How to Read a Geophysical Survey Report

A geophysical survey report is the foundational document for any serious borehole project. It tells you what is beneath the ground before a single metre of drill rod has turned — and it informs where, how deep, and at what location the borehole should be sited to give the best chance of striking a productive water-bearing zone. But for most property owners, the report arrives as a technical document full of colour-coded cross-sections, resistivity values in ohm-metres, and interpreted subsurface geology that can feel impenetrable.

This article demystifies the geophysical survey report. You do not need a geology degree to understand the key elements, ask the right questions of your geophysicist, and make informed decisions about proceeding to drill. Here is what the document contains, what each element means, and how to read it intelligently.

What a Geophysical Survey Report Contains

A standard geophysical survey report prepared for borehole siting in South Africa typically includes several key components. The exact format varies between geophysicists and survey methods, but most competent reports contain all of the following elements:

• Site description and survey context: A brief description of the property, its location, the date of survey, and the purpose of the investigation — establishing the physical and hydrogeological context for the data that follows.
• Survey method and equipment description: Which geophysical method was used (resistivity traversing, electromagnetic survey, or a combination), what equipment was deployed, and the survey line layout on the property. This section tells you how the data was acquired.
• Raw data profiles: The measured field data — resistivity traverse profiles or electromagnetic response curves — usually presented as line graphs or colour sections for each survey traverse. These are the uninterpreted measurements.
• Interpreted subsurface section: The geophysicist's interpretation of what the raw data means in terms of subsurface geology — identifying distinct layers, anomalies, fractured zones, and conductive or resistive features. This is the most important section of the report.
• Drill target recommendation: A specific recommendation for where to drill, at what depth the water-bearing horizon is expected, and any qualifying comments about confidence, alternative targets, or risk factors.
• Site plan: A map or sketch showing the property, the survey traverses, and the recommended drill point location, allowing the driller to locate the target on the ground.

If your report is missing any of these components — particularly the interpreted section and the drill target recommendation — ask the geophysicist to supply them before accepting the report as complete.

Understanding Resistivity: The Core Measurement

The most widely used geophysical method for borehole siting in South Africa is electrical resistivity tomography (ERT) — also called resistivity traversing. Understanding resistivity is the key to reading the report's central data.

Electrical resistivity is a measure of how strongly a material resists the flow of electrical current. It is expressed in ohm-metres (Ω·m). Different geological materials have characteristic resistivity values:

• High resistivity (typically above 500 Ω·m): Dry, competent rock — granite, quartzite, and other hard crystalline formations in their unweathered, unfractured state. High resistivity means the material is electrically resistant, which typically indicates dry, solid rock with no water-filled pore spaces or fractures.
• Moderate resistivity (typically 50–500 Ω·m): Partially weathered rock, dry sandy soils, or rock with limited moisture content. The exact range depends heavily on the local geology.
• Low resistivity (typically below 50 Ω·m): This is where the interpretation becomes critical and nuanced. Low resistivity can indicate two very different things: water-saturated fractures or weathered zones — which is what you are hoping for — or clay-rich material, which is electrically conductive but not a useful water source.

The colour coding on the resistivity cross-sections in your report usually follows a consistent convention: warm colours (reds and oranges) represent high resistivity, while cool colours (blues and greens) represent low resistivity. A zone of blue or green at depth, surrounded by orange or red, in a hard-rock environment, is a classic visual signal of a potential water-bearing fracture zone.

The Interpreted Subsurface Section

The interpreted section is where the geophysicist converts the raw resistivity data into a working model of what lies beneath the property. This is the most important and most technically demanding part of the process, and it requires both the data analysis and local hydrogeological knowledge to execute correctly.

The interpreted section typically appears as a cross-sectional diagram showing estimated depth on the vertical axis and horizontal distance along the survey traverse on the horizontal axis. Colour-coded or shaded zones represent different interpreted materials — weathered rock, fresh rock, clay layers, fractured zones, and anomalous features of interest.

