The coastal geology around Coffs Harbour is dominated by metamorphic rocks like slate and phyllite, often overlain by transported soils and alluvial deposits along creeks. These conditions create significant lateral variability in subsurface conditions. Electrical resistivity and VES (Vertical Electrical Sounding) surveys provide a rapid, non-intrusive way to map soil layers, locate the water table, and estimate bedrock depth without drilling. For deeper targets, combining resistivity with a georradar GPR survey helps image shallow utilities and stratigraphy in built-up areas of the city.
Resistivity surveys map the soil–rock interface in Coffs Harbour's weathered slate terrain, reducing the need for multiple boreholes and lowering overall investigation cost.
Method and coverage
We follow AS 1726:2017 for geotechnical site investigation and use the Schlumberger array for VES soundings, achieving penetration depths up to 50 m depending on electrode spacing. In Coffs Harbour, where weathered rock profiles can be irregular, resistivity surveys help define the soil–bedrock interface before foundation design. Key parameters measured include:
Apparent resistivity (Ω·m) for each sounding
Interpreted true resistivity and layer thickness
Depth to groundwater or perched water tables
Weathered zone thickness in slate/phyllite terrain
Data is processed using RES2DINV to produce 2-D resistivity models that support decisions on shallow foundations or slope stability. This method is particularly useful for linear infrastructure projects along the Pacific Highway corridor.
Technical reference image — Coffs Harbour
Regional considerations
In Coffs Harbour, we frequently see shallow groundwater tables after heavy rain, especially in low-lying suburbs like Toormina and Sawtell. If a resistivity survey is skipped, the risk is underestimating the water table depth, leading to buoyancy issues in basement slabs or unexpected seepage in excavations. Another common pitfall is misinterpreting a thin, high-resistivity layer as competent bedrock when it is actually a dry, weathered cap over saturated clay. A VES survey with proper inversion processing avoids these costly mistakes. For high-risk slopes, we recommend combining resistivity with a falla taludesanalysis to assess potential failure surfaces.
True resistivity, layer thickness, depth to bedrock
Processing software
RES2DINV, IPI2WIN
Applicable standard
AS 1726:2017 (site investigation)
Related services
01
Vertical Electrical Sounding (VES)
One-dimensional sounding using increasing electrode spacing to map vertical layering. Ideal for locating the bedrock profile and water table depth on single-lot residential sites in the Coffs Harbour region.
02
2-D Electrical Resistivity Tomography (ERT)
Multi-electrode profile along a line, producing a cross-section of subsurface resistivity. Best for linear infrastructure, slope stability assessments, and identifying anomalous zones such as buried channels or voids.
03
Soil Resistivity for Corrosion Assessment
Measurement of soil resistivity at multiple depths to evaluate corrosion potential for buried steel pipelines, tanks, or grounding systems. Essential for industrial and infrastructure projects in Coffs Harbour's coastal environment.
Standards that apply
AS 1726:2017 – Geotechnical site investigations, AS 4678:2002 – Earth retaining structures, AS 1289 – Standard test method for field measurement of soil resistivity
Top questions
What is the difference between VES and 2-D ERT?
VES (Vertical Electrical Sounding) uses a fixed center point with expanding electrode spacing to model vertical layering at that single location. 2-D ERT (Electrical Resistivity Tomography) uses a multi-electrode array along a line to produce a cross-sectional image, revealing both vertical and lateral changes. For most Coffs Harbour projects, we recommend starting with VES for point-specific depth targets and upgrading to ERT when lateral variability is suspected.
How deep can a VES survey penetrate in Coffs Harbour soils?
With a maximum electrode spacing of 200 m, we typically achieve penetration depths between 30 and 50 m. The actual depth depends on the soil conductivity: in the conductive clay-rich sediments common around Coffs Harbour's floodplains, penetration may be shallower due to signal attenuation. For deeper targets, we can use larger spacings or combine VES with a seismic refraction survey.
How much does an electrical resistivity survey cost in Coffs Harbour?
The cost for a VES survey in the Coffs Harbour area ranges from AU$1.090 to AU$1.450 per sounding, depending on access, required electrode spacing, and whether on-site interpretation is included. For a 2-D ERT profile, expect between AU$2.500 and AU$4.000 for a 200 m line. All prices include NATA-accredited field work and a certified report.
