Сейсмик толқындармен Жер қойнауын бейнелеу техникасы

Imaging technique used in seismology
Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P , S , and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to better understand core, mantle, and plate tectonic processes.

Tomography is solved as an inverse problem. Seismic travel time data are compared to an initial Earth model and the model is modified until the best possible fit between the model predictions and observed data is found. Seismic waves would travel in straight lines if Earth was of uniform composition, but the compositional layering, tectonic structure, and thermal variations reflect and refract seismic waves. The location and magnitude of these variations can be calculated by the inversion process, although solutions to tomographic inversions are non unique. Seismic tomography is similar to medical x ray computed tomography (CT scan) in that a computer processes receiver data to produce a 3D image, although CT scans use attenuation instead of traveltime difference. Seismic tomography has to deal with the analysis of curved ray paths which are reflected and refracted within the Earth, and potential uncertainty in the location of the earthquake hypocenter. CT scans use linear x rays and a known source.

Seismic tomography requires large datasets of seismograms and well located earthquake or explosion sources. These became more widely available in the 1960s with the expansion of global seismic networks, and in the 1970s when digital seismograph data archives were established. These developments occurred concurrently with advancements in computing power that were required to solve inverse problems and generate theoretical seismograms for model testing. In 1977, P wave delay times were used to create the first seismic array scale 2D map of seismic velocity. In the same year, P wave data were used to determine 150 spherical harmonic coefficients for velocity anomalies in the mantle. Initial models had resolution of ~3000 to 5000 km, as compared to the few hundred kilometer resolution of current models. Seismic tomographic models improve with advancements in computing and expansion of seismic networks. Recent models of global body waves used over 107 traveltimes to model 105 to 106 unknowns. Wide angle tomography is similar, but with a wide source to receiver offset. This allows for the detection of seismic waves refracted from sub crustal depths and can determine continental architecture and details of plate margins. These two methods are often used together. Local earthquake tomography is used in seismically active regions with sufficient seismometer coverage. Given the proximity between source and receivers, a precise earthquake focus location must be known. This requires the simultaneous iteration of both structure and focus locations in model calculations.

Regional to global scale tomographic models are generally based on long wavelengths. Various models have better agreement with each other than local models due to the large feature size they image, such as subducted slabs and superplumes. The trade off from whole mantle to whole Earth coverage is the coarse resolution (hundreds of kilometers) and difficulty imaging small features (e. g. narrow plumes). Although often used to image different parts of the subsurface, P and S wave derived models broadly agree where there is image overlap. These models use data from both permanent seismic stations and supplementary temporary arrays. First arrival traveltime P wave data are used to generate the highest resolution tomographic images of the mantle. These models are limited to regions with sufficient seismograph coverage and earthquake density, therefore cannot be used for areas such as inactive plate interiors and ocean basins without seismic networks. Other phases of P waves are used to image the deeper mantle and core. In areas with limited seismograph or earthquake coverage, multiple phases of S waves can be used for tomographic models. These are of lower resolution than P wave models, due to the distances involved and fewer bounce phase data available. S waves can also be used in conjunction with P waves for differential arrival time models. Surface waves can be used for tomography of the crust and upper mantle where no body wave (P and S) data are available. Both Rayleigh and Love waves can be used. The low frequency waves lead to low resolution models, therefore these models have difficulty with crustal structure. Free oscillations, or normal mode seismology, are the long wavelength, low frequency movements of the surface of the Earth which can be thought of as a type of surface wave. The frequencies of these oscillations can be obtained through Fourier transformation of seismic data. The models based on this method are of broad scale, but have the advantage of relatively uniform data coverage as compared to data sourced directly from earthquakes. Attenuation tomography attempts to extract the anelastic signal from the elastic dominated waveform of seismic waves. The advantage of this method is its sensitivity to temperature, thus ability to image thermal features such as mantle plumes and subduction zones. Both surface and body waves have been used in this approach. Ambient noise tomography cross correlates waveforms from random wavefields generated by oceanic and atmospheric disturbances. A major advantage of this method is that unlike other methods, it does not require an earthquake or other event to occur in order to produce results. A disadvantage of the method is that it requires a significant amount of time, usually a minimum of one year but several years of data collection are also common. This method has produced high resolution images and is an area of active research. Waveforms are modeled as rays in seismic analysis, but all waves are affected by the material near the ray path. The finite frequency effect is the result the surrounding medium has on a seismic record. Finite frequency tomography accounts for this in determining both travel time and amplitude anomalies, increasing image resolution. This has the ability to resolve much larger variations (i. e. 10–30%) in material properties.

Seismic tomography can resolve anisotropy, anelasticity, density, and bulk sound velocity. They found a strong low velocity body from ~30 to 250 km depth beneath Yellowstone, and a weaker anomaly from 250 to 650 km depth which dipped 60° west northwest. The authors attribute these features to the mantle plume beneath the hotspot being deflected eastward by flow in the upper mantle seen in S wave models. The Hawaii hotspot produced the Hawaiian–Emperor seamount chain. Tomographic images show it to be 500 to 600 km wide and up to 2,000 km deep.

Subducting plates are colder than the mantle into which they are moving. This creates a fast anomaly that is visible in tomographic images. Both the Farallon plate that subducted beneath the west coast of North America and the northern portion of the Indian plate that has subducted beneath Asia have been imaged with tomography.

Global seismic networks have expanded steadily since the 1960s, but are still concentrated on continents and in seismically active regions. Oceans, particularly in the southern hemisphere, are under covered. Tomographic solutions are non unique. Although statistical methods can be used to analyze the validity of a model, unresolvable uncertainty remains. is a collection of web based tools for the interactive visualisation, analysis, and quantitative comparison of global scale, volumetric (3 D) data sets of the subsurface, with supporting tools for interacting with other, complementary models and data sets. EarthScope Education and Outreach: Seismic Tomography Background. Incorporated Research Institutions for Seismology (IRIS). Retrieved 17 January 2013. Tomography Animation. Incorporated Research Institutions for Seismology (IRIS). Retrieved 17 January 2013.