Seismische Verfahren
Glacial seismic. © M. Behm

Seismic methods are based on the laws of elastic wave propagation in the ground. The propagation velocity and characteristic of seismic waves is predominately affected by mechanical properties of the soil (bulk modulus, shear modulus, density). The goal of all seismic methods is to derive the distribution of seismic velocities and thus the ground’s structure from the observation and analysis of the seismic wave propagation. Seismic waves can be generated by earthquakes – the corresponding science is called Seismology, whereas Seismic refers to a procedure in which artificially generated waves are employed.

Seismic Methods

Seismic surveys are of enormous economic importance and is particularly valuable in the search for raw materials (e.g. hydrocarbons) and in geotechnical engineering. Seismic data acquisition starts with the artificial excitation of elastic waves. Depending on the required areal extent of the waves, the source can range from a hammer to mechanical/pneumatic weight-drops, vibrators or explosives. Seismic waves are measured using geophones which can be arranged in a straight line (2D seismic) or over a whole area (3D). Depending on the acquisition system, the output of the geophones is sent as an analogue or digital signal to the recording unit. Exploration depths vary as a function of the excitation strength between tens to thousands of metres.  

Seismic determines the wave propagation velocities underground, where the analysis of compressional waves provides an image of the P-wave velocities and the analysis of shear waves gives information on S-wave velocities. In practice it is usually only P-waves that are evaluated. However, S-waves give valuable additional information concerning e.g. compressive strengths of the soil or groundwater exploration. Furthermore, surface waves can also deliver information on the ground.

The propagation of elastic waves in the ground is an extremely complex process. For this reason several different seismic methods have evolved which all concentrate on the acquisition and processing of different wave types. The most important methods are reflection seismic, refraction seismic, seismic tomography and wave field inversion.

Seismology

Seismology as such is not a method, but the study of vibrations. In general, this concerns earthquakes which are a natural source of vibration, including tectonic quakes, volcanic quakes and collapse quakes. Special vibration sources are gaining ever greater interest, as the seismograms usually allow unusual causes such as rapid landslides, rock falls, rock bursts or explosions to be recognised.

Seismology not only serves the purpose of identifying vibration sources, but mainly aims to record natural earthquakes in order to estimate the earthquake hazard of a region. Seeing as precise instruments for the measurement of ground motion have only been used since 1900, a significant part of the knowledge on earthquakes and their consequences relies on historic earthquake investigations. One of the tasks of the seismological service is thus the development of an earthquake hazard map and expert reports on the location for strategic infrastructure, such as power supplies and transport routes.

Earthquake networks today encompass a number of survey stations, which often span across national borders. The Austrian Seismological Service at the Central Institute for Meteorology and Geodynamics in Vienna, for example, also uses stations from neighbouring countries in order to improve the accuracy of earthquake analysis in the vicinity of the border. Detecting earthquakes worldwide is also an aspect of seismology, which is not only used for the investigation of the Earth’s inner structure but also for assisting aid organisations in case of emergencies. The Austrian army dispatches search and rescue troops to the affected regions.

Besides statistical methods – for the localisation and rate of occurrence – seismology also uses geomechanical principles in order to investigate the processes and mechanisms in the hypocentre. It also continuously develops new concepts for explaining various phenomena such as soil liquefaction which is often concurrent with quakes.

The national data centre for the surveillance of the comprehensive test ban treaty is also based at the Central Institute for Meteorology and Geodynamics.