From Seismic Arrays to Single-Station Wavefield Gradiometry: Results from a Multi-Scale Experimental Campaign

Seismological studies are traditionally based on the observation of ground motions recorded by translation sensors. However, to assess a comprehensive description of any wavefield produced by seismic sources, the three components of rotation are as important as the three translation components. Due to improvements in instrumentation through the last two decades, the ground rotational motion is now an observable. Several recent publications show that 6 degrees of freedom (-dof) seismic station, recording the 3 translation and 3 rotation components of the ground motion, provides valuable information to locate events or to infer their source mechanism.

Rotation motions can be recorded directly via dedicated sensors, or numerically derived via a dense array of seismometers. The formers primarily use technologies based on fiber optic gyroscopes, liquid-based systems, or mechanical principles. However, these broadband instruments do not have the high sensitivity required to detect "weak" ground movements. Conventional sensor arrays, on the other hand, use finite differences techniques to estimate reliable indirect rotation measurements, but these are limited at high frequencies (wavelength > 4 × array aperture).

Since the end of 2025, a temporary experimental campaign is set at the Low Noise Underground Laboratory (LSBB) in Rustrel, France. Two rotational rate sensors, namely a BlueSeis 3A and a Eentec R3, are deployed in the underground galleries, jointly with five seismometers which complete the permanent seismic array. The dense seismic array co-located around the rotation sensors is used to compute array derived rotations (ADR), and validate the direct observations of rotational ground motions. Furthermore, six seismometers are deployed at the surface to form an array with an aperture of around fifty kilometers. It has been designed for long-term observations of regional and global seismicity and array processing analysis, such as beamforming techniques. This campaign allows to compare the performance of classical seismic array processing with innovative gradiometric approaches based on a single 6-dof station, focusing on detection, location, and characterization of seismic events within a common frequency band. A sensitivity study on rotation signals, in terms of instrumental conditions (array geometry, station quality) and processing parameters (signal duration, filtering), is carried out with the help of numerical full waveform modelling in order to quantify the uncertainty of the estimated source parameters. We present the benefit of such multi-component seismic wavefield recording, illustrated on several events of interest such as the recent (2025/07/29) Kamchatka event (Mw 8.8).