Key features to look for in the interpreted section include:

• The weathered zone (regolith): The upper portion of the subsurface is typically a zone of chemically weathered material — softer, often clay-rich, and electrically conductive. This zone holds limited borehole potential on its own but is important context for what lies below.
• The weathered-fresh rock interface: Below the regolith, the transition to fresh, competent bedrock is often marked by a resistivity increase. This interface — and the zone immediately below it — is frequently where productive fractures occur in South African hard-rock aquifer systems.
• Fractured or anomalous low-resistivity zones in fresh rock: These appear as localised zones of lower resistivity within a generally high-resistivity bedrock setting. In the correct geological context, these zones represent fractures, faults, or dyke contacts that can carry significant groundwater.
• Dykes and intrusions: Dolerite dykes are common across much of South Africa and appear as distinct vertical or near-vertical features on the interpreted section. Dykes themselves are typically dry — but their margins, where the hot intrusion fractured the surrounding rock as it cooled, are often productive water zones. The geophysicist should identify these specifically.

A well-interpreted section will annotate these features clearly and explain the reasoning behind the interpretation. If the section is simply a colour plot with no explanatory text, ask the geophysicist to annotate and explain the key features before accepting it.

The Drill Target Recommendation: What It Means and How to Use It

The drill target recommendation is the practical output of the survey — the answer to "where should we drill and how deep?" A clear drill target recommendation should specify:

• Location on the property: The target point should be marked on the site plan included in the report, with a description relative to identifiable landmarks (buildings, fence lines, established trees). The geophysicist may mark this physically on the ground during the survey, often with a peg or paint mark.
• Recommended drill depth: An estimated maximum recommended depth, based on where the interpreted water-bearing horizon is expected. This is typically expressed as a range, not a fixed number — for example, "the anomaly is interpreted at 40–60 m depth; recommend drilling to 80 m to penetrate the target zone with sufficient margin." Everest Drilling guarantees the depth of the borehole as quoted and drilled.
• Expected water-bearing horizon: The specific feature — fractured zone, dyke contact, weathered zone interface — that the drill target is aimed at, and at what depth it is expected to intersect.
• Confidence level: A statement of how confident the geophysicist is in the target, often expressed qualitatively (good, moderate, low confidence) and sometimes quantified. This is particularly important where the survey data is ambiguous or where multiple interpretations are possible.

The drill depth in the report is a recommendation, not a guarantee of what will be encountered. The geophysical survey identifies the most likely location and depth of water-bearing features — it cannot see the actual fractures or confirm water is present. The borehole, drilled to the recommended depth at the recommended location, is the test of the interpretation. Depth is site-specific and a geophysical survey is required to determine the appropriate depth for any given site. Drilling depths at Everest Drilling can extend up to 250 m where the survey and geology warrant it.

What to Ask Your Geophysicist

Receiving the survey report is not the end of the conversation. A good geophysicist will welcome questions — and the answers will help you understand the risk profile of the proposed drill programme. The following questions are worth asking in every case:

• "What is your confidence level in this target, and why?" The answer should reference specific features in the data that support the interpretation — not a general reassurance. If the geophysicist cannot explain why the target is compelling, that is important information.
• "Are there alternative targets on the property?" A single-target survey may miss a better location. If the property is large enough, the geophysicist may identify a primary and secondary target — useful if the primary borehole does not perform as expected.
• "Could the low-resistivity anomaly be clay rather than water?" This question tests the interpretation directly. The geophysicist should be able to explain the geological reasoning that distinguishes the two — and where there is genuine ambiguity, should say so rather than overclaiming.
• "Are there any site features that limit the survey confidence?" Overhead powerlines, buried infrastructure, fill material, and certain subsurface geology can reduce the reliability of resistivity data. If these factors are present, ask how they were accounted for.
• "What yield can I realistically expect?" Note that the geophysical survey cannot reliably predict yield — it identifies the structural target, not the volume of water. A geophysicist who gives a confident yield prediction from resistivity data alone is overstepping what the method can deliver. Yield is only established by drilling and airlift or pump testing after the borehole is complete.

Common Misunderstandings About Survey Reports

Several persistent misunderstandings about geophysical survey reports are worth addressing directly, because they affect how property owners interpret the document and make decisions based on it.

Misunderstanding 1: Low resistivity always means water. As described above, low resistivity indicates electrically conductive material — which can be water-saturated rock, but equally can be clay. Distinguishing between these interpretations requires geological context and expertise. A report that says "low resistivity zone interpreted as water-bearing" without explaining why it is not clay should be questioned.

Misunderstanding 2: A good survey guarantees a productive borehole. The survey identifies the best available drill target on a given site. It does not