Oilfield Glossary
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delta

Schematic diagram of depositional environments
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1. n. [Geology]
An area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems.
2. n. [Geophysics]

An anisotropy parameter that describes near-vertical P-wave velocity anisotropy and the difference between the vertical and small-offset moveout velocity of P-waves.

δ = ½{[(C13 + C44)2 − (C33C44)2] / [C33 (C33C44)]}

Anisotropy parameter for near-vertical P-waves. Delta (δ) describes near-vertical P-wave velocity anisotropy and the difference between the vertical and small-offset moveout velocity of P-waves. C33 is the vertical P-wave modulus (parallel to the symmetry axis), C44 is the modulus for a vertically traveling and horizontally polarized S-wave (parallel to the symmetry axis) and C13 is the modulus of dilation in the vertical direction induced by compression in the horizontal direction.

Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966.