Geophysics

Geophysics measures the ground without breaking it open. That matters in two situations: when the site cannot be disturbed — live services under a road reserve, a heritage setting, an urupā or cemetery, contaminated ground — and when the question is about the whole site rather than a handful of points. A borehole tells you what is under the borehole. A survey tells you what happens between them.

It is not a replacement for intrusive investigation and we do not sell it as one. The two answer different questions, and used together they are usually cheaper than either alone: the survey shows where the ground changes, and the rig goes to those places instead of a grid.

Depth to rockhead and weathering profile across a footprint, not at points

Shear-wave velocity and site subsoil class for seismic design

Buried services, tanks, voids and structures before anything is dug

Groundwater, fill and contamination extent

Targeting and scoping of the intrusive investigation that follows

Method selection is driven by the question, the ground and the access. Most projects use more than one, because the failure mode of geophysics is a single technique used past the point where it can resolve anything.

Ground penetrating radar (GPR) — services, voids, reinforcement, buried structures and features

Seismic refraction — depth to rockhead, layer velocities and weathering profile

MASW surface-wave surveying — shear-wave velocity profiles and site subsoil class

Wireline (downhole) geophysics — logging the borehole itself to calibrate surface results

Electrical resistivity imaging — groundwater, fill, contamination and buried features

Magnetometry and EM surveys — buried metal, tanks, drums and archaeology

Electromagnetic induction for full underground service location

Site subsoil class drives the seismic design of almost every building in New Zealand, and on soft or variable ground it cannot safely be assumed from a map. Seismic refraction and MASW measure it on the site: refraction gives the layering and depth to rock, MASW gives the shear-wave velocity profile that the classification actually depends on.

Because the survey covers a footprint rather than a point, it also shows where the classification changes across a site — which is the case that catches out a design based on one borehole in the corner of the block.

Wireline geophysics logs the borehole itself. A probe run down the hole records the properties of the material it passes through continuously, rather than at sample intervals, and that log is what ties a surface survey to known strata.

The pairing is the point. A surface survey over a wide area is an interpretation until something calibrates it; a downhole log calibrates it. That is what makes a geophysical result defensible in a design report or a consent application rather than indicative.

We have used ground penetrating radar to define the extent of a cemetery affected by flooding, identifying both marked and unmarked graves so the area needing protection could be established before any intrusive work was scoped.

On a proposed building development at Dent Street in Whangārei we combined ground penetrating radar with electromagnetic induction and other geophysical equipment to complete a full underground service location, mapping known and unknown services through the berm, footpath and road reserve and surveying the results into CAD for the design team.

Geophysics runs out of our ten North Island offices and travels — from Kerikeri in the far north to Whanganui — alongside our Explore ground investigation, service location and CPT drilling teams. Survey crews mobilise nationally for larger programmes.

Results are surveyed to real coordinates, delivered as GIS-ready layers or CAD for the design team, and interpreted by the same engineers who write the geotechnical report — so the survey answers a design question instead of arriving as a standalone dataset.