Under AS 1726 and AS 4678, electrical resistivity surveys provide critical data for projects in Mackay. The city sits on Quaternary alluvial deposits overlying Permian volcanics, creating complex layering that standard boreholes often miss. For coastal developments near the Pioneer River, saline intrusion is a real risk. We use Vertical Electrical Sounding (VES) to map resistivity contrasts down to 50 m depth, distinguishing freshwater aquifers from saltwater wedges. This is especially relevant for residential subdivisions in North Mackay and industrial sites at Paget. Before any deep excavation or piling, combining VES with a georradar GPR survey clarifies the contact between alluvial sands and the underlying bedrock.
Resistivity imaging in Mackay's alluvial plains revealed a buried paleochannel at 12 m depth, avoiding a costly foundation redesign.
Methodology and scope
Our field setup uses a Syscal Pro 72-channel system with a Schlumberger array for VES soundings. Electrode spacing ranges from 1 m to 100 m, giving us effective investigation depths between 5 m and 50 m. In Mackay, we deploy this across sites from the coastal plain at Bucasia to the rural blocks at Eton. The equipment injects low-frequency DC current and measures potential difference to compute apparent resistivity. We then invert the data using Res2DInv software, generating 2D profiles that show lithological boundaries. For shallow foundation works, we often cross-reference resistivity anomalies with ensayos CPT to validate soil behavior type. Typical parameters we record include:
Apparent resistivity (ohm·m) at each electrode spacing
Inverted true resistivity model with RMS error < 5%
Depth to interpreted bedrock or water table
Resistivity contrast ratio between layers
Technical reference image — Mackay
Local considerations
A common mistake we see in Mackay is relying solely on boreholes for geotechnical models. The Pioneer River floodplain has rapid lateral variability — a borehole 15 m apart can show completely different stratigraphy. Without electrical resistivity, contractors often miss buried gravel lenses or perched water tables. We had a case at the Mackay Marina where a resistivity survey identified a saline wedge at 8 m depth that standard auger holes had missed. That finding changed the corrosion protection specs for steel piles entirely. Don't let a single borehole dictate your foundation design. Use VES to see the big picture.
Multi-electrode arrays producing continuous 2D profiles. Ideal for mapping landfill boundaries, groundwater contamination plumes, and bedrock topography. We configure line lengths from 50 m to 400 m depending on site access.
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Vertical Electrical Sounding (VES) for Deep Profiling
Single-point soundings with expanding electrode spacing to 100 m. We use this for deep groundwater exploration and locating the fresh-saline interface in coastal Mackay. Each sounding takes about 2 hours in the field.
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Resistivity Monitoring for Time-Lapse Studies
Repeated surveys over weeks or months to track changes. Useful for dewatering projects, leachate migration, and tidal groundwater fluctuations. We install permanent electrode lines for rapid repeat measurements.
Applicable standards
AS 1726 – Geotechnical site investigations, AS 1289 – Standard guide for using the direct current resistivity method, AS/NZS 1170 – Structural design actions (supplementary for coastal loading)
Frequently asked questions
How deep can electrical resistivity surveys reach in Mackay's soil conditions?
With a 100 m electrode spread using Schlumberger array, we achieve investigation depths of 30 to 50 m. The actual depth depends on the resistivity contrast between layers and the surface noise level. In Mackay's alluvial deposits, we routinely image down to the bedrock contact at 25-40 m depth.
What is the difference between VES and 2D ERT?
VES measures resistivity at a single point with increasing depth, giving a 1D layered model. 2D ERT uses multiple electrodes along a line to produce a continuous cross-section. For Mackay sites with lateral variability — like the river terraces — ERT is preferred. For deep groundwater assessment in flat terrain, VES is more cost-effective.
How much does an electrical resistivity survey cost in Mackay?
A standard VES survey with 5 soundings and interpretation typically ranges from AU$830 to AU$1,680 depending on site access and data density. Full 2D ERT profiles with 400 m line length and inversion modeling are at the higher end. We provide a fixed quote after a site walkover.
Can resistivity surveys detect groundwater quality in Mackay?
Yes. Resistivity is very sensitive to dissolved solids. Freshwater typically shows 10-100 ohm·m, while saline water drops to 0.2-5 ohm·m. We have used this in the Mackay coastal plain to map the freshwater lens and identify upcoming from deeper aquifers. The method cannot replace chemical sampling but provides spatial context for planning monitoring bores.
