*last updated October 1999*

Software is available upon request. Send requests to hole@vt.edu.

Each of the following algorithms is available separately. I will send whichever package(s) you request via email. The package will be suitable for a unix machine - tell me if you are not using unix.

The finite difference algorithm is the basis for computing times in all of the subsequent algorithms. It is included within each package.

The codes should work in 2 dimensions simply by setting ny=1.

**3-Dimensional Finite Difference Travel Times**

Computer algorithm to compute 3-D first arrival travel times using finite difference operators.
This algorithm is an improvement of John Vidale's original fd algorithm (Vidale, Geophysics, 1990). It allows the proper calculation of travel times in the presence of large, sharp velocity contrasts, without significantly sacrificing the superior speed and accuracy of the original algorithm. I thank John Vidale for providing his code and permitting its re-distribution in altered form.
The program is described in:

*Hole and Zelt, 1995,** Geophys. J. Int., 121, 427-434.*

**Inversion Algorithm for a 3-Dimensional Refracting Interface**

Assumes a velocity model, and inverts travel time residuals to find the depth z(x,y) of an interface across which there is a velocity contrast. Iterative solution of the nonlinear problem. Described in:

*Hole, Clowes and Ellis, 1992,** J. Geophys. Res., 97, 3417-3429.*

**3-Dimensional Tomographic Inversion**

Uses simple backprojection to invert first arrival travel times for 3-D velocity structure. Iterative solution of the nonlinear problem. Described in:

*Hole, 1992,** J. Geophys. Res., 97, 6553-6562.*

**3-Dimensional Finite Difference Reflection Travel Times**

Algorithm to compute 3-D reflected travel times using finite difference operators. Allows the reflector to exist at an arbitrary depth between grid nodes. Described in:

*Hole and Zelt, 1995,** Geophys. J. Int., 121, 427-434.*

**Inversion Algorithm for a 3-Dimensional Reflecting Interface**

Assumes a velocity model above the reflector, and inverts travel time residuals of a reflected arrival to find the depth z(x,y) of an interface. Iterative solution of the nonlinear problem. Includes the reflection forward-modelling package above. Described in:

*Zelt, Ellis, Clowes, and Hole, 1996,** J. Geophys. Res., 101, 8503-8529.*

**COMING SOON...**
**Earthquake Hypocenter Inversion**

Assumes a velocity model and inverts for hypocenter location and time. Iterative solution of the nonlinear problem. Can be used with the tomography package above to simultaneously invert for velocity and earthquakes. Described in:

*Hole, Brocher, Klemperer, Parsons, Benz, and Furlong, 2000,** J. Geophys. Res., XX, in press.*