Cross-section construction and balancing: examples from the Spanish Pyrenees

Cross-sections are used by geologists to represent the structure of the Earth’s subsurface. To construct a cross-section, observations from the Earth’s surface and other available information are projected onto an underground vertical plane to interpret how data relate to each other. The main challenge is transforming the scattered data into a reasonable geological geometry. Interpretations are not unique, so geologists need validation rules to assess their results. A cross-section is considered valid if: (1) it fits the available data and geological knowledge (admissible cross-section); (2) it is retro-deformable (restored cross-section); and (3) the volume of material between the deformed and the restored state remains constant (balanced cross-section). This chapter explains the general methodology to construct a cross-section from a geological map and the standard techniques to balance it (bed length, area, and area-excess). For each method, a practical problem is presented. Problems are real examples from the Spanish Pyrenees.