Hydroacoustic stations can of seismometers to measure T-waves from possible underwater explosions instead of hydrophones. These tremors were discovered using deep borehole seismometers that avoid surface noise. Pendulums are also used in scientific instruments such as accelerometers and seismometers. Earthquakes are measured using observations from seismometers. The best success has come with detailed micro-seismic monitoring, involving dense arrays of seismometers. Long-period pendulums enable seismometers to sense the slowest waves from earthquakes. In reflection seismology, an array of seismometers image sub-surface features. USGS seismometers and GPS receivers operated by UNAVCO form the monitoring network for Mount Shasta. Various outbuildings support instrumentation such as ionosondes, seismometers and upper atmospheric experiments. Routinely, waves that break can be recorded on seismometers. Geophones however are much cheaper than seismometers. Delivered seismometers come with two styles of output&colon; analog and digital. Some drum seismometers are still found, especially when used for public display. Information about moonquakes comes from seismometers placed on the Moon from 1969 through 1972.