# Dictionary:Double-square-root equation

{{#category_index:D|double-square-root equation}}
The traveltime surface of a point diffractor in offset space where raypaths are straight and the source and receiver are not coincident; see Figure D-20b.

If *x*=midpoint location and *y*=offset (source-geophone distance), it is the surface *t*(*x*,*y*)

where *m*=inline distance from diffracting point to the midpoint and *h* its depth. The surface is called a Cheops pyramid (ke’ ops). This equation contrasts with the hyperbolic **single-square-root equation** for a zero-offset (CMP) section,

*DMO processing* transforms a Cheops pyramid so that a cylindrical hyperbola (see Figure D-20d) is obtained after normal moveout correction with the correct velocity. After a transformation *y*=*Ut* that constitutes slicing the pyramid by radial planes containing the *x*-axis, NMO can be applied correctly.

## References