Passive seismic monitoring

Deployment of Tromino with differential GPS

Typical field placement

In 2012 the BGS acquired a modern, passive seismic measurement unit (Tromino) to complement the existing suite of near-surface geophysical equipment (Figures 1 and 2). The Tromino measures passive seismic (acoustic, elastic wave) noise, which comprises both natural (<1 Hz) and artifical (>1 Hz) frequencies. It comprises a single, lightweight, mobile seismometer incorporating a compact, three-component electrodynamic sensor (velocimeter).

We have been deploying the Tromino in a number of different geological and geomorphological settings across the country (Figure 3) and have established that this can be used very effectively as a reconnaissance survey tool for a range of applications. These include basement mapping, superficial thickness mapping, beach thickness surveys and mapping of geohazards such as landslides, liquefaction potential and buried hollows.

Case study 1: Carsington Pasture, Derbyshire

This case study exemplifies the application to fracture mapping.

The foundation excavations for four wind turbines at Carsington Pasture, near Wirksworth in the Derbyshire Peak District, exposed buried, sediment-filled hollows in the dolomitised bedrock (Figure 4). The depth to competent rock is variable as a consequence of weathering related to early Carboniferous fluid flow (hypogene) processes on the southern edge of the carbonate platform. Furthermore, the dolomitised limestones are faulted. The difficult ground conditions have significant implications for the engineering design. The BGS used the Tromino to measure the seismic noise at five-metre intervals across one of the wind turbine footings to characterise the near-surface hollows and bedrock fracturing (Figure 5).

.Wind turbine installation.
Profile generated from processed Tromino data at Carsington Pasture

Additional case studies and pictures

Case study 2: East Yorkshire coast

The Tromino was used to map till thickness on the coast of East Yorkshire.

Mapping till thickness

Case study 3: south coast of England

The Tromino was used to map beach thickness on the south coast of England

Mapping beach thickness
Profile across a spit in the south of England

Case study 4: Charnwood Forest

The Tromino was used to map the thickness of superficial cover and Triassic deposits over Charnian bedrock at Rise Rocks in Charnwood Forest.

Rise Rocks
Typical output of processed data

Case study 5: south nottinghamshire

A survey was carried out in south Nottinghamshire to map part of the Mercia Mudstone sequence

Mapping part of the Mercia Mudstone sequence
Profile from south Nottinghamshire

Contact

For further information please contact Dave Morgan or Vanessa Banks.