There are several types of remote sensing techniques used to study volcanic activity, including[1][2]:

1. Ground surface deformation and topographic change mapping: This technique involves using satellite data to measure changes in the shape of the volcano’s surface, which can indicate the movement of magma and the potential for an eruption.
2. Earthquake analysis: Seismic data can be used to detect and analyze earthquakes, which can indicate the movement of magma and the potential for an eruption.
3. Thermal anomaly mapping: Thermal remote sensing using infrared or thermal infrared can detect thermal anomalies in high spatial resolution, which can indicate the presence of magma or changes in volcanic activity.
4. Gas and ash detection: Remote sensing can be used to detect, measure, and track volcanic gases and ash from eruption plumes and clouds, which can provide valuable data for forecasting volcanic activity.
5. Optical and radar imaging: Optical and radar images can be used to observe changes in surface morphology, detect ground deformation, and monitor the growth and collapse of volcanic edifices[3].

These techniques, when used in combination, can provide valuable data for understanding and forecasting volcanic activity. However, remote sensing has limitations, including limited real-time data, inaccessibility to remote areas, and the inability to detect all precursory signals[4][5].