Question 1

How are geophysical investigations different from geotechnical investigations?

Accepted Answer

They investigate the same thing, what is happening below the ground, but in different ways.
 
Geotechnical investigations, such as boreholes, CPTs and test pits, provide detailed information at specific test locations. They tell you a lot about the ground where testing was carried out, but much less about what happens between those points. 
Geophysics provides broader coverage. It surveys across an area and shows how ground conditions change between test locations and across the wider site. Because it measures physical properties indirectly rather than sampling the ground itself, it is less precise at any single point than a borehole or CPT.

The best results often come from using both together. Geophysics gives the wider ground model and helps identify where conditions change. Boreholes or CPTs then confirm the detail in the right places. Used together, this can provide a more complete understanding of the site, with intrusive testing targeted where it adds the most value.

Because Cook Costello has both geophysics and geotechnical engineering in-house, we can plan the investigation as one coordinated exercise from the start, rather than treating them as separate pieces of work.

Question 2

Will councils accept geophysical data for consents?

Accepted Answer

Yes, when it is properly interpreted and directly relevant to the consent requirements. Councils are not usually interested in raw geophysical data on its own. What matters is whether the investigation answers the question being asked, and whether the findings are presented as clear, defensible engineering evidence.

Geophysical data can support consent applications where it helps address specific requirements, such as seismic site classification, Vs30, site subsoil class, depth to rock, subsurface conditions or natural hazard risk. It is most useful when the results are interpreted and explained in the context of the proposed development, rather than simply attached as survey output.

In many cases, geophysics is strongest when combined with direct geotechnical investigation, such as boreholes, CPTs, test pits or laboratory testing. The geophysics provides broader coverage across the site, while the intrusive testing confirms the ground conditions at selected locations. Used together, they give councils a more complete and defensible basis for assessment.

The quality of interpretation also matters. A good report should explain the method used, what was found, how confident the interpretation is, and what the results mean for design, consent or further investigation.

This is becoming increasingly important for seismic site classification, where councils may ask for measured site parameters rather than assumptions. Geophysics is well suited to providing this evidence when it is properly planned, interpreted and supported by appropriate geotechnical input.

Our geophysics and geotechnical engineering teams can prepare combined, interpreted evidence that is shaped around the consent question from the start. Tell us what your council is asking for, and we will advise the most suitable investigation approach.

Question 3

Can geophysics help with early planning for subdivisions?

Accepted Answer

Yes. Early planning is often where geophysics provides the greatest value. Before you commit to a site layout, purchase price or development strategy, a geophysical survey can quickly and non-invasively build a clearer picture of the ground across the site, while there is still time to act on the findings.

At subdivision planning stage, geophysics can help you:

Understand ground variability across the whole site. 
A geophysical survey can show how soil conditions, rock depth and groundwater may vary across the land, helping inform feasibility, lot layout, earthworks and the location of developable areas. 
Identify constraints before the design is fixed. 
Soft ground, shallow rock, high groundwater, old fill, buried features or voids can all affect what can be built, where it can be built and what it may cost. Finding these constraints early allows the subdivision to be planned around them, rather than redesigned later. 
Target geotechnical testing where it matters most. 
Geophysics can help identify the best locations for boreholes, CPTs or test pits, so intrusive investigation is focused on the areas that will provide the most useful information. This often gives a better overall ground model than drilling alone. 
Support seismic and natural hazard assessment. 
Early information on ground conditions, rock depth and site variability can support seismic site classification, hazard screening and consenting strategy before detailed design begins.

The main benefit is timing. Ground constraints are much easier to manage when they are found before the layout, budget and consent pathway are locked in. A constraint found early can become a design decision. The same constraint found during construction can become a delay, variation or redesign.

With our geophysics, surveying and engineering teams working together, the findings can feed directly into the feasibility assessment and subdivision design, rather than sitting in a separate report. If you are assessing land for subdivision, tell us about the site and we can advise whether an early geophysical survey is likely to help, and what it could tell you.

Question 4

Can you help with seismic site classification under NZS 1170.5 or TS 1170.5?

