Analysis of volcanic activity patterns using MODIS thermal alerts

Abstract

We investigate eruptive activity by analysis of thermal-alert data from the MODIS (moderate resolution imaging spectrometer) thermal infrared satellite instrument, detected by the MODVOLC (MODIS Volcano alert) algorithm. These data are openly available on the Internet, and easy to use. We show how such data can plug major gaps in the conventional monitoring record of volcanoes in an otherwise generally poorly documented region (Melanesia), including: characterising the mechanism of lava effusion at Pago; demonstrating an earlier-than-realised onset of lava effusion at Lopevi; extending the known period of lava lake activity at Ambrym; and confirming ongoing activity at Bagana, Langila and Tinakula. We also add to the record of activity even at some generally better-monitored volcanoes in Indonesia, but point out that care must be taken to recognise and exclude fires.

Appendix: Transfer of MODVOLC data from Web site to spreadsheets and further processing

We extracted MODVOLC data for each target volcano from the MODIS Web site (http://modis.higp.hawaii.edu/). By using the option Summarize by month and then downloading the related Text alert file (.txt format) we obtained the data for one entire month are displayed in rows. Each row contains the information for a single alert pixel. The information consists of the time of the satellite’s overpass, the latitude and longitude of the centre of the alert-pixel, the detected radiance in five MODIS channels, the satellite zenith angle and azimuth relative to the alert-pixel cent, the solar zenith angle at the alert-pixel centre, and the value of the alert ratio.

By saving the Text alert file in an Excel spreadsheet we were able to order and analyze the data by column. Sorting by latitude and longitude in turn made it convenient for us to delete all the alert-pixels falling beyond the area of interest (usually a 10-km box around the summit). We then added each month’s spatially filtered data to a master spreadsheet for the volcano, containing all that volcano’s alerts for the entire reporting period. From this, we generated three different worksheets: one for analysis of the NTI or alert ratio (ratio); one for the number of alert pixels (n_alert); and one for the total 4.0-µm spectral radiance (radiance). The time-series graphs presented in this paper were generated from these.

In the case of daytime data, the 4.0-µm thermal signal is always contaminated by reflected sunlight, so we applied the same empirical correction used by the MODIS team to generate daytime NTI (R. Wright 2003, personal communication). We identified as daytime all those data acquisitions for which the solar zenith angle was <90° and for these we made a solar correction to the MODIS band 21 (4.0 µm) spectral radiance by subtracting from it 4.26% of the MODIS band 6 (1.6 µm) spectral radiance.

In the ratio worksheet, we ordered the data by date and time (for this we generated an extra data column, called date-time, containing year, month, day, hour, and minute of the satellite’s overpass) and we plotted the value of alert ratio versus date-time in a graph covering all the monitored period.

To analyse the number of alert pixels and the total spectral radiance recorded by each satellite overpass, it was necessary to distinguish groups of pixels sharing the same date and time (date would be insufficient at high latitudes, where there may be data from several overpasses on a single date). We achieved this using a function in Excel called “subtotals” (Menu Bar – Data). For each change in the column date-time, we used the formula “count the cells with the same value in the date-time column” (to count the number of pixels) and “sum the value of 4.0-µm radiance of the cells with the same value in date-time column” (to calculate the total spectral radiance at 4.0 µm). We thus obtained (for each satellite overpass) the number of alert-pixels and the total (solar-corrected) spectral radiance at 4.0 m. We used the latitudes and longitudes of alert-pixel centres in the spreadsheets to plot the spatial distribution of the alerts, which we used as overlays on maps of each volcano.