Distributed acoustic sensing for quick clay monitoring

Distributed Acoustic Sensing (DAS) is becoming increasingly popular due to its high spatial and temporal resolution. DAS holds great potential for geohazard applications as, in principle, anything affecting the strain on a fibre optic cable section can be measured. Examples are passing seismic surface waves and ambient temperature changes.  This presentation demonstrates the feasibility of DAS for quick clay monitoring, and presents data from a field trial in Rissa, Norway.

In Norway, almost all landslides in clays that have serious consequences are caused by the instability of quick clay. Examples include the landslides Trögstad (1967), Rissa (1978), and recently Gjerdrum (2020).

A research field site was established at Rissa by the Centre for Geophysical Forecasting (CGF). Long term monitoring with DAS over several months is carried out to monitor changes in the geophysical parameters of the soil before and after road construction work.

Due to the close relation between elastic parameters controlling seismic wave propagation and the petrophysical properties of the sediment, which determine the strength, DAS measurements from seismic waves, mainly Rayleigh waves, can be used to investigate the soil stability.

The Rayleigh waves of interest travel with a velocity that is approximately 0.9 times the shear wave velocity (Vs) and may have wavelengths of only a few meters. The shear modulus, which is the main geomechanical parameter controlling the stability and shear strength, can be approximately inferred from Vs. Therefore, observation of changes in Vs can be used to detect changes in shear strength of clay formations.

One of the main challenges for this application lies in the detection of seismic surface waves of shorter wavelengths. Commonly used methods for quick clay monitoring suffer either from lower spatial resolution or limited area coverage, and we also seek to address these challenges.

Alcatel Submarine Network Norway developed an interrogation technology (OptoDAS) enabling long-range measurement over 100km. Spatial sampling intervals as small as 1m can be chosen. It is, however, the gauge length and the spatial sampling that determines the spatial resolution. The gauge length varies from 40m to 2m, and is analogous to receiver (group or node) separation in conventional seismic methods. 

Due to the inherent properties of DAS interrogation the SNR is lower for very small gauge lengths. Although the data quality is adequate, we strive to improve the SNR further to make DAS well suited for the analysis of seismic waves with wavelengths even shorter than 4m.

A cost-effective solution for increasing the data quality could be found by introducing fibre loops into the acquisition design. The gain of these optimization will be presented, and it will be demonstrated that data quality can be improved by stacking over multiple similar fibre optic pathways.

Results will be presented for seismic signals from passive sources – such as passing cars on the nearby road, and from an active source, a seismic hammer and plate shot.

The pros and cons of using long-range high-resolution DAS technology for soil monitoring will be discussed along with potential areas for future advances.