April 23, 1990: In hopes of detecting and tracking frozen mud flows as they moved down the Drift River scientists installed 3 seismometers sensitive to ground vibration at relatively high frequencies along the edge of the valley. The sensors are called acoustic-flow monitors (AFM) to distinguish them from traditional seismometers. The AFM's were installed at increasing distances from the volcano – the nearest station at the base of the volcano was installed about 5.5 miles from the dome.