The Seismograph

The Pendulum

In order to measure ground motions, the seismograph must remain steady when the ground moves. Various types of pendulums have been used to obtain this steady state. The simplest type of pendulum is a heavy mass suspended by a wire or rod from a fixed point, similar to a grandfather clock. Other forms are the inverted and horizontal pendulums. The inverted pendulum has a heavy mass fixed to the upper end of a vertical rod pointed at its lower end, and the horizontal pendulum has a rod with a mass on its end which is suspended at two points so it swings on a horizontal plane.

Recording Pendulum Motion

The Mechanical Method:

A sheet of smoked paper is wrapped around a rotating drum, mounted to move with the Earth. A moving pen connected to the pendulum presses lightly on the paper. As time passes, the drum rotates so that the recorded lines are not superimposed on each other. Defleciton of the pendulum is commonly magnified mechanically by single or double multiplying levers so that the graph is easier to see.

This method is simple and econmical; however, the seismograph must have a heavy mass to overcome the friction between the pen and paper. Consequently, some mechanical seismographs weigh one ton or more.

The Optical Method:

The optical method still uses a pendulum motion to record the ground movements. However, to overcome friction, mirrors are used to refelect the light onto photosensitive paper wrapped on a drum.

Noise: With any free oscillation of a mechanical motion, there is a certain amount of background noise created by the machine, which can mask the proper recording of Earth movements. The simplest way to reduce the free oscillation of a pendulum is to suspend it in a viscous liquid of which the resisting force is proportional to the velocity of the pendulum. In practice, the required resisting force is exerted by a special device called a damper. An air damper produces its resisting force by the use of a piston moving in a cylinder.

High-Precision Pendulum Seismometers:

Technological developments, notably in electronics, have given rise to high-precision seismometers and sensors of ground motion. In these electromagnetic instruments, a coil is fixed to the mass of a pendulum and moves in a magnetic field. The electric current, generated in the coil, operates a galvanometer exactly as a dynamo operates a motor. A mechanical damper is not needed because a dampening force can be produced by electrical currents induced in a copper plate moving in a strong magnetic field. The voltages produced by motions of the pendulum are passed through electronic circuitry to amplify the ground motion for more exact readings.

The LEO Seismometer: Broadband Seismic Station

The Lehigh Earth Observatory uses Guralp CMG-3T broadband seismic station. It is designed for vault, portable, and downhole applications. The components are specially selected for low noise during long periods. The standard output is a flat velocity from 0.01 to 50 Hz. In addition to the velocity output, a mass position output is provided.

The CMG-3T sensor has the USNSN solid body mechanical system and circular electronics board. All external metal parts are stainless steel. A microprocessor - controlled mass recentering system is used.

A 24-bit digitizer module with up to 16 megasamples of RAM, model DM24, can be added to the CMG-3T. When installed on the Guralp sensor, a clean, isolated, pressure and temperature stable environment is ideal for a wide-dynamic range digitizer. Additionally, the sensor output cannot be contaminated from analog noise sources, as the output is in digital form.