BOREHOLE-TO-BOREHOLE IMAGING
Crosshole Seismic Tomography
Image seismic-velocity variation between boreholes using controlled source–receiver measurements, careful borehole geometry and engineering interpretation.
ENGINEERING OVERVIEW
Imaging the ground between boreholes
Crosshole seismic tomography uses travel times from a seismic source in one borehole to receivers in another to infer variations in wave velocity within the intervening ground. Multiple source and receiver depths provide crossing ray paths for a two-dimensional velocity model. This is different from a conventional crosshole test that reports P- or S-wave velocities between boreholes at selected common depths.
When surface surveys cannot resolve a critical zone or drilling reveals complex lateral variability, measurements between existing or newly drilled boreholes may provide additional constraints. The method can help test assumptions about variable rock quality, disrupted ground or stratigraphic continuity, while remaining dependent on borehole placement and usable ray coverage.
MEASUREMENT & INTERPRETATION
What the method actually tells you
Measured quantity
The principal observation is seismic-wave travel time. The inverted result describes P-wave or, where a suitable acquisition system supports it, S-wave velocity—not compressive strength or rock quality designation directly.
Geometry controls evidence
Source–receiver paths must sample the area of interest. Poor angular coverage and long unsampled zones can create apparent features from regularisation rather than observed contrast.
Crosshole testing is not tomography
A conventional crosshole test commonly derives velocities at discrete depths. Tomography combines many offset depth combinations and solves a spatial inverse problem.
Ground model remains essential
Core logging, borehole deviations, water levels and local geology are necessary to interpret what a velocity anomaly may represent.
PRACTICAL SURVEY FLOW
From site question to defensible interpretation
Position boreholes
Select borehole spacing and geometry around the target; measure collar coordinates and downhole deviation where material.
Confirm coupling and access
Check source deployment, receiver response, water or casing requirements and practical depth intervals.
Acquire crossing paths
Record repeatable waveforms at planned source–receiver elevations, preserving timing and acquisition metadata.
Model velocity and uncertainty
Pick first arrivals, assess ray coverage, invert travel times and compare velocity anomalies with borehole records and other measurements.
WHERE IT ADDS VALUE
Engineering applications and use cases
Ground variability between boreholes
Investigate spatial differences that may affect tunnelling, excavation or foundation design assumptions.
Rock and fractured-ground studies
Explore zones of contrasting seismic velocity to support targeted engineering verification.
Existing asset investigations
Support investigations adjacent to important structures where intrusive disturbance or surface access is constrained.
MEASUREMENT BOUNDARIES
Limitations and conditions of use
Interpretation quality depends as much on survey suitability and independent evidence as on processing software.
- Resolution is uneven; regions with sparse ray paths should not be presented with the same confidence as densely sampled regions.
- Borehole location errors or deviation uncertainty can produce travel-time bias.
- Travel-time tomography is sensitive to first-arrival picking, source coupling and starting-model assumptions.
- Velocity anomalies are not unique geological interpretations; they do not independently prove voids, fractures or load-bearing capacity.
METHOD SELECTION
How to choose the right approach
Tomography versus conventional crosshole
Tomography seeks a velocity image across a borehole panel; conventional crosshole testing is a more direct velocity-at-depth investigation.
Tomography versus surface seismic refraction
Crosshole geometry targets a confined inter-borehole volume; surface refraction offers broader access-dependent coverage but different resolution and hidden-layer risks.
Tomography versus electrical resistivity imaging
Seismic tomography tracks elastic-wave propagation, while resistivity imaging responds to electrical properties. Complementary anomalies require geological reconciliation.
Select tomography when borehole access and target geometry justify its additional effort and when the project question needs a spatial velocity model. If only a representative depth–velocity profile is needed, conventional crosshole testing or a surface-wave method may be more proportionate.
ENGINEERING ASSURANCE
Field quality, interpretation and review
A traceable project record should retain the following information where applicable:
- Surveyed collar coordinates and borehole deviation where relevant
- Instrument timing, source/receiver coupling and waveform repeatability
- Arrival-pick quality flags, rejected picks and error assumptions
- Ray-coverage plots, inversion sensitivity and fit diagnostics
PROJECT DELIVERABLES
What a useful technical report should contain
01 · Project output
Borehole geometry, source/receiver depth schedule and survey log
02 · Project output
Waveform archive with travel-time picks and QC annotations
03 · Project output
Velocity tomogram and ray-coverage representation
04 · Project output
Interpretation, confidence limits and recommendations for targeted confirmation
COMMON QUESTIONS
Frequently asked questions
Does crosshole tomography provide 3D information?
Can the image identify rock fractures?
Are boreholes always necessary?
TECHNICAL REFERENCES
Further technical reading
- US EPA — General Crosshole Procedures
- US EPA — Seismic Borehole Geophysics
- US EPA — Borehole Geophysical Methods
References describe the underlying methods and recommended data-quality principles; interpretation for any particular site must address its own evidence and constraints.
PROJECT ENQUIRIES
Discuss your ground investigation challenge.
GEOOE · GEOORIGIN ENGINEERING LIMITED
Share the site location, investigation objective, expected depth range, available boreholes and decisions the results need to support. We can discuss appropriate technical methods, limitations and project-specific collaboration.