First, a PIXEL is a shorthand expression for "Picture Element", and is defined to be the smallest digital or graphical element of a two-dimensional picture, or image. Valuable digital (and many newspaper-graphical) pictures are composed of from tens to thousands of uniformly arranged, tiny PIXELs. To see some examples of PIXELs, look at a newspaper photo with a magnifying glass, or look up-close-and-personal at an active television screen. Clusters of dots (PIXELs) can form a fairly clear image when viewed from afar. Each PIXEL carries multiple visual properties of (at least) Red-Green-Blue color, hue, and intensity, but black-and-white (or gray-scale) intensity sometimes replaces the color information.
Next, a VOXEL is shorthand for a "Volumetric PIXEL", defined to be the smallest element of a three-dimensional (usually digital) volume, matrix, or computerized virtual cube. Such a digital volume can be thought of as a regular stack of 2D pictures or images (like a deck of cards, or a toy Rubik's Cube), in which each VOXEL sits at the center of a small, 3-by-3-by-3 matrix (27 VOXELs), and is in contact with each of the 26 surrounding VOXELs; either at the surface of one of its six faces; at the point of one of its eight corners; or along the line of one of its twelve edges. In general, VOXELs can also carry the same color-intensity properties as PIXELs, plus the additional important property of Opacity (or Transparency, from 0-1, so that VOXELs hidden “behind them” can be seen, if the blocking VOXEL Opacities are less than 1.0).
Finally, a D3D (Diagnostic-3D) VOXEL is defined to be a Volumetric PIXEL whose center is a fully D3D-processed seismic trace sample, and whose (amplitude-color-intensity) value is the D3D-(relative-natural-logarithm-of-the) Acoustic Impedance (AI) "amplitude value" at that (X, Y, TWT). AI is defined more fully under the next button (but it is the product of the local average Bulk Density , and the vertical compressional wave Velocity). This small sub-volume has the map-view dimensions of the final, D3D-migrated (X, Y) trace spacing (55x55-feet?), and the vertical dimension of the vertical Two-Way-Time (TWT, for example 0.002 seconds, or 2 milliseconds). Its vertical 3D position in seismic TWT-space, can be converted to its measurable drilling Depth (in feet or meters), by using an appropriate D3DSP-provided TWT-Depth relationship. The D3DSP recommends that this T-D relationship be taken from an Estimated Depth "Time Cube" (or "T'ube"), where the EDT looks exactly like a D3D-impedance seismic volume except that each VOXEL in a D3D-EDT carries the estimated true vertical depth to that sample's TWT, whereas each VOXEL in the seismic T'ube carries a D3D-impedance value. More simply put, a D3D VOXEL is the volumetric name for a D3D-impedance (digital seismic trace) sample.
In summary, AI is the key rock property that governs transmission and reflection of compressional sound waves propagating in the subsurface. It is an immensely important empirical fact that higher hydrocarbon saturations (and/or rock porosities) lead to lower AI. VOXELs shown in the examples on this website, range in size from 55 ft x 55 ft x 1 ms (~0.35 acre-feet) to 82 ft x 82 ft x 4 ms (~3.1 acre-feet), at depths from 3,500 ft to 12,000 ft.
Incidentally, there is no "Amplitude Versus Offset" (AVO)-derived information contained in a D3D VOXEL, because the D3DSP is an inherently Offset Variations Absent (or “OVA”) technique. The D3DSP attempts to remove (mute out) any samples with amplitudes, or associated wavelet characteristics, substantially altered by their large shot-to-receiver-distances (which result in non-normal/perpendicular-incidence acoustic ray paths). The D3DSP is a very robust, high-resolution "Acoustic-Earth model" method of imaging, measuring, and evaluating CIO's. It does not use the more difficult to interpret "Elastic-Earth model" techniques (e.g., AVO, Zoeppritz’ equations, “elastic impedance”, etc.).