An innovative and fluid-sensitive prospection method: Deployment of Nodal Ambient Noise Tomography (NANT) in Muara Laboh geothermal system, Sumatra

Muara Laboh is a geothermal system located in central Sumatra along a sector of the Great Sumatra Fault and neighboring the Kerinci volcano. The area is characterized by diffused surface manifestations, including hot springs and fumarolic activity at various sites along the valley. In this geological context, the fluid migration is expected to be controlled by the lithological and tectonic discontinuities that characterize such a heavily faulted region.

To identify subsurface fluid pathways and reservoirs, a large region of nearly 400 km² was investigated deploying a network of 212 3-components seismic nodes. 182 instruments were placed within a central 25 km area around the populated centre of Muara Laboh, while 6 external antennas (composed of 5 seismic nodes each) were deployed at 40 km SE from the main network to better constrain the depth within the central region. From the seismic records, we extracted cross-correlation functions and Rayleigh wave group-velocity dispersion curves to perform Nodal Ambient Noise Tomography (NANT). We derived a 3D S-wave velocity model that can be used to identify domains characterized by magmatic, tectonic, sedimentary, structural, hydrothermal, and geothermal features. Empirical Green's Functions (EGFs) were derived from ambient noise cross-correlations for Rayleigh waves using standard data processing methods. Dispersion curves were subsequently determined via the Frequency Time Analysis (FTAN) technique. Nonlinear multi-scale inversion was applied to produce group velocity maps for different periods. Finally, a transdimensional Bayesian approach was utilized for depth inversion, resulting in a 3D S-wave velocity model.

Our S-wave velocity model highlights the occurrence of fast and slow domains, often marked by sharp variations. Transition zones of intermediate-velocity are located at the shoulders of the low-velocity zones and are potentially interpreted as the boundaries of intrusive bodies displaying high shear wave velocities. One of these domains is located below the main Muara Laboh geothermal system. Other similar areas can be identified from the 3D tomography data and represent ideal targets for geothermal energy harnessing.

NANT is a non-invasive and cost-efficient method that provides high-resolution subsurface 3D images of the first 5 km of the upper crust. This approach is ideal to identify subsurface fluid storage and pathways along e.g., seismically active regions where tectonic discontinuities are broadly distributed. These discontinuities are often suggested to control the laterally extensive diffused hydrothermal fluid migration that is targeted for geothermal energy harnessing.