**Stefan Bredemeyer 1,\*, Franz-Georg Ulmer 2, Thor H. Hansteen <sup>1</sup> and Thomas R. Walter <sup>3</sup>**


Received: 28 August 2018; Accepted: 18 September 2018; Published: 21 September 2018

**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 (H2O) 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 (SO2) emission measurements from a scanning UV spectrometer (Mini-DOAS) station installed at Láscar volcano, which were scaled by H2O/SO2 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.

**Keywords:** gas emission monitoring; X-band InSAR; scanning Mini-DOAS; Multi-GAS; volcanic gases; precipitable water vapor; radar path delay; Láscar volcano

#### **1. Introduction**

Volcano monitoring is effectively based on multi-parametric datasets, often characterizing surface deformation, degassing, seismicity and temperature changes in time and space. However, even though comparison of independent datasets is common, they are only rarely integrated into a common evaluation scheme to enhance the relative strengths of the individual applied methods. Seismic and deformation data for instance are increasingly being integrated to improve spatio-temporal localization of magma movement beneath a volcano, which enables to better constrain the geometries of magma pathways and reservoirs, and to derive more precise physical models of the associated processes [1,2]. Gas emission and deformation data, however, have not yet been deeply considered as integrated information, though it has been shown that variations in degassing and deformation are often intimately coupled [3]. A combined analysis of time series measurements of volcanic degassing and deformation rates proved to be a powerful tool for the evaluation of volumetric changes in magma reservoirs due to the accumulation or discharge of volcanic gases (e.g., [4,5]). Dynamics of degassing are furthermore critical for understanding pressure fluctuations in a magmatic and hydrothermal system, which additionally may manifest as temporary deformation of the volcanic edifice [6–8]. However, these datasets are commonly considered separately, and the fate of gas emissions downwind of the emission source is typically not taken into account, which is why the contribution of gas emissions on the radar signal used for deformation measurements has not been comprehensively demonstrated. Differential interferometric synthetic aperture radar (DInSAR) measurements allow for the detection of mm-scale line of sight (LOS) displacements of the observed surface [9]. It has been well demonstrated that the accuracy of satellite radar ground deformation measurements is affected by changes in atmospheric refractivity, in particular by changes which can be attributed to the highly variable distribution of water vapor in the observed atmospheric column [10,11].

Atmospheric contributions to DInSAR data often have similar magnitudes and wavelengths as the actual ground deformation signal, and thus they need to be removed from interferograms, when deformation measurements are the purpose of monitoring. There have been two main strategies for this, (i) time-space-based filtering, and (ii) modeling of atmospheric contribution [12]. In particular, the latter is highly successful, as interfering contributions from a moist atmosphere can coarsely be predicted and compensated by means of high-resolution numerical weather models (e.g., MM5, and WRF), which provide the information required to predict the atmospheric phase delay in the LOS of an interferometric measurement [13–16].

Degassing volcanoes; however, produce their own atmospheric disturbances, which are typically not well captured by weather models [17,18]. The large and variable amounts of water vapor in volcanic plumes may cause differential phase errors in interferometric measurements due to reduction of radar propagation velocity within the plume above and downwind of the volcano [19–22].

The leeward sector of a degassing volcano is hence prone to be affected by pronounced differential phase signatures, which can be misinterpreted as a deforming ground surface, and may thus obscure the real ground deformation information of an interferogram. Rosen et al. [19] described the possible consequences that such propagation delays would have on DInSAR measurements using the examples of phase signatures that occurred on Kilauea volcano, Hawaii. Similarly, Wadge et al. [20] observed enhanced tropospheric delays in radar data of Soufrière Hills. Instead of using weather models for the correction of their DInSAR data, these authors exploited data from Global Positioning System (GPS) measurements on the leeward side of the volcano. Based on local SO2 flux and gas compositional measurements, and assuming a homogeneous distribution of water vapor inside the volcanic gas plume, they however obtained an estimate of the gas plume related radar delay of only about 0.05 mm, which is far below, and thus negligible with respect to InSAR resolution. Gas plume related disturbances are nevertheless also identified elsewhere in interferograms of degassing volcanoes (e.g., at Pico do Fogo as described in González et al. [21]), but remained to be quantitatively demonstrated and validated for a selected showcase.

To reach this aim, the precipitable water vapor (PWV) content in the plume at the time of SAR acquisitions has to be determined. PWV is the amount of water vapor (in kg·m−<sup>2</sup> or mm) vertically integrated in an atmospheric column. Space borne radar interferometric delay measurements in principle can be used to map the spatial distribution of water vapor in the atmosphere [23–25], if the contributions from topographic phase and ground deformation are known, or when independent PWV estimates are used as a reference. Accordingly, those measurements also enable to map the spatial extent of a volcanic gas plume when the strength of the degassing source is known, and independent PWV estimates are integrated into analysis.

Even though water vapor by far is the largest and most abundant component in the gas emissions of most volcanoes, volcanic water vapor emissions to date only rarely have been measured and quantified by means of remote sensing (and preferentially by means of indirect methods as e.g., in [26]), due to the missing contrast relative to the high water vapor concentrations in the ambient atmosphere, which commonly preclude its quantification in the volcanic cloud. During the present decade, however, some progress has been made in this respect, and volcanologists have adopted and adjusted several technologies known from meteorological applications to remotely map and quantify volcanic water vapor emissions despite a variable background atmosphere. Fiorani et al. [27] for instance, successfully measured water vapor fluxes of Stromboli volcano in Italy using a ground-based LiDAR system, and Bryan et al. [28] introduced a combined millimeter-wave radar/radiometer system (also ground-based), which enables 3-dimensional mapping of the spatial distribution of ash and water vapor in eruptive clouds. These methods have in common that they use active sensors, which transmit a signal that is able to penetrate the volcanic cloud and thus allow measuring the reflection, refraction, or scattering of the signal occurring inside the cloud. Such methods thus are largely superior to methods deploying passive optical imaging sensors (such as UV- and IR-spectroscopical methods), which typically are strongly affected by the presence of liquid water and aerosols [29], and therefore fail to work in very dense clouds. Ground-based active sensors are, however, rather exotic, and are cost- and labor-intensive, and thus have not yet routinely been used for permanent gas emission monitoring purposes.

We thus used a combination of more common sensors, which enable unsupervised continuous monitoring. Because direct measurements of volcanic water vapor emissions by means of optical remote sensing are challenging [30], we measured SO2 column density profiles in conjunction with molar H2O/SO2 ratios in this work, in order to estimate the apparent PWV contents in the gas plume above a volcano. These estimates were integrated as an a-priori knowledge into the analysis of DInSAR observations in order to investigate the propagation delay that was generated by volcanic gas emissions. We demonstrate the repeated contribution of the volcanic gas plume to the radar delay in the interferometric measurements and thus determine the plume-induced delay variations which can be falsely interpreted as a deformation signal.

### **2. Study Area**

The study was conducted at Láscar volcano, one of the most active volcanoes of the central Andes [31,32], which has been identified as the second largest emission source of volcanic gases in Northern Chile [33]. The volcano is located east of the Salar de Atacama basin (23.37◦S, 67.73◦W), on the western margin of the Puna plateau (Figure 1a), which is one of the driest areas on Earth. Much of the plateau around the 5592 m high Láscar volcano has an altitude of more than 4 km above sea level. Background atmospheric PWV over the region is very low most of the year, and generally less than 1 mm total water column [34]. Cloudless skies, an extremely low atmospheric water content, and the high altitude lead to an exceptionally high atmospheric transparency [35], which in general is conducive for remote sensing applications and hence for the purpose of our study.

The volcanic edifice of Láscar comprises two truncated intersecting composite cones, termed Western and Eastern edifice hereafter, and hosts 5 nested craters, which are aligned along an ENE–WSW trending lineament [31,36,37]. During the past two decades Láscar exhibited several periods of cyclic

dome growth and collapses, which were punctuated by occasional explosive vulcanian to plinian eruptions [38]. Recent activity is characterized by gradual subsidence [39,40] and persistently strong fumarolic degassing from the remnants of a dacitic dome [41] located in the westernmost crater of the Eastern edifice (Figure 1b,c). The dome remnants cover an area of about 40,000 m2 (about 230 m in diameter) in the central depression of the 800 m wide and 400 m deep crater. Numerous high temperature fumaroles (about 300 ◦C) are distributed on top of the dome and the surrounding inner crater walls [41–43]. Several low temperature fumaroles and diffuse vent sites are located in the eastern craters of the Eastern edifice and on the southern crater rim, but the high temperature gases ascending from the dome and the fumaroles in the active crater are the main emission source [41]. These hot gas emissions produce a persistent thermal anomaly in the middle of the crater, which has been subject of numerous remote sensing studies using infrared satellite imagery (e.g., [44–48]). Gas emissions from Láscar volcano are very moist in contrast to the dry atmospheric background. Water vapor emissions from Láscar account for more than 90% of its total gas emissions, approaching rates of several metric kilotons per day [33]. The persistently degassing, and largely unvegetated stratovolcano (Figure 1d) thus provides ideal conditions to examine radar propagation delays caused by refractivity variations in volcanic gas.

**Figure 1.** Láscar and adjacent Aguas Calientes volcanoes. The location of the scanning Mini-DOAS station is indicated by the *white dot* on the southern flank of Aguas Calientes volcano. (**a**) Aster visible range composite of 29 April 2013 draped onto SRTM-1 grid; (**b**) Close-up of high temperature fumaroles in the active crater; (**c**) View from the southern crater rim towards NNW into the active crater of Láscar showing fumarolic activity during the Multi-GAS survey on the Southern crater rim on 2 December 2012. The *white bounding box* framing the area shown in (**b**) roughly spans 200 m in width and 100 m in height; (**d**) Panoramic view from South towards NNW showing a dispersed gas plume emanating from Láscars' active crater on 5 December 2012, and drifting towards SE, which corresponds to the main transport direction during daylight time. Distance between Mini-DOAS in the foreground and active crater in the background of the image is about 6 km.

#### **3. Data and Methods**

In this section, we will introduce a method that enables us to isolate and map gas plume related phase delay contributions in interferometric radar measurements by means of correlating a-priori information on the temporal variations of PWV contents in the volcanic gas plume with the signal strength variations at each range azimuth position of a DInSAR time series. Application of the method further requires involving several different prior constraints encompassing the temporal variations of all other processes that potentially contribute to total DInSAR phase in order to derive an estimate of the gas plume related interferometric signal in the DInSAR time series. In the following we will specify which data were used for our case study spanning the period October 2013 to February 2014 (later often simply referred to as the considered period), present the techniques used to derive the necessary a-priori knowledge, and describe how the estimation technique works.
