(bās’ m∂nt) The level below which changes in rock properties do not affect observations significantly, or the lowest level of interest. Different types of basement may not coincide. 1. Geologic basement is the surface beneath which sedimentary rocks are not found; the igneous, metamorphic, granitized, or highly folded rock underlying sedimentary rocks. 2. Petroleum economic basement is the surface below which there is no current exploration interest, even though some sedimentary units may lie deeper. 3. Magnetic basement is the upper surface of igneous or metamorphic rocks whose magnetization is so much larger than that of sedimentary rocks that their effects get lost in the noise. 4. Electrical basement is the surface below which resistivity is very high so that variations below this surface do not affect electrical-survey results significantly. 5. Acoustic basement is the deepest more-or-less continuous reflection. 6. Gravity or density basement is where a very large density contrast exists so that anomalies resulting from deeper contrasts are lost in the noise. 7. Hydrologic basement is the deepest point where significant porosity exists.