Seismic tomography
The seismic tomography (also Erdtomographie ) is a method of examination of seismology and used to determine velocity anomalies of the seismic wave propagation in the Earth's interior . In terms of method, a basic distinction is made between three research approaches: local quake tomography, teleseismic tomography and damping tomography.
Theoretical background
Tomographic examinations are usually carried out using natural source signals, the earthquakes . In addition to natural waves , artificially generated seismic waves can also be used.
The basic principle of the method is that the transit time of a seismic space wave from its place of origin - in the case of earthquakes, this is the hypocenter - to a measuring station depends on the propagation speed along its path . Tomography now tries to draw conclusions about the speed distribution of the ground traveled from the transit times of the wave field determined at various measuring stations. As with all tomographic methods, the basis of seismic tomography is the Radon principle, which states that the values of a manifold can be completely determined from its projections. The Radon transformation used here is usually implemented numerically.
For this purpose, the part of the earth's body through which the seismic waves radiate is divided into volume elements. At the beginning, a so-called start model is developed, which is based on previous measurements (e.g. reflection or refraction seismics ) or on geological observations. Among other things, analytical methods are used to create models . A density model can be calculated using the Adams-Williamson equation or similar methods. The volume elements of the investigation area are then assigned slowness values in accordance with the start model. Using the beam geometry , theoretical transit times can now be calculated as an integral or sum over the transit times (slownesses) in the cells passed through and compared with the measured data.
The differences between the observed and theoretical transit times (the so-called transit time residues ) arise from local deviations of the actual speeds in the investigation area from the specified speed model (the speed anomalies ). If the observed transit time is longer than expected, the seismic wave has passed through slower areas and vice versa. By gradually adapting the values in the volume elements, the residuals should be minimized so that in the end the distribution of the velocity anomalies in the subsurface can be reproduced as precisely as possible.
The runtime residuals always reflect the sum of all effects along the route. However, since a wave can pass through several both positive and negative anomalies one after the other, coverage is of crucial importance for successful tomography . I.e. The investigation area must be traversed by as many wave beams as possible from as many different directions as possible in order to achieve optimal detection of the volume elements in various combinations. This is the only way to correctly localize anomalies.
Local tremor tomography
With this approach to tomography, the area to be examined is examined with signals from a short distance. The use of local earthquake events has the advantage that, due to the spatial proximity to the recording seismometer , the effects of the path originate solely from the investigation area. On the other hand, the method relies on high seismicity and is therefore limited to seismically active areas. Alternatively, the excitation of seismic waves with artificial sources such as e. B. Explosions are carried out. However, these are associated with high costs, so that use is rarely made of them due to the large number of blasts required and at the same time limited penetration depth into the earth's body.
Teleseismic tomography
Teleseismic tomography, on the other hand, uses earthquakes from a greater distance. Since these are recorded worldwide, this method is subject to far fewer spatial restrictions and can be used almost anywhere. Another advantage lies in the beam geometry: teleseismic earthquake waves also traverse deeper layers of the earth down to the deep lower mantle and therefore also allow investigations in these regions of the earth's body.
However, the resolving power of teleseimic tomography is usually very low at greater depths. In addition, the database is also restricted here by the limited spatial distribution of earthquake sources, which often does not allow optimal coverage. In addition, runtime residuals can also flow in here, the origin of which is not in the investigation area but close to the earthquake foci, but which are included in the tomographic inversion when the velocities are adjusted in the volume elements.
Attenuation tomography
In damping tomography (amplitude tomography as opposed to transit time tomography ), the damping behavior of the subsurface flows into the examination, i.e. the decrease in wave energy along the path due to damping. Since damping effects also depend on the elastic properties of the rock traversed by the seismic wave, their anomalies also allow conclusions to be drawn about the investigated area. Since the total attenuation along a wave beam is not the sum (integral) of the attenuation in the individual cells, but rather its product, logarithmic attenuation values can be assumed, which then add up to the total attenuation. Other elements of the decrease in amplitude with distance, such as the geometric decrease in amplitude, must be taken into account.
Interpretative approaches
Anomalies of the seismic velocities and also of the attenuation are often due to temperature changes, as they occur e.g. B. by hot magma or partial melt in volcanic areas or by cold lithospheric fragments or plunging plates in subduction zones .
However, changes in the elastic parameters can also have other geological or mineralogical causes. The pore filling of rocks, for example with petroleum, water or other fluids , but also slight chemical changes in the minerals can play a role here. The interpretation of tomographic results is therefore mostly done against the geological background of the study area.
literature
- K. Aki, W. Lee (1976): Determination of three-dimensional velocity anomalies under a seismic array using first P arrival times from local earthquakes, Part 1. A homogeneous initial model. Journal of Geophysical Research. Volume 81, pp. 4381-99. (English)
- K. Aki (1993): Seismic Tomography: Theory and Practice. (Ed. H. M. Iyer and K. Hirahara), Chapman and Hall, London, p. 842. (English)
- E. Kissling (1988): Geotomography with local earthquakes. In: Reviews of Geophysics. Volume 26, pp. 659-698. (English)
- M. Koch (1982): Seismicity and structural investigations of the Romanian Vrancea region: Evidence for azimuthal variations of P-wave velocity and Poisson's ratio. In: Tectonophysics. Volume 90, pp. 91-115. (English)
- M. Koch (1985): A theoretical and numerical study on the determination of the 3D-structure of the lithosphere by linear and nonlinear inversion of teleseismic travel times. In: Geophys. JR Astr. Soc. Volume 80, pp. 73-93. (English)
- CH Thurber (1993): Local earthquake tomography: velocities and v p / v s theory ; in: Seismic Tomography: Theory and Practice. (Ed. H. M. Iyer and K. Hirahara), Chapman and Hall, London pp. 563-583. (English)
swell
- Mirjam Bohm (2004): 3-D local quake tomography of the southern Andes between 36 ° and 40 ° S. Dissertation, Freie Universität Berlin (= GFZ Potsdam, Scientific Technical Report STR 04/15), urn : nbn: de: kobv: b103-041559 (full text access also possible via the DNB )
- Benjamin S. Heit (2005): Teleseismic tomographic images of the Central Andes at 21 ° S and 25.5 ° S. An inside look at the Altiplano and Puna plateaus. Dissertation, Freie Universität Berlin (= GFZ Potsdam, Scientific Technical Report STR 06/05), urn : nbn: de: kobv: b103-06052 (full text access also possible via the DNB , English)