Activity at the Soufrière Hills Volcano is characterized by explosive activity associated with the growth and destruction of andesitic lava domes. Monitoring changes to the lava dome and the fumaroles and mapping the deposits from the activity is the responsibility of the Dome Volume and Geology program and involves the application of a wide range of tools and techniques including:

• Visual observations and photography from the ground and from a helicopter;

• Monitoring the activity with remote digital cameras;

• Use of remote sensing imagery, photogrammetry and field mapping for deposit mapping.

Visual Observations

Visual observations of the volcano are extremely important, both for monitoring escalating volcanic events as they develop and for tracking change to the lava dome over the long term.

One of the key techniques is the ability to make observations from regular helicopter flights around the volcano. During these observation flights, phenomena such as dome growth/lava extrusion, rockfall and pyroclastic flow activity, ash venting, and changes to the behaviour and distribution of fumaroles are recorded with digital photography.

As a result, MVO has an extensive archive of photographs captured over the years since the eruption began in 1995, which serves both as an amazing visual record of the eruption and a valuable scientific resource.

Remote Cameras

Remote cameras are one of MVO’s most important tools for monitoring the Soufrière Hills Volcano. MVO currently uses dual sensor cameras from Mobotix that have a thermal and a night-time sensor that is sensitive to low light levels. The combination of the two sensors allow MVO to capture footage of the volcano 24 hours day, and permits for rapid interpretation of the imagery.

In addition to capturing still images, the cameras continuously record video to an internal memory card at 1 frame per second, with up to 10 days-worth of video being kept on the camera. This means that MVO staff can review the video for any signs of activity, such as rockfalls or changes in fumarole activity, that may be associated with other activity, e.g., increases in seismicity.

The two images shown here are pairs of still images taken from two of MVO’s cameras, showing the image from the night-time sensor on the left and the thermal sensor on the right. In both cases, the nigh-time sensor produces a greyscale image during the day time, and fumaroles (shown as hotspots on the thermal image) on the lava dome have been highlighted.

The MVOT images show the view from the observatory, while the FERG images show the view of the southwest side of the lava dome from Fergus mountain.

Thermal Imagery

Thermal images reveal features and processes not seen by normal digital cameras or are even visible to the naked eye. Thermal images of a lava dome can highlight the active areas, and map the distribution of fumaroles, vents and fractures all while recording temperature.

The Mobotix cameras include a thermal sensor capable of recording still and video imagery allowing us to capture the onset of events, such as explosions, that might otherwise be missed, particularly at night.

In addition, MVO also uses a handheld FLIR T650sc thermal IR camera during observation flights, to acquire detailed images of the lava dome and measure the temperature of numerous high-temperature fumaroles and other thermal features on and around the lava dome that are impossible to access directly.

In-situ Fumarole Monitoring

Fumaroles are features that emit gases such as carbon dioxide (CO₂), sulphur dioxide (SO₂) and hydrogen sulphide (H₂S) as well as steam at temperatures that vary from less than 100 °C to more than 600 °C.

Monitoring the temperature of fumaroles can provide some early warning as precursory activity to new or renewed activity at the Soufrière Hills volcano and also offer some insight into any shallow hydrothermal system processes with the volcanic edifice.

At MVO, in-situ temperature monitoring is carried out at a few low-temperature fumaroles using temperature probes buried to a depth of at least 40 cm. A data logger records the temperature every 10 minutes and the data is averaged on an hourly and daily basis to identify long-term trends.

Aerial Photogrammetry

Photogrammetry is the process of using photographs to measure distances between objects and generate 3D surfaces. At MVO, we use aerial photogrammetry to map and measure the lava dome and deposits from activity on the surrounding flanks of the volcano.

Our aerial photogrammetry workflow consists of collecting images with a Sony a6000 mirrorless camera with a 20 mm lens. This is mounted to the underside of a helicopter in a custom housing and captures an image every one second during survey flights to maximise area coverage and image overlap. For mapping smaller areas at higher resolutions, a DJI Phantom 4 Pro sUAV or “drone” is used.

The images and associated GPS data are combined in the structure-from-motion software Agisoft Metashape Professional to produce both orthophoto mosaics and 3D digital surface models. These can then be used for mapping and volume calculations.

Deposit Mapping

Mapping the products from the activity of the Soufrière Hills volcano is a key component of the work of volcanologists at MVO. As soon as it is safe to do so, staff go into the field to map the distribution of deposits and collect samples. Usually, a wide variety of deposits are identified in the field, including pumice fall; ashfall; block-and-ash flow deposits; pumice flow deposits; ballistics and lahar deposits, depending on the activity.

Small samples from the deposits are collected and analysed for grain size distributions and componentry. Some samples are sent to labs in the UK and USA for geochemical analysis. All this helps to understand the conditions in the volcanic plumbing system or lava dome prior to the event, and also to understanding the behaviour and hazards of pyroclastic flows, dome growth and explosions.

Ash Sampling

Ash is generated during explosive events and dome growth. Sampling is important, particularly during the early stages of an eruption when it is important to determine how much of the ash is juvenile lava or older, non-juvenile lava is and thus whether there is magma close to the surface that may soon erupt as lava.

Ash can be collected from any flat surface, or in buckets, trays or other containers that have been left out for the specific purpose of collecting ash. Ash collection occurs as soon as possible after an explosion or venting episode, to ensure that the ash is fresh and not contaminated or washed away by rain or blown by wind.

Parameters such as the amount by volume or weight per unit area, and thickness of ash are collected, depending on whether the ash is on a flat surface or in a container. When collecting from flat surfaces, areas of 1 m2 are collected. Determining the thickness and amount of ash per unit area from many sites allows scientists to determine the total volume of ash erupted in a given event.

Satellite Imagery

MVO makes occasional use of satellite imagery to identify and track changes to the lava dome and surrounding flanks of the volcano. Typical sources of imagery used by MVO includes ASTER, Landsat, Sentinel-2 multispectral and thermal imagery and radar data from the TerraSAR-X and Sentinel-1 satellites.