Generally any seismic receiver device could be used as a seismometer, but typically seismometer refers specifically to a broadband earthquake monitoring receiver.
Most seismometers are precision instruments not used in exploration seismology because they are usually quite bulky and require careful set-up and calibration. They are accelerometers, much like relative gravimeters, measuring ground acceleration from the force on a proof mass. Seismometers can detect frequencies in a very broad band, on the order of 0.001 Hz to 500 Hz: that's 19 octaves!
- Geophysical stamps 3: Geophone — blog post