Anatomy of an Observation Well
Each of the observation wells is equipped with an electronic data recorder that is essentially a dedicated computer. It is programmed to activate every 30 minutes, measure the depth to water in the well, record that value in its memory, and then wait until it is time to collect the next data point. In most of the wells, the depth to water is measured using a digital encoder and float. The float rests on the water surface. A flat stainless steel tape goes from the float to a pulley on the encoder, then back into the well. Pins on the encoder pulley mate with holes in the stainless steel tape to prevent the tape from slipping on the pulley. A counterweight holds tension on the tape. The float moves up and down as the water level changes, causing the pulley on the digital encoder to rotate. A sensor in the encoder measures the amount of movement that occurs and sends this information to the recorder when the recorder is ready for a water-level measurement. The observation well installations are powered by 12-volt batteries that are recharged from photovoltaic panels.
A few of the wells use a pressure transducer rather than the float-digital encoder combination to measure the water level. A pressure transducer is a pressure-sensitive instrument that is suspended below water level. It measures water level by measuring the pressure change that occurs as the height of water above the transducer increases and decreases. Each foot of water level change increases or decreases the pressure exerted on the transducer membrane 0.433 pounds per square inch. The information is send to the data recorder electronically via cable.
The data recorders contain a low-power (about 7 watts) ultrahigh frequency (uhf) radio transmitter that sends the data stored in the recorder to one of two GOES (Global Operational Environmental Satellite) satellites that are in geosynchronous orbit some 26,000 miles from Earth above the equator. The data are re-transmitted to Earth and received at a station operated by the U.S. Geological Survey in Little Rock Arkansas, and then into the ADAPS computer data storage system. The entire process from data leaving the observation well to being input into ADAPS requires only a few seconds. Weather disturbances and other factors can sometimes interfere with data transmissions. By sending the last eight hours of data every four hours, there is redundancy in the system that helps decrease the amount of lost data due to transmission errors.