Accepted Answer

Yes. Seismic site classification is a core part of our geophysics work. We help determine the site subsoil class or site classification parameters needed for seismic design under NZS 1170.5 or TS 1170.5, using measured shear wave velocity data rather than relying on broad assumptions.

The method depends on the site, the consenting requirements and the level of certainty needed. Common approaches include:

MASW and HVSR surveys. These are non-invasive surface methods that help estimate the shear wave velocity profile and site period across a site without drilling. 
Seismic CPT. In combination with Geocivil, we can collect shear wave velocity data alongside standard CPT data in a single investigation, which is efficient where CPT testing is already planned. 
Downhole seismic testing. Where a borehole is available, or where greater certainty is required, downhole testing can measure shear wave velocity directly at depth.

In practice, we often combine methods. Surface geophysics gives broader coverage across the site, while CPT or borehole-based testing can confirm the profile at key locations. This provides a seismic site classification supported by measured data, which is increasingly important for building consent and structural design.

Tell us about your site and consenting requirements, and we can recommend the most suitable investigation approach.

Question 5

Can you scan for voids, sinkholes or soft ground?

Accepted Answer

Yes. Void detection is one of the main uses of geophysics. We can survey beneath and around buildings to help identify likely voids, sinkholes, soft ground, collapsed zones or areas of ground loss without excavation.

The right method depends on what you are looking for: 
Voids in the ground: These may include sinkholes, cavities, soft or collapsed zones, old mine or quarry workings, failed fill, karst features or areas where water has washed material away. We use non-invasive methods such as ground penetrating radar and electrical resistivity, selected to suit the ground conditions and the likely depth of the target. 
Voids or defects within concrete: These may include gaps, honeycombing, reinforcement locations or deterioration within slabs, foundations or structural elements. We can scan concrete using high-frequency GPR without damaging the structure.

In both cases, the survey is non-invasive and can often be carried out quickly. Where the findings matter for design, repair or safety decisions, geophysical results are best verified with targeted investigation. For example, on a Northland limestone quarry project, our void interpretation was checked against targeted drilling, giving the operator confidence in the subsurface model.

Geophysics is effective where there is a strong contrast between the void and the surrounding ground or structure, but no method finds everything in every condition. Some soils, materials and depths limit what a survey can reliably detect. We will explain these limitations upfront, so you know what the scan can and cannot establish for your site.

If your building is showing signs of subsidence, cracking or movement, the void may be part of a wider ground or structural issue. Our engineering teams can investigate further to assess what may be causing it and advise on the next steps.

Question 6

Can you locate underground utilities before drilling or excavation?

Accepted Answer

We can help identify likely buried utilities and other subsurface features, but it is important to understand what the survey can and cannot confirm.

Cook Costello uses high-frequency ground penetrating radar, or GPR, to scan for buried services before drilling or excavation. It is a fast, non-invasive method that can help build a clearer picture of what lies beneath a site, particularly where services are large enough and there is a strong contrast between the utility and the surrounding ground.

GPR does not detect everything. Smaller services, gas pipes, electrical cables, plastic pipes and services in difficult ground conditions can be hard to identify reliably. In some soils, the signal may also not penetrate deeply enough to provide reliable coverage across the full site. A GPR survey can reduce the risk of an unexpected service strike, but it should not be treated as a clearance to dig.

For that reason, GPR is one part of a safe-digging process. Before excavation, you should still obtain utility records, such as through beforeUdig, and physically verify services where required using careful hand-digging or hydro-excavation. GPR works best alongside these steps by helping identify services that may not appear on records and highlighting areas that should be checked before work begins.

If you tell us about your site and what you are planning to drill or excavate, we can advise whether a GPR survey is likely to add value, what it can reasonably detect, and what other checks may be needed before work starts.

Geophysical Surveys

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How geophysical surveys are delivered

We deliver answers, not just data

Selected projects across New Zealand

See what's underground before decisions get expensive

Geophysics FAQ

How are geophysical investigations different from geotechnical investigations?

Will councils accept geophysical data for consents?

Can geophysics help with early planning for subdivisions?

Can you help with seismic site classification under NZS 1170.5 or TS 1170.5?

Can you scan for voids, sinkholes or soft ground?

Can you locate underground utilities before drilling or excavation?

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