PASSCAL’s short-period sensors can be either feedback seismometers requiring power or conventional, passive seismometers requiring no external power.

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