Photo Caption Conducting an audio-magnetotelluric (AMT) survey in the southern San Pedro River basin in northern Sonora State, Mexico. This kind of survey allows geophysicists to acquire a shallow vertical sounding - a virtual image of earth conductivity - beneath the measurement station. Depth of penetration can be as much as a kilometer. In the background is the Sierra an José, a mountain never previously geologically mapped. This work is part of a two-nation study of the ground water that supplies the San Pedro National Riparian Conservation Area in southeastern Arizona. Water flow in the San Pedro River is critical to the continued viability of a major component of one of four major North American migratory bird fly-ways. =Jeff Wynn, USGS