Seismic tomography

The seismic imaging ( or tomography ) is a method of investigation of Seismology and used for determination of the velocity anomalies of seismic wave propagation in the earth. Basically methodically three study approaches in tomography are distinguished: local earthquake tomography, teleseismic tomography and loss tomography.

Theoretical background

Tomographic studies are i.d.R. based on natural source signals, the earthquake, performed in rarer cases, this artificially generated seismic waves are used.

The basic principle of the method is that the running time of a seismic wave space from their place of origin - during earthquakes, this is the hypocenter - depends to a measuring station of the propagation velocities along its path of travel. The tomography is now trying zurückzuschließen from the determined at various stations running times of the wave field on the velocity distribution of the run through the subsurface.

For this, the irradiated part of the seismic waves of the earth is divided into volume elements. At the beginning of a so-called initial model is developed, based on previous measurements (eg reflection or refraction ) or based on geological observations. Come to modeling, among other analytical methods used. Thus, for a density model with the Adams - Williamson equation or similar methods to calculate. The volume elements of the study area velocity values ​​are then allocated according to the starting model. Using the beam geometry theoretical terms can now be calculated and compared to the measured data.

The differences that may exist between observed and theoretical terms ( the so-called time residuals ) caused by local deviations of the actual velocities in the study area from the predetermined velocity model ( the velocity anomalies). Is the observed runtime is greater than expected, the seismic wave has passed through slower areas and vice versa. Through a gradual adjustment of the values ​​in the voxels ultimately minimizing the residuals is to be achieved, so that at the end of the distribution of velocity anomalies can be reproduced in the underground as accurately as possible.

The time residuals always quote the sum of all effects along the travel path again. However, since a wave can move successively through several both positive and negative anomalies, is the coverage of crucial importance for a successful tomography. That is, the study area must be passed by as many wave beams from as many different directions in order to achieve an optimal collection of volume elements in various combinations. Only so anomalies can be correctly located.

Local earthquake tomography

In this approach, the imaging area to be examined is examined with signals from a short distance. The use of local earthquake events has the advantage that Laufwegeffekte due to the proximity to the registering seismometer originate solely from the study area. On the other hand, the method is dependent on a high seismicity and thus restricted to seismically active areas. Alternatively, the excitation of seismic waves with artificial sources such as could be carried out blasting. However, these are associated with high costs, so that thereof due to the required number of explosions at the same time limited penetration depth into the Earth's body rarely exercised.

Teleseismic tomography

The teleseismic tomography contrast, uses earthquake from a greater distance. Since these are recorded worldwide, this method is subject to a much lower spatial limitation and can be used almost anywhere. Another advantage lies in the beam geometry: teleseismic earthquake waves pass through even deeper soil layers to the deep lower mantle and therefore can also investigations in these regions of the globe to. The resolving power of teleseimischen tomography, however, is at greater depths is usually very small. In addition, the data base is limited in scope by the limited spatial distribution of earthquake foci, which often does not allow an optimal coverage. In addition, can also incorporate time residuals, whose origin is not in the study area but close to the earthquake epicenters, but miteinfließen in the tomographic inversion at adapting velocities in the volume elements here.

Attenuation tomography

When Dämpfungstomopraphie the damping behavior of the subsurface flows in the investigation, so the decrease in wave energy along the path of travel. Since damping effects on the elastic properties of the traversed by the seismic wave rock depend on their anomalies allow us to draw conclusions on the studied area.

Approaches to interpretation

Anomalies of seismic velocities and the attenuation are often due to temperature changes, as they can be triggered in subduction zones such as by hot magma or partial melt in volcanic areas or cold lithosphere fragments or subducting plates.

However, changes in the elastic parameters can also have other geological or mineralogical causes. The pore filling of rocks, such as with oil, water or other fluids, but also slight chemical changes in the minerals can play a role here. The interpretation of tomographic results, therefore, takes place mostly before the geological background of the study area.