Seismological methods

Seismological methods use, among others, earthquakes for the acoustic sounding of the earth’s crust. The methods used in BGR are classified below:

• Array seismology
  The interconnection of several similar seismological measuring stations (seismometers) spaced relatively close together is called an array. An array is able to sound seismic signals (earthquake signals), i. e. to determine the direction of the incoming waves like an antenna and to reduce the natural noise interfering with the earthquake signals. The property of noise suppression is used for the detection of weak signals, the directional information is used to allocate the measured signals to the releasing seismic events
  
  
• Broadband seismology
  Seismic signals from earthquakes and other natural and artificial sources can create seismic waves in a very broad frequency range of less than one thousandth Hz up to about 100 Hz. Seismometers that are able to record this frequency range are called broadband seismometers. Broadband seismology is generally understood as the acquisition and utilization of these broadband signals for scientific purposes. This includes the examination of the internal structure of the earth, the determination of seismic focus parameters and the investigation of the influence of external seismic sources (e. g. atmospheric disorders) on the solid earth.

In addition to the frequency range, also the dynamic range, i. e. the range from the smallest to the strongest measurable soil movement, plays an important role in broadband seismology. Modern broadband seismometers cover a range from about one nanometre up to some centimetres. The 24-bit analogue/digital-transducers available today allow to continually record the dynamic and frequency ranges required for broadband seismology and file them in mass storages.

Seismology investigates properties of earthquakes and the crust:

• Focal mechanics
  Focal mechanics is the investigation of the earthquake’s focus, i. e. which event triggered the earthquake. The simplest model is the assumption of a plane fracture with both sides separated by the fracture moving into opposed directions. Determination parameters are the situation of the surface and the released kinetic energy. From the energy, the magnitude of moments Mw can be calculated. More complicated models consider superpositions of several movements and include volume changes (explosion, subsidence earthquake). Modern methods also allow statements on size and geometry of the fracture plane and on details of the thrusting movement.
  
  
• Structure investigation
  Seismology provides the most important tools for the investigation of the earth’s inner structure. Earthquake waves, stimulated by natural or artificial processes serve as a primary data source. They propagate through the earth or run along the earth’s surface and are influenced on their way by structural inhomogeneities. If these waves are recorded by as much seismometers as possible distributed evenly on the earth’s surface, modern broadband seismology methods can yield information on seismic velocities, inhomogeneities of density and temperature, structural boundary planes, anisotropic regions and general information on the internal structure of the earth. For this reason, seismic tomography for the mapping of the earth’s interior can be vaguely compared to x-ray tomography.

What makes seismic tomography difficult, is the uneven distribution of seismic sources (earthquakes) and receivers (seismometers). These disadvantages can be compensated partially by systematic large-scale experiments.