Seismic Tomography (Refraction/Reflection) for Site Investigation in Portlaoise

With a sledgehammer source on the compact glacial tills typical around Portlaoise, we reliably image to 25–30 metres depth. The limiting factor is usually the velocity inversion: if weathered limestone sits beneath faster till, the refraction method can miss the lower-velocity layer. That's why we always review the geological model before selecting the technique. For deeper targets, we switch to seismic reflection or combine refraction with electrical resistivity tomography.

Detection depends on cavity size relative to depth and the velocity contrast with the surrounding rock. An air-filled void of 1 metre diameter at 10 metres depth can produce a measurable travel-time anomaly when geophone spacing is tight enough. Clay-filled cavities are harder to spot because the velocity contrast is smaller. Cross-hole tomography between boreholes gives the best resolution for sub-metre features, and we recommend it for critical load-bearing elements.

For a standard refraction survey covering the footprint of a commercial building or residential development in Portlaoise, the cost ranges from €2,260 to €5,430 depending on line length, number of spreads, and whether reflection or cross-hole components are added. Each quote is site-specific because the array geometry depends entirely on the target depth and the space available on site.

A single refraction line of 100 metres takes about 3 hours in the field with a two-person crew. Processing and interpretation add two working days. We need a cleared corridor roughly 2 metres wide along the survey line; the geophone cable can cross hardstanding but not active traffic areas. For reflection surveys, the source is louder so we coordinate with adjacent properties. The final deliverable includes velocity tomograms in PDF and SEG-Y format, plus a written interpretation cross-referenced with any borehole data available.