8Roy A, Apparao A. Depth of investigation in direct current methods. Geophysics, 1971,36(5):943-959.
9Bhattacharya B B, Sen M K. Depth of investigation of collinear electrode arrays over homogeneous anisotropic halfspace in direct current methods. Geophysics, 1981,46 ( 5 ) : 768-780.
10Barker R D. Depth of investigation of collinear symmetrical four-electrode arrays. Geophysics, 1989, 54(8) : 1031-1037.