Sensitivity of Rayleigh waves to saturation distribution and water table depth within hydrothermal systems

CASTROMÁN, GABRIEL A.; ROSAS-CARBAJAL, MARINA ; ZYSERMAN, FABIO I.; BARBOSA, NICOLÁS D.

Congreso; XXIX Reunión Científica de la Asociación Argentina de Geofísicos y Geodestas (AAGG2021); 2021

Hydrothermal systems exert an important control on fluid flow and heat transfer in volcanoes,which, in turn, affects the analysis of their activity and hinders the detection of eruptionprecursors. In particular, hazardous volcanic activity such as, for example, phreatic eruptions,partial or total collapse of the volcanic edifice, spontaneous release of toxic gases and lateralexplosions, can be directly related to the circulation, change of state, and pressurization of thepore fluids. Given their difficult access conditions and complex dynamics, the study of volcanoesrequires combining observations at specific points on the surface (e.g., temperature and gascomposition) with methods that allow the remote monitoring of their shallower part. In this context,geophysical methods based on active sources that can estimate the distribution of fluids in thesubsurface entail acquisition times and operational costs that are large and sometimesdangerous. Seeking to overcome these limitations, in recent years a very promising techniquebased on passive sources has shown a great potential for monitoring volcanic hydrothermalactivity: the seismic noise interferometry.Seismic interferometry is based on the correlation of seismic traces recorded at differentlocations to determine the impulse response of the medium between them. Continuousmonitoring of this response allows observing relative variations in the velocity of the seismicwavefield, which can be associated with changes in the properties of the medium and, thus,contribute to its characterization. In particular, seismic noise interferometry is a passive techniquethat uses the recordings of small random and continuous vibrations characteristic of theobservation site, the so-called ambient seismic noise. Given that this signal consists mainly ofsurface waves, most of the studies regarding the use of ambient noise interferometry employ thecharacteristics of these seismic waves in general and of Rayleigh waves in particular. The relativeseismic velocity variation typically exhibits an inverse correlation with the water table depth inhydrothermal systems, and it is often interpreted considering relatively simple models in whichhomogeneous body wave velocities are assumed for the unsaturated zone of the system.However, it is expected that the water content within the unsaturated zone will not be uniformlydistributed. This, in turn, may have some impact on the computation of the surface wavevelocities.In this work, we explore the sensitivity of surface wave velocities to the spatial distribution ofwater saturation considering different saturation profiles and water table depths withinhydrothermal systems. To do so, we consider a geological model representative of the dome ofLa Soufrière de Guadeloupe volcano. The model consists of a 200 m thick layer composed by ahighly altered andesite overlaying a half-space made of a relatively non-altered andesiterepresenting the volcanic basement (Figure 1a). The corresponding elastic properties were takenfrom laboratory measurements in samples from this volcano. The water table lies within thealtered layer and, based on previously reported observations, it is assumed to vary its depthbetween 75 m and 125 m. Considering this model, we compute the phase and group velocitydispersion of the fundamental mode of Rayleigh waves as a function of the degree anddistribution of water saturation. This is done assuming that the system is fully saturated with aneffective fluid mixture whose properties are determined by the harmonic average of thecorresponding properties of each constituent phase, which in this study are water and air. Above the water table, we consider different spatial distributions of water saturation, namely a constantvalue and an S-shaped profile obtained through a Van Genuchten model [1]. For each saturationscenario, the body wave velocities of each layer are computed according to Gassmann’sequations [2]. Once the Rayleigh wave dispersion relations are computed, relative velocityvariations are obtained for each saturation profile by comparing the velocities for each water tableposition with respect to a reference scenario in which the water table depth is 100 m.Within the frequency range usually employed in ambient seismic noise interferometry, weobserve that the relative velocity variation curves obtained for the different saturation profiles varysignificantly from each other. The differences observed between them not only have magnitudescomparable to the relative velocity variations that are normally associated with fluctuations of thewater table depth, but also strongly depend on the frequency range examined. Therefore, sincethe characteristics of the selected saturation profiles rely on certain parameters that depend onthe type of rock considered, their probable determination through the use of seismic data could bevery useful both for the hydraulic characterization of the medium, as well as for the differentiationbetween saturation profiles and the determination of the water content distribution in the system.The preliminary results of this work thus indicate that Rayleigh waves can be sensitive not only tothe water content in hydrothermal volcanic systems, but also to its spatial distribution which, inturn, shows the potential that the continuous monitoring of ambient seismic noise has for itsestimation. Future work will focus on combining seismic interferometry data with other types ofgeophysical data such as muon radiography. The latter will allow us to constrain densityvariations and, thus, to reduce the uncertainty that may arise in the computation of the spatialdistribution of fluids and elastic moduli. Finally, we conclude that to determine the depth of thewater table within hydrothermal systems by means of seismic noise interferometry, care must betaken if a constant velocity is assumed for their unsaturated zone, which is the most commonlyused procedure for this kind of studies.