Microseismic monitoring of wave propagation through heterogeneous media: a tool for brownfield site investigation?

Gheith Alfakri¹   Emmanouil Parastatidis¹  Stella Pytharouli¹

Abstract: Previous studies present evidence that microseismic monitoring could be a favourable potential technology for brownfield land site investigations, e.g. in the identification of buried objects in the shallow subsurface (< 3 m). More specifically, the presence of buried objects change the characteristics (amplitude and frequency) of a mechanical wave that propagates through a medium where this object lies. These changes have to-date only been observed at recordings from stations that are located directly above the buried object. To investigate whether a buried object can be ‘seen’ by more sensors located in the vicinity above the object, we carry out a series of numerical simulations. We examine the propagation of a sine wave emitted by a point source on the surface of a medium and study the frequency, amplitude and emitted energy from that sine wave and how these are affected by local changes in the mechanical properties of the model. For the duration of each simulation, we record the velocity history at a number of points on the free surface of the model. Numerical simulations are carried out in FLAC3D. First, we look on how the distance between the source and the monitoring points changes what we record. We examine two cases : In Case A, the monitoring stations and the buried object are at a distance less than 30 meters from the seismic wave source. In Case B, the monitoring stations and buried object are at a distance more than 30 meters from the seismic wave source. We apply spectral analysis to the resultant seismic velocity time histories as recorded at a number of monitoring stations at the free surface of the model. Our results for Case A show that an object can be detected at a monitoring station located directly above the object to a depth of 1-2 meters. Results for Case B show that an object can be detected at the monitoring station that is deployed directly above the object to a depth of up to 4-5 meters, and it can also be detected at neighbouring stations, at distances approximately equal to the depth of the object. In addition, we study factors having an impact on the amount of energy of the seismic wave emitted, i.e. depth of the object from the surface and its mechanical properties. Our analysis indicates that by increasing the depth of the object, the amount of reflected seismic energy decreases. The changes in the mechanical properties of the materials lead to a change in seismic wave propagation velocity and frequency. Results from our numerical simulations present evidence that microseismics can be used as a complementary, low-cost site investigation tool for applications where very shallow depths are of particular interest such as those at brownfield sites. This can have significant implications on the way site investigations on brownfield sites are carried out, with microseismics providing an alternative to sites where traditional non-intrusive methods such as GPR and/or resistivity tomography are limited due to ground properties.