Scientists monitor phreatic eruptions using various techniques, including[1][2][3][4][5]:

1. High-frequency gas monitoring: This method involves the use of instruments to monitor gas emissions, such as water vapor, carbon dioxide, and sulfur dioxide, which can provide valuable data for forecasting phreatic eruptions.
2. Seismic monitoring: Seismometers are used to detect and analyze seismic activity, including small earthquakes, which can be indicative of potential phreatic eruptions.
3. Deformation monitoring: This technique involves measuring changes in the shape of the volcano, which can indicate pressure build-up and potential phreatic activity.
4. Gas composition monitoring: Scientists monitor changes in the composition of volcanic gases, such as water vapor, carbon dioxide, and sulfur dioxide, to track degassing and detect potential phreatic eruptions.

These monitoring techniques, when used in combination, can provide valuable data for understanding and forecasting phreatic eruptions, although the lack of clear precursors for some phreatic eruptions remains a challenge.