Introduction Geophysical methods can be used to obtain an image of the near surface of the earth. This image will display the spatial variation in the geophysical properties of the subsurface. In general, our interest is not in these geophysical properties, but in the physical, chemical, and biological properties and processes in the earth. To extract this type of information from our geophysical data, we need to transform our geophysical image into an image of these material properties.