Geophysical Testing for ground investigation and verification
Geophysical testing measures physical properties of the ground from the surface or from boreholes and turns them into profiles or sections across a site. Seismic methods such as Multichannel Analysis of Surface Waves (MASW) and seismic refraction measure wave velocities, which reflect stiffness and depth to rock; electrical resistivity tomography maps changes in soil and rock type, moisture and cavities; ground-penetrating radar images shallow features and services. Geophysics covers large areas quickly and guides where boreholes and CPTs are most needed, and repeated surveys can show stiffness gained by ground improvement.
When the test is used
Typically used for…
- Site investigation over large areas before boreholes are placed.
- Depth to rock and lateral changes in ground conditions.
- Locating cavities, buried features and services.
- Shear-wave velocity for seismic site classification.
- Before-and-after comparison of ground improvement.
Limitations
- Results are indirect and must be calibrated against boreholes or CPT.
- Resolution decreases with depth and depends on the method.
- Noise and vibration on active sites, and buried services, affect some methods.
- Interpretation requires experienced specialists.
The test type, number and locations are agreed with the consultant for each project.
How the test works
Sensors such as geophones or electrodes are laid out along survey lines. A seismic source, such as a hammer or drop weight, or an electrical current is applied, and the response is recorded along the line.
The data are processed and inverted into velocity or resistivity sections, interpreted together with borehole and CPT data, and presented as profiles and plans.
Test planning inputs
- Purpose: stiffness, layering, rock depth, cavities or verification.
- Survey lines and depth of interest.
- Existing borehole and CPT data for calibration.
- Site noise, access and buried services.
- Acceptance criteria where used for verification.
Calibration and equipment checks
- Test line over a borehole or CPT location to confirm the method suits the ground.
- Equipment checks and noise recording before the survey.
- For verification, a baseline survey before treatment.
Test execution controls
- Sensor spacing and line geometry surveyed.
- Source and acquisition settings recorded.
- Data quality checked in the field and repeated where needed.
- Line positions referenced to site coordinates.
Interpretation and reporting
The test plan, the raw record and the interpretation are kept separate, so it is always clear what was specified, what was measured and what it shows.
What is specified
Survey scope, methods and, where used, verification criteria agreed with the consultant.
What was measured
Raw data, processing steps and inverted sections.
What it shows
Interpretation calibrated against boreholes and CPT, in the survey report.
Deliverables
- Survey plan
- Raw and processed data
- Velocity or resistivity sections
- Interpretation report
- Before/after comparison where used for verification
Discuss the testing your project needs.
Share the project stage, the ground conditions and the specification or acceptance criteria.