Seismic tomography

Seismic tomography uses digital seismographic records to image the interior of the Earth.

The basic scheme is to first localize and characterize a set of significant earthquakes. These earthquakes are then considered to "illuminate" the interior of the earth with seismic waves.

The time that the waves arrive at seismic stations can then be used to calculate the waves' speed through the Earth. By combining analyses from many earthquakes, in different places around the Earth, a three dimensional map of wave speed through the Earth can be constructed.

Seismic tomography can be roughly categorized into traveltime tomography and full-waveform tomography. Traveltime tomography uses the high-frequency contents of the seismic data while the full-waveform tomography uses the whole seismic data including amplitudes and phases. Seismic tomography is an inverse problem, in which the seismic data are given and the goal is to determine the attributes of the medium such as wave speed and medium density. Seismic tomography is a nonlinear inverse problem where the traveltime tomography is much less nonlinear than the full-waveform tomography.

Computed tomography

Computed tomography (CT) is a medical imaging method employing tomography. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).

Computed tomography was originally known as the "EMI scan" as it was developed at a research branch of EMI, a company best known today for its music and recording business. It was later known as computed axial tomography (CAT or CT scan) and body section roentgenography.

Electrical impedance tomography

Electrical Impedance Tomography (EIT), is a medical imaging technique in which an image of the conductivity or permittivity of part of the body is inferred from surface electrical measurements. Typically conducting electrodes are attached to the skin of the subject and small alternating currents applied to some or all of the electrodes. The resulting electrical potentials are measured, and the process repeated for numerous different configurations of applied current.

Atom-probe field-ion microscopy (APFIM)

The atom probe made one-dimensional compositional maps by combining time-of-flight spectroscopy and field ion microscopy (FIM). The instrument allows the three-dimensional reconstruction of up-to hundreds-of-millions of atoms from a sharp tip (corresponding to specimen volumes of 10,000-1,000,000 nm3).

As in FIM, a sharp tip is made, placed in ultra high vacuum at cryogenic temperature (typically 20-100 K). A region of the tip's surface is selected (sometimes from an FIM image) and placed over a "probe hole" by moving the tip. The atoms at the apex of the tip are ionized, either by a positive pulsed voltage or a laser. These ions are repelled from the tip electrostatically and those passing through the probe hole reach a detector. A fast timing circuit is used to measure the time taken between the pulse and the impact of the ion on the detector, thus allowing the mass-to-charge ratio of the ion to be calculated and; therefore, the corresponding element (or elements) to be identified. From the collection of many of these ions, a chemical profile of the sample can be made with relative position accuracy of less than one atomic spacing.

Tomography

Tomography is imaging by sections or sectioning. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in medicine, archaeology, biology, geophysics, oceanography, materials science, astrophysics and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction. The word was derived from the Greek word tomos which means "a section" or "a cutting". A tomography of several sections of the body is known as a polytomography.