8 1 10 Manual on Subsurface Investigations AASHTO Primary Author mixed material America AASHTO 1988 WhatsApp_Image_2026-01-13_at_13.47.04.jpeg.jpeg ASTM D 6430 - 99 (Reapproved 2005) Standard guide for using the gravity method for subsurface investigation ASTM Primary Author mixed material ASTM 2006 ASTM D 6431 - 99 ( Reapproved 2005) Standard guide for using the direct current resistivity method for subsurface investigation ASTM Primary Author mixed material ASTM 2006 ASTM D 6432 - 99 (reapproved 2005) Standard guide for using the surface ground penetrating radar method for subsurface investigation ASTM Primary Author mixed material ASTM 2006 ASTM D 6639 - 01 Standard guide for using the frequency domain electromagnetic merthod for subsurface investigations ASTM Primary Author mixed material ASTM 2006 ASTM D 6820 - 02 Standard guide for use of the time domain electromagnetic method for subsurface investigation ASTM Primary Author mixed material ASTM 2006 ASTM D 7046 - 04 Standard guide for use of the metal detection method for subsurface investigation ASTM Primary Author mixed material ASTM 2006 ASTM D 7128 - 05 Standard guide for using the seismic reflection method for shallow subsurface investigation ASTM Primary Author mixed material ASTM 2006