Technical library

What belongs in a soil-resistivity report

A useful report connects the engineering purpose to the survey geometry, readings and interpretation. Specify these sections in the appointment:

1. Project purpose, site location and revision record.
2. Test plan, positions, directions and access constraints.
3. Instrument identification and calibration evidence.
4. Conditions, geometry and unedited readings.
5. Calculation method and resistivity-versus-spacing plots.
6. Model interpretation, fit and limitations where included.
7. Conclusions, unresolved inputs and next actions.
8. Preparation and review record.

This is a report-content guide, not a completed client report or a design certificate.

What a single-spacing test can establish

A result at one spacing documents the response of that measurement arrangement. It does not establish how apparent resistivity varies with spacing or uniquely identify a layered soil model. Repeating one spacing at different positions can investigate lateral variation, but is not equivalent to a multi-spacing sounding.

Keep historical records useful by describing exactly what they contain. Specify a new investigation when the engineering decision requires information the record cannot provide.

Wenner and Schlumberger: specify the geometry

Wenner and Schlumberger are different four-electrode survey arrangements. Their electrode geometries and calculation factors differ. A data sheet must identify the actual arrangement and distances used; readings cannot be interpreted using the name of another array.

Choose the investigation method and spacing plan for the design question, available site extent and interpretation approach. Ask for the survey plan before fieldwork.

Why site conditions matter

Moisture, ground condition and recent weather can affect soil measurements. Record the test date and relevant conditions, and consider whether the observations represent the design conditions that matter for the installation.

Do not apply a generic seasonal correction without an engineering basis. Where variability matters, define additional investigation or a justified design allowance.

Why low resistance is not a complete safety result

Installed earth resistance is one observation. Fault current, duration, bonding, geometry and accessible potential differences also matter. Ask what acceptance criteria were used and whether the tested arrangement represents the operating system.

Related reading: earthing design inputs and installed-system verification.

Standards and further reading

These notes explain procurement questions; they do not reproduce standard tables or replace a project assessment. Consult the licensed standards and the agreed project basis.

• IEEE 81-2025: grounding measurements
• IEEE 80: AC substation grounding
• SABS: obtain applicable South African standards

Discuss the work package

Send the location, available drawings, engineering decision and required date. We will clarify the inputs and issue a defined scope.