Seismic instruments used for registration of ground motion caused by earthquakes are essential for studying seismology. Without instruments, we would have little knowledge of earthquakes and the interior of the Earth. The following will describe a bit about seismic instruments, both those shown in the exhibition and a few others.
The seismometer together with the unit recording the signal is called a seismograph. The seismometer senses the ground vibration and converts this to a signal that can be recorded. Modern seismographs can measure movements smaller than one nm (one millionth of a millimetre)
How it works: When the ground is moving rapidly, the spring ("Fj�r") suspended mass ("Masse") will keep quiet due to inertia and we will get a measurement on the scale ("M�leskala") to the right. This is the principle of the mechanical seismograph. The seismograph in this figure measures the vertical ground motion. In newer seismometers there are electrical coils around the mass, which is magnetic, so that an electrical signal is generated when the mass moves.
Earthquakes generate both vertical and horizontal motions. In order to measure a horizontal movement, we need a mass which can swing in the horizontal plane.
A simple horizontal pendulum. When the ground moves to the right, the mass will swing to the left and the ground motion will be recorded on the paper, which moves down. The mass can swing in all directions and must be suspended on a very long string in order to be able to record low frequencies. In order to avoid this, a �garden gate� or inverted pendulum is used (see next figures).
Inverted pendulum. The mass can swing in all horizontal directions. This is the principle of the Wiechert seismograph used in Bergen from 1921 to 1968, see figure below.
The Wiechert seismograph. The mass horizontal motion is captured by two arms that, by using a system of levers, can amplify the motion and record it on two rotating drums (R). The seismograph can record horizontal motions in East-West and North-South directions at the same time.
"Garden gate" pendulum. The mass only swings horizontally in one direction. It hangs at an inclined angle in order to make it swing more slowly (like a door hanging at an angle). This principle is used in the Bosch seismograph.
In new sensors, the mass barely moves. The mass is suspended by a spring ("Fj�r"). Departure from the mass center position is measured with a distance measurement device ("Avstandsm�ler"). As soon as the mass tries to move, the distance measurement device will send a current through the coil ("Spole"), which will oppose the motion so that the mass remains stationary. The larger the force on the mass, the larger the current. The size of the current will therefore be a measure of the ground motion (more correctly the ground acceleration). Such instruments can be built compact and very sensitive and are called accelerometers. They are also widely used for other purposes, such as releasing
Accelerometer on an electronic chip. The distance measurement device is a capacitor, the spring is a torsion bar and the mass is the upper capacitor plate. The chip has a dimension of 2x2 mm.
Seismometers measure signals with frequencies between 0.001 Hz and 100 Hz It is relatively simple to construct seismometers that measure the higher frequencies higher than 0.1 Hz (short period seismometer) Seismometers that measure the low frequencies (less than 0.1 Hz) are more difficult to make (long period seismometers) Modern (and expensive) seismometers measure both low and high frequencies (broad band seismometers). Technically they are based on the principle of the accelerometer.
The sensor generates a signal. This was recorded mechanically on older seismographs. All modern sensors give out an electrical signal that can be recorded in several different ways. A recording on paper, usually lasting 24 hours, is called a seismogram.
Optical recording. The electrical signal is sent to a galvanometer with a mirror. A light beam is reflected from the mirror and recorded on a rotating drum with light-sensitive paper. This system was used from early 1900 up to a few years ago.
Pen recording. Instead of a �lightbeam� pen, an electrical pen, also recording on paper on a rotating drum, is used. The pen can record with ink, scratch on smoked paper, or develop heat and write on heat-sensitive paper.
Digital recording. Today, nearly all seismographs record digitally. The electrical signal is transformed to a digital signal that can be recorded by a computer.
Optical recording. The electrical signal is sent to a galvanometer with a mirror (H). A light beam (L) is reflected from the mirror (H), and recorded on a drum with optical paper (R). In front of the drum, there is an optical lens focusing the beam.
Pen recording of seismic signals. The signal from the seismometer is amplified ("Forsterker") and sent to a drum recorder ("Trommel til registrering"). The time signal from a GPS (Global Positioning System) receiver is used to generate minute and hour pulses recorded together with the signal. Before time signals were available by radio, time pulses were generated by a mechanical clock.
Analog to digital converter. The electrical input signal is continuous: In other words, we know the size (amplitude) of the signal at any time. The analog to digital converter (AD) measures the amplitude at regular time intervals (?t) and gives out the numerical values for the amplitude as a sequence of numbers. These are then read by the computer. For seismic signals, the amplitudes are usually read 100 times per second. For digital music recorded on CD, we have ca 44 000 values per second.
Digitial seismic station. The signal is sent to the analog to digital converter ("Analog til digital omformer"), which converts the signal to a digital signal. It is then transferred to a computer ("Datamaskin"), where it is recorded and stored ("Datalagring"). Via Internet, the signal is sent from there to a central data center. "GPS tidsmarkering" means making a time stamping by GPS, and "Str�mforsyning", power supply.