We still see contractors in Norwich ordering a borehole every ten metres across a site, hoping to catch a chalk pipe or dissolution hollow by sheer luck. The geology here doesn't cooperate with guesswork. The Upper Chalk beneath the city is riddled with flint bands, buried valleys and solution features that a point investigation will miss nine times out of ten. Seismic tomography, both refraction and reflection, gives you a continuous profile instead of a stab in the dark. When we pair it with a targeted test pits campaign on the weathered zone, the ground model tightens up considerably. And for projects near the River Wensum floodplain, understanding the chalk–alluvium interface matters before anyone pours a foundation.
A velocity anomaly below 1600 m/s in Norwich chalk almost always points to a dissolution feature that needs ground-truthing before foundation design.
Methodology applied in Norwich
Local geotechnical conditions in Norwich
Norwich sits at a modest elevation, roughly 25 metres above sea level near the city centre, but the real story is underneath. The Cretaceous chalk bedrock has been dissolving for millennia, creating an unpredictable karst-like landscape of buried pipes and crown holes that can open up with very little warning. In 1988 a crown hole collapsed on Earlham Road, swallowing part of the carriageway overnight, a reminder that the ground here is never truly dormant. Seismic refraction and reflection surveys pick up the low-velocity signatures of these voids and the loosened halo of material around them before they reach the surface. Skip that step and you are designing foundations over a Swiss cheese geology where differential settlement can tear a building apart within the first five years. The British Geological Survey has mapped extensive dissolution features across the Norwich Chalk, and every local engineer worth their salt has a story about a site that turned out to be a paleochannel nobody saw coming.
Our services
Every seismic tomography campaign we run in Norwich ends with a deliverable that the design team can actually use. The velocity tomograms are annotated with interpreted lithological boundaries and any anomalous zones flagged for intrusive follow-up. Below are the two core survey configurations we deploy across East Anglia.
Seismic refraction tomography
We lay out a 115-metre geophone spread and record first-arrival travel times from multiple shot positions, then invert the data to produce a 2D P-wave velocity cross-section. This method excels at mapping the top-of-chalk profile, the thickness of the weathered zone, and the lateral extent of infilled dissolution pipes across a site.
Seismic reflection profiling
Using a tighter geophone array and a higher-energy source, we capture reflected energy from internal chalk stratigraphy, flint bands, and the base of glacial deposits. The processed section reveals subtle offsets and sag structures that refraction alone cannot resolve, giving the structural engineer a clearer picture of buried faulting and deeper dissolution.
Questions and answers
How much does a seismic tomography survey cost in Norwich?
For a typical commercial site in the Norwich area, a seismic refraction campaign with a 115-metre spread and multiple shots generally runs between £2,030 and £3,730, depending on the line length, access conditions, and whether we need to combine refraction with reflection acquisition. We offer a fixed-price quote after reviewing the site plan and the target depth.
What depth can seismic tomography reach in chalk geology?
With a standard 115-metre spread and a weight-drop source, P-wave refraction tomography typically resolves the top 25 to 40 metres in Norwich chalk, which covers the weathered zone and the first competent chalk bench. Reflection profiling can image deeper flint bands and structural features down to 60 metres or more, depending on the acoustic contrast between layers.
Does seismic tomography work on busy urban sites in Norwich?
It can, but we need to be upfront about the limitations. Traffic vibration and buried utilities generate noise that masks the weaker seismic arrivals. We usually schedule urban acquisitions during quieter periods, early morning or Sunday, and we run a noise test beforehand. On very tight city-centre sites we sometimes recommend combining a shorter seismic line with a complementary intrusive method to ground-truth the results.
How long does a seismic survey take from fieldwork to final report?
Fieldwork for a single refraction line with three shot positions typically takes one day on site, assuming reasonable access. The data processing and tomographic inversion add another three to five working days. We deliver a PDF report with annotated velocity sections and a written interpretation, plus the raw SEG-2 files if the client's geophysicist wants to run their own processing.