Deep-ground and downhole surveys for construction projects

Downhole magnetometry for defined depth envelopes

Downhole magnetometry requires the borehole starting point, creation of the survey path and surveying to be planned together. Under BFR KMR, the first borehole is advanced in sections and surveyed after each section. It is continued only where results show no anomalies. Unexpected resistance stops drilling and triggers a fresh local assessment. Construction and sequencing of neighbouring boreholes depend on the established survey range and borehole grid.

Conclusions apply to the planned borehole geometry and the subsurface envelope it covers. Borehole depth, spacing, arrangement, survey system and backfilling must therefore be defined together before execution.

Measurements are taken at controlled depth intervals and linked to position, final depth and borehole condition. Control surveys and plausibility-checked processing help distinguish instrument offsets, probe movement or local interference from reproducible anomalies.

Drilling and survey records link identifiers, actual coordinates, elevation reference, achieved depth and surveyed path to the depth profiles. Unreached areas, dominant interference and probe-movement restrictions remain identified explicitly. Documentation states the subsurface envelope that can actually be assessed.

Deep-ground surveys along piles and shoring alignments

Piled foundations, sheet-pile alignments, shoring piles, masts and other deep structural elements affect clearly defined working envelopes. Surveys are referenced to the location, inclination, target depth, diameter and construction method of these interventions.

An area-wide survey at ground level does not automatically answer this question. What matters is whether the selected survey geometry covers the subsequent intervention envelope with traceable overlap.

For closely spaced piles or long shoring alignments, assessment checks whether the survey boreholes provide appropriate overlapping coverage. Edge points, changes in alignment and interfaces with existing structures are considered separately because their geometric and magnetic conditions often differ.

The results plan assigns profiles and assessment limits to current pile locations, shoring alignments or anchor corridors. Edge points and geometric gaps remain visible. Changes in position, diameter, inclination or depth are checked against this spatial assessment again.

Establish historical suspicion before the borehole grid

Aerial photographs, maps, previous uses, known finds and existing statements indicate which targets and depth horizons may be relevant. This information determines whether an area-wide borehole grid, individual control points or investigation along a suspect alignment is required.

Historical assessment does not replace technical surveying at depth. It does establish survey objectives, priorities and spatial boundaries, preventing borehole locations being distributed without a reliable investigation question.

Assessment of suspicion may produce different priorities: a uniform area-wide grid, closer spacing around a suspect point or a linear arrangement along a structure. This reasoning makes the borehole plan technically traceable and verifiable for tendering.

The source register, image dates, suspect-area map and data gaps support selection of targets and investigation areas. Historical observations remain distinct from present-day survey findings. Their spatial reference and uncertainty are carried into borehole planning.

Surface surveys as preliminary area investigations

Magnetometry detects ferromagnetic anomalies, but steel, reinforcement and scrap may mask them. It helps define magnetic interference zones and relevant targets before deep survey boreholes are positioned.

TDEM measures electromagnetic responses from conductive objects. Utilities, large metal structures and survey geometry may limit interpretation. Any supplementary use is assessed for the additional information it can provide beyond the magnetic data.

In suitable ground, GPR can show layer boundaries, utility corridors and other structures. High ground conductivity and metal installations may limit conclusions. A radar reflection does not identify explosive ordnance.

Near-surface and downhole investigations address different depth and geometry questions. Combining them is useful where each step reduces a clearly defined uncertainty.

A preliminary survey can also identify areas where utilities, scrap or substantial installations prevent safe or meaningful use of drilling equipment. Alternative points or measures can then be included early in the construction sequence.

Surface profiles, sensor arrangement, positioning data and assessed anomalies are linked to the proposed borehole schedule. Survey gaps, masked zones and potential utility conflicts remain visible. A surface survey showing no anomalies is not extended to the full drilling depth without assessment.

Drilling method, casing and survey path

The drilling method is tailored to ground, groundwater, access, shoring, existing utilities and risk assessment. Casing and internal diameter must permit probe movement along the proposed survey path with little interference.

Utility and cable records are checked before work begins. Backfilling, groundwater protection and subsequent use of the borehole location must also be considered to keep surveying compatible with geotechnical requirements.

We select casing to guide the probe while minimising its effect on measurements. It must subsequently be removable properly or able to remain in the agreed condition. Differences between planned and achieved final depth are stated as actual survey limits, not replaced by calculated values.

The drilling record and casing information document the actual path, final depth, obstacles and condition of the survey path. These are compared with probe profiles and control measurements. Casing removal or retention and backfilling are referenced to the agreed completion condition.

Follow-up measures for survey limitations or anomalies

Ferromagnetic installations, casing, structures, utilities or unsuitable borehole geometry may restrict survey coverage. These limits are identified spatially rather than hidden by a blanket conclusion.

Depending on the situation, follow-up may include additional boreholes, another survey system, target investigation, area clearance, ordnance recovery or supervision tailored to the actual excavation.

Each limitation leads to a project-specific decision: an additional survey borehole, relocation of the structural element, controlled exposure, excavation in layers or reassessment after a design change. This identifies prerequisites still to be met before each deep intervention.

The follow-up decision remains linked to the point identifier, affected depth interval, survey limit and work performed. Exposure or area clearance adds findings and clearance records. Unresolved points and inadequately investigated areas are identified before the next construction activity.

For any subsequent area clearance, the clearance objective, working limits, depth and protective measures are defined. Access and interfering objects may restrict execution. Where surveying during construction is technically suitable, technical investigation and a specialist decision precede each stage of soil removal. Excavation then proceeds in layers under additional visual supervision, with repeated surveying to the proposed formation level.