Radar Path Delay Effects in Volcanic Gas Plumes: The Case of Láscar Volcano, Northern Chile

Abstract

Modern volcano monitoring commonly involves Interferometric Synthetic Aperture Radar (InSAR) measurements to identify ground motions caused by volcanic activity. However, InSAR is largely affected by changes in atmospheric refractivity, in particular by changes which can be attributed to the distribution of water (H 2 O) vapor in the atmospheric column. Gas emissions from continuously degassing volcanoes contain abundant water vapor and thus produce variations in the atmospheric water vapor content above and downwind of the volcano, which are notably well captured by short-wavelength X-band SAR systems. These variations may in turn cause differential phase errors in volcano deformation estimates due to excess radar path delay effects within the volcanic gas plume. Inversely, if these radar path delay effects are better understood, they may be even used for monitoring degassing activity, by means of the precipitable water vapor (PWV) content in the plume at the time of SAR acquisitions, which may provide essential information on gas plume dispersion and the state of volcanic and hydrothermal activity. In this work we investigate the radar path delays that were generated by water vapor contained in the volcanic gas plume of the persistently degassing Láscar volcano, which is located in the dry Atacama Desert of Northern Chile. We estimate water vapor contents based on sulfur dioxide (SO 2 ) emission measurements from a scanning UV spectrometer (Mini-DOAS) station installed at Láscar volcano, which were scaled by H 2 O/SO 2 molar mixing ratios obtained during a multi-component Gas Analyzer System (Multi-GAS) survey on the crater rim of the volcano. To calculate the water vapor content in the downwind portion of the plume, where an increase of water vapor is expected, we further applied a correction involving estimation of potential evaporation rates of water droplets governed by turbulent mixing of the condensed volcanic plume with the dry atmosphere. Based on these estimates we obtain daily average PWV contents inside the volcanic gas plume of 0.2-2.5 mm equivalent water column, which translates to a slant wet delay (SWD) in DInSAR data of 1.6-20 mm. We used these estimates in combination with our high resolution TerraSAR-X DInSAR observations at Láscar volcano, in order to demonstrate the occurrence of repeated atmospheric delay patterns that were generated by volcanic gas emissions. We show that gas plume related refractivity changes are significant and detectable in DInSAR measurements. Implications are two-fold: X-band satellite radar observations also contain information on the degassing state of a volcano, while deformation signals need to be interpreted with care, which has relevance for volcano observations at Láscar and for other sites worldwide.

Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, evidenced by multiparametric data

“…Wooster (2001) described the rapid cooling behavior of the Lascar dome as precursor activity following the 1993 eruption, while a recent study of the 2013 explosion utilized seismic wave interferometry to depict variations in the seismic velocity and consequently speculated about the preeruptive deformation of a magmatic/hydrothermal reservoir (González et al, 2016). A shallow hydrothermal system as depicted by magnetotelluric and seismic data (Díaz et al, 2012;Hellweg, 2000) influences the degassing activity of Lascar (Bredemeyer et al, 2018;Tassi et al, 2009). These degassing processes have been suggested to be the source of the tremors observed during the 1994-1995 period of unrest (Hellweg, 2000), and they are probably associated with the increase in long-period (LP) events preceding an eruption (González et al, 2016).…”

Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, monitored by multiparametric data

“…Matthews et al (1997) proposed a continuous deepening of the crater floor associated with a high rate of degassing from fumaroles within the active crater; this degassing phenomenon was subsequently identified by an InSAR investigation (Pavez et al, 2006) and further confirmed by continuous monitoring using the high-resolution German satellite TerraSAR-X (Richter et al, 2018), which also showed that the deformation rate appeared to be largely unaffected by the most recent explosive eruptions in 2013 and 2015. A shallow hydrothermal system depicted by magnetotelluric and seismic data (Díaz et al, 2012;Hellweg, 2000) seems to influence the active degassing at Lascar (Bredemeyer et al, 2018;Tassi et al, 2009). These degassing processes have been suggested to be the source of the tremors observed during the 1994-1995 period of unrest (Hellweg, 2000), and they are probably associated with the increase in longperiod (LP) events preceding an eruption (González et al, 2016).…”

“…This also favours the mechanism of degassing transporting more energy through hot fumarolic gases as a result of a higher magmatic gas flux into the atmosphere. The entire gas pulse accompanying the eruption expelled at least 170 metric tons of SO2 in only 100 minutes, which is approximately half the amount of SO2 that Lascar volcano usually emits throughout a whole day in a non-eruptive period (Tamburello et al, 2014;Bredemeyer et al, 2018). Using the molar ratios of Tamburello et al 2014, we additionally obtain equivalents of at least 5,440 metric tons of H2O and 230 metric tons of CO2 that were released during these 100 minutes.…”

“…Finally, most of the methods exposed in this study are broadly and successfully used to monitor volcanoes; for instance, drones are employed to map morphological structures (e.g., Darmawan et al, 2018a), mini-DOAS scanners are used to measure SO2 emission rates (e.g., Bredemeyer et al, 2018), satellite data are utilized to detect thermal anomalies (e.g., González et al, 2015), and in situ sensors are used to record CO2 concentrations and fumarole temperatures (e.g., Zimmer et al, 2017). In addition, the use of optical images from permanently installed cameras in this study was shown to be an alternative way to detect pre-eruptive changes through pixel brightness analysis and to estimate the variations in and the altitude of the eruptive plume using kymographic analysis, which will be very useful for forecasting steam-driven eruptions at Lascar and other volcanoes taking into account the limitations of this approach.…”

“…While SO2 fluxes were measured globally from the entire volcano and were largely dominated by gas contributions from the plume emitted mainly from the magmatic dome, the CO2 concentration was measured locally at a fumarole located along the crater periphery. The scrubbing of SO2 emissions is thus less likely to occur at Lascar since the gases emitted by the dome do not have to pass through the aquifers of the surrounding hydrothermal system (Bredemeyer et al, 2018), whereas the CO2 concentrations on the crater rim may indeed have been affected by water infiltration of the volcanic edifice. In the long term, anyway, both parameters (i.e., the SO2 and CO2 concentrations) did not display significant variations that deviated from the normal previously observed behaviour.…”