I recently released open-source 1D CSEM forward and inverse modeling codes under the GPL v3.0 license.  Both Fortran2003 codes can be downloaded at:

http://marineemlab.ucsd.edu/Projects/Occam/1DCSEM

About:

• Both OCCAM1DCSEM and DIPOLE1D are licensed under the GNU General Public License, Version 3.0. They are open source---enjoy!
• Uses a Cartesian coordinate system
• Models all 3 field components (x,y,z) of both the electric and magnetic fields
• Supports inversion of real and imaginary, amplitude and phase, and polarization ellipse data
• Receivers can be located anywhere in the model
• Receiver sensor axes can be given a fixed rotation (supports full 3D rotation)
• Option to solve for the receiver sensor orientations (supports solving for the full 3D rotation if z component data available)
• Transmitters can be located anywhere in the model
• Transmitter antenna can be rotated both horizontally and vertically
• Model layers are parameterized in absolute depths, not thickness
• There are no assumptions of fixed layers (you have to input the air and sea layers)
• Handles stratified seawater conductivity layering
• Can invert for layer conductivities both above and below the receivers and transmitters
• Includes 1D MT inversion (alone or jointly with CSEM inversion)

Not a programmer?  

You can run the forward code and view response plots using only your web browser at:

http://marineemlab.ucsd.edu/WHAM