Mining Publication: Methods of Determining the Orientations of Bedrock Fracture Systems in Southwestern Pennsylvania and Northern West Virginia
This Bureau of Mines report describes photo-lineaments obtained from stereoscopic examination of vertical aerial photographs and by the ronchi grating study of aerial photoindex sheets. The photo- lineaments are compared with the trends of oriented fracture systems procured by measuring the compass direction of joints in bedrock. The relative reliability of the methods is shown, and the results are presented in graphical, tabular, and written form. Analyses of field measurements of strikes of bedrock joints in the project area indicate for the most part two intersecting sets of major joints oriented n 76 deg w = n 15 deg e, and n 57 deg w = n 27 deg e. The analyses of lineaments determined by stereoscopic examination of aerial photographs show dominant trends of lineament zones of weakness to be an average of n 65 deg w = n 20 deg e. Analysis of lineaments found by ronchi grid viewing of aerial photoindex sheets indicate dominant trends of n 70 deg w = n 27 deg e. The three methods give similar findings and verify the work of previous authors. The ronchi grid method is a useful guide, but it cannot be substituted for the two more painstaking methods.