Surfer GRD files are two-dimensional uniform lattices (elevation grids) in the Surfer Grid file format. Both ASCII and binary variants are supported.
Files with the .GRD extension are Surfer grid files. To preserve faulting information and to use double precision values, be sure to save the grid in the Surfer 7 format. If you need to use a grid in Surfer 5 or Surfer 6, save the grid as GS ASCII .GRD or GS Binary .GRD. Keep in mind that these two formats do not retain fault information or double precision values.
Surfer 7 Grid File Format
Surfer 6 Grid File Format
ASCII Grid Files
File Format Chart