MAGNETIC EXPLORATION FOR WATER IN BASALTIC TERRAIN

Basaltic terrain is ideal for magnetic exploration. Basalt generally has high remanence as well as high magnetic susceptibility. Strong magnetization contrasts exist between basalt and other materials present such as soil or sedimentary rock. Commonly, different flows, having erupted at different geologic times, have different directions and intensities of magnetization. Flow contacts and feeder dikes are then easily detectable with a magnetometer. Faults in basalt generally produce magnetic anomalies because of the offset of flows as well as the modification of the magnetite grains due to frictional heating, grinding, and hydrothermal alteration. The high magnetic susceptibility of coarse sediment clasts eroded from basalt can result in fluvial and landslide deposits with anomalous magnetization. Anomalous magnetization is extremely useful for locating potential aquifers in the Columbia Plateau of the northwestern United States where groundwater flow is controlled primarily by flow morphology and by volcanic, tectonic, and sedimentary features that crosscut lava flows. Buried basalt flow tops are frequently aquifers containing abundant water. Columnar jointing in flows may also affect groundwater flow. Aquifers may also be found at the contact between basalt and older, underlying crystalline rocks, or in buried stream channels incised into basalt flows. Groundwater flow through aquifers is frequently influenced by the location of faults, which act as channels for flow. Groundwater is also affected by the presence of feeder dikes for younger flows that crosscut the older basalts and act as effective barriers to flow.