Pioneering the use of nanoseismics as a site investigation tool

"Field evidence shows that faults and fractures can act as focused pathways or barriers for fluid migration. As a result, detailed imaging, i.e. location, orientation and hydraulic properties, of fracture networks not visible at the surface, is crucial for modern engineering applications, such as CO2 storage, geothermal energy exploitation, tunnelling. To-date, such information is not available at the required level of detail and accuracy. Currently, methods for the imaging of fractures at a resolution of less than 100m are based on seismic reflection/refraction and surface waves. These methods require a large number of sensors (usually over 100 making their application costly) and their success highly depends on the elastic properties of the rock and the inclination (dip) of the underlying strata. The fractures can be identified at a maximum depth of 100 m below ground surface.

The main aim of this project is to (1) develop a new site investigation tool that utilises micro-seismicity for the identification of individual fractures that accommodate flow at depths down to 4km and (2) build the skills and knowledge necessary to implement it within industrial applications directly related to fluid flow systems at depth. This will result in the publication of a new protocol for microseismic borehole stimulation to image fractures that accommodate flow over large spatial scales.

A new monitoring technology will be used and further extended: short-period seismometers with the combination of Hypoline software. The latter can help distinguish between weak signals and noise and reduces the signal-to-noise ratio to 1:1. The combination of sensors and software is known as nanoseismics (NS). Although new and very promising, this technique has a limitation: it can provide a location, time of origin and magnitude for the recorded events, however, its does not provide an estimation of the location error, nor the moment tensor that would allow determination of fracture orientation, two very important parameters for providing a detailed image for engineering applications. This project will pursue to further extend this capability for the determination of individual fracture orientations and quantification of fracture permeability.

There is a unique opportunity to found both the testing of the efficiency of NS as a site investigation tool, and the development of the analysis technique, by recording data at an existing site with high potential to act as geothermal resource in NE England. Permission has been granted to conduct a series of injection tests at a borehole on the site in order to record the induced microseismicity and locate and image the fractures that accommodate the flow at a depth of up to 2km.
The outcomes of this research will contribute to a better understanding of sub-surface processes through characterisation of heterogeneous flowing fracture networks over large spatial scales."

This project has shown that nanoseismics can record very weak seismic signals representing material failure such as crack formation and propagation in a landslide process. Signals can be distinguished even in records with a low signal-to-noise ratio, highlighting the potential of nanoseismics to become a significant part of an early warning system.