The buried valleys and Pennine Coal Measures beneath Wigan create a subsurface riddled with velocity contrasts that conventional boreholes often miss. Drift deposits here can exceed 30 metres in places like Ince or Pemberton, masking abandoned mine workings and sandstone channels where seismic velocity shifts abruptly from under 800 m/s in soft alluvium to over 2,500 m/s in competent rock. Seismic tomography (refraction/reflection) maps these transitions continuously, providing the lateral coverage essential for foundation design near known fault lines such as the Wigan Coal Measures syncline. Where drilling alone leaves gaps, spt-drilling provides point-specific N-values that tomography velocity models then help extrapolate across the full site footprint.
Velocity tomography resolves the rockhead profile beneath Wigan's drift cover where drilling alone would interpolate across tens of metres of uncertainty.
Local context
A common error on brownfield sites around Wigan is relying solely on SI boreholes spaced 20–30 metres apart, assuming the rockhead is planar between them. Glacial erosion of the Pennine Coal Measures has carved irregular channels into the bedrock, and a piled foundation terminating on a sandstone high can be flanked by soft infill just two metres away. Undetected, this differential stiffness produces unacceptable settlement gradients in the superstructure. Seismic tomography (refraction/reflection) eliminates that interpolation gamble by delivering a near-continuous rockhead profile, and the velocity data itself feeds directly into B-value calculations for dynamic modulus estimates, particularly important for structures subject to vibration from the West Coast Main Line or local industrial plant.
Quick answers
What depth can seismic refraction reach in the Wigan area?
With a 115-metre spread length and a weight-drop source, we typically image the refractor at 30 to 40 metres depth through the glacial till. In favourable conditions, reflection processing on the same dataset can extend usable coverage to 60 metres or more, sufficient to reach the sandstone bedrock across most of the borough.
How much does a seismic tomography survey cost for a typical Wigan site?
A single-line refraction profile with 24 geophones and tomographic processing falls between £2,410 and £3,990, depending on spread length, source type, and terrain access. Multi-line or 3D grid surveys are priced per linear metre after an initial review of the site plan and mining records.
Can seismic methods detect old mine workings beneath the site?
Seismic velocity alone cannot directly image small voids, but the tomographic grid often reveals velocity anomalies consistent with collapsed workings or fractured roof rock. The most reliable approach combines seismic tomography for rockhead mapping with electrical resistivity profiling to cross-validate air-filled or water-filled cavity signatures.
