Mud flow dynamics at gas seeps - Nirano Salse, Italy -

Abstract

The Nirano Salse, known since the Roman Times, are one of the most beautiful and scenic mud volcanoes areas of Italy with thousands of visitors every year. In this work, we apply novel (for the context) hydrogeological techniques to characterize mud levels in the Salse by means of GPS-RTK positioning and continuous level logging within mud conduits. This is important to quantify the gas–liquid ratio in the conduits and evaluate the potential for dangerous abrupt mud eruptions. The results presented suggest that different mud levels in mud volcanoes clusters are due to the different gas–liquid ratio in the conduits and not necessarily exclude interconnection at depth, a hypothesis, on the other hand, that seems strengthened by mud level time series correlations. The presence of shallow aquifers at a depth of 5 to 30 m is also supported by our field data and allows us to delineate the boundaries of the shallow mud reservoir—pipes system and its overall shape. The shallow aquifers may provide a temporary storage for the ascending gas and when fluid pressure in these aquifers exceeds the tensional strength of the sedimentary rock, leakage of fluids to the surface would occur. In this case, if the gas–liquid ratio is high, mud volcanoes develop into tall gryphons and tend to have a discontinuous activity with sudden eruptions of mud after long periods of quiescence. This, together with the knowledge of shallow conduits localization has an important implication for site safety in proximity to the mud volcanoes. Our inferences based on mud level relationships to mud extrusion dynamics can be applied to lower risk in other mud volcanoes areas of the world with high geo-tourist visits, such as those of Trinidad, Azerbaijan, and Colombia.

Fluid Conduits and Shallow Reservoir Structure Defined by Geoelectrical Tomography at the Nirano Salse (Italy)

“…9). The conductive layered structure toward NE is interpreted as the presence of multiple aquifers (low resistivity areas, indicated in figure 9 by blue dashed lines), that well agree with the presence of sandy turbidite deposits in the 380 FAA ( Castaldini et al, 2017;Giambastiani et al;2022) and are locally interconnected between vents MV2 e MV3. A shallow sub-horizontal connection also exists between vents MV1 and MV2 (labelled as "C" in figure 9), although characterized by lower resistivity values.…”

Gravity Data Allow to Image the Shallow‐Medium Subsurface Below Mud Volcanoes

“…This would be possible in the damage zone of a fault with large matrix and fracture porosity or where some fluid filled mud/gas cracks and feeders intruded the rock. Considering the densities of both mud ( ρ = 1,200 kg/m 3 , Giambastiani et al., 2022) and rocks, and the gas content within them, we can easily obtain the inferred density anomalies assuming volumetric ratios (volume of gas/volume of mud and/or rock) in the range 0.55–0.70. By examining the isosurfaces computed for density contrasts |Δ ρ | = 300 kg/m 3 in the inverted gravity model (Figure 4a), we can infer that: (a) there is a low‐density zone (Δ ρ = −300 kg/m 3 ), which is laying on a vertical NW‐SE striking plane and (b) the top of the high density zone (Δ ρ = +300 kg/m 3 ), which forms a flexure, is structurally higher in the South‐SW than in the North‐NE area.…”

“…In Italy, mud volcanoes are characterized by the almost continuous, but relatively quiet, expulsion of material (Giambastiani et al., 2022). In the present work we focus on the mud volcanoes located in the northern Apennines Italy (e.g., Borgatti et al., 2019; Capozzi & Picotti, 2002), near the city of Fiorano Modenese (Emilia‐Romagna region).…”

“…therein, the reservoirs are put into communication with the surface due to the episodical reactivation of pre-existing faults or pipes. In this view, Lupi et al (2015) and Giambastiani et al (2022) also underline the importance of the shallow aquifers (tens of meters of depth), which may act as buffers where rising gas (methane) is trapped and temporarily stored. This configuration could lead to the localization of small groups of gryphons, mud pools, and mud cones (Figure 1a) just above the shallow reservoirs.…”

“…Within them, gas and connate water coming from depth may accumulate and remain at rest until overpressure building up opens the conduits to the surface. According to Giambastiani et al (2022) and ref. therein, the reservoirs are put into communication with the surface due to the episodical reactivation of pre-existing faults or pipes.…”