Geophysical surveys

How bridges get built · Site investigation and testing · Geophysical surveys

Also called: seismic refraction, electrical resistivity, MASW, marine sub-bottom profiling

Geophysical surveys — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Define the targets (rock head, buried channels, cavities, soft pockets, obstructions, shear-wave velocity for site class) and choose methods by the physical contrast each target offers.
Run a trial line over an existing borehole to confirm the method can see the target before committing to the full survey.
On land, lay seismic refraction, MASW or resistivity lines along the alignment and across each support, with the spread length set by the required depth.
Over water, survey with multibeam bathymetry, side-scan sonar, a sub-bottom profiler and a magnetometer on a grid with cross lines, logging RTK position and tide.
Process and invert the data, stating resolution and known artefacts; tie each section to boreholes at line intersections.
Feed the results into the ground model: place the next boreholes on anomalies and use the sections to interpolate between holes.
For seismic design, measure the shear-wave velocity profile to at least 30 m with MASW, downhole, crosshole or seismic CPT tests.

Key numbers

Depth reachedroughly one fifth to one third of the spread length for refraction and resistivity lines.
Site class by Vs30 (Eurocode 8)A above 800 m/s, B 360–800 m/s, C 180–360 m/s, D below 180 m/s.
Sub-bottom profilerschirp systems resolve decimetres over the top 5–30 m of soft sediment; boomers and sparkers reach 50 m or more at metre resolution.
Output1–3 km of seismic or resistivity line per day on land; tens of line-km per day for marine surveys.

Where it fits

Long alignments, karst, buried channels, marine sites before the drilling campaign.
Sites where access for rigs is limited.

Where it does not

Using geophysics as the only evidence for a foundation level.

Choosing it

Use geophysics before drilling on long alignments, over water and in karst: it puts boreholes where the ground changes.
Prefer more boreholes on short crossings with only a few closely spaced supports, where geophysics adds little.
Choose MASW or seismic CPT for stiffness and site class, resistivity or microgravity for cavities, and sub-bottom profiling for marine sediment thickness and channels.
Never fix founding levels on geophysics alone: inversions are not unique and must be checked by drilling.

Plant, pace and money

PlantGeophones and a seismic source, resistivity cables, survey boat with profiler.

ProductivityKilometres per day on land; several line-kilometres per day at sea.

CostLow per metre; a few percent of the investigation budget.

Risks and controls

What goes wrong

Ambiguous inversions; a thin hard layer over soft can hide the soft.

Quality assurance

Calibration boreholes at line intersections; interpretation reported with confidence levels.

Origins

Seismic refraction moved from oil prospecting (Mintrop's work around 1919–1920) to dam and foundation sites in the 1920s and 1930s; resistivity sounding dates from Wenner (1915) and the Schlumberger brothers; MASW was introduced by the Kansas Geological Survey in 1999.

Examples

Sub-bottom profiling is routine before marine bridge investigations in the Gulf and Southeast Asia.

Case studies

Channel TunnelUnited Kingdom / France · 1994Marine seismic profiling between boreholes traced the Chalk Marl across the strait, which allowed the tunnels to be kept within that stratum.

Fehmarnbelt Fixed LinkDenmark / Germany · late 2000s (investigation)Marine geophysics of the whole corridor was combined with boreholes and CPTs to map glacial deposits and the highly plastic Palaeogene clay along the crossing, serving both the bridge and immersed tunnel options before the tunnel was chosen in 2011.