USGS scientists create new method for detecting lahars
Until recently, most warnings of a lahar rushing down a valley toward a town or city were based on visual sightings either from video cameras or from observers located along the valley. Maintaining video cameras in remote valleys and in extreme weather conditions is very costly, and the incoming images have to be watched continuously by a person. Relying on many observers in several valleys for extended periods of time is extremely time consuming, and sometimes the warnings come too late or not at all. A new method developed by a USGS scientist for detecting lahars has proved to be very reliable, durable, and inexpensive. It is now used downstream of several volcanoes in the United States, Indonesia, Philippines, Ecuador, Mexico, and Japan.
How does the lahar-detection system work?
The new automated detection system relies on a series of acoustic-flow monitor (AFM) stations installed downstream from a volcano. Each station consists of a seismometer (sensor in illustration) that senses ground vibrations from an approaching and passing lahar and a microprocessor that analyzes the signal. A radio at the station sends and receives information from a base station, usually a volcano observatory. A battery and solar panel powers the system.
Every second, the microprocessor samples the amplitude of the vibration detected by the seismometer. At regular intervals (usually 30 minutes) data is sent to the base station -- an emergency message is sent any time the vibrations exceed a programmed threshold for longer than 40 seconds. The microprocessor continues to send alert data every minute for as long as the amplitude stays above the threshold level.
Ground-vibration response of an acoustic-flow monitor
An acoustic-flow monitor (AFM) is a seismometer sensitive to ground vibration with higher frequencies than a typical seismometer used for recording earthquakes and volcano activity. An AFM has a frequency response of 10-250 Hz whereas a typical seismometer has a frequency response of <2 Hz. Ground vibration generated by lahars (debris flows) is predominantly in the frequency range of 30-80 Hz, whereas vibrations generated by earthquakes, volcano tremors, and an explosive eruptions are predominantly <6 Hz. This difference in frequency response allows scientists to distinguish between lahars and other natural ground-vibrating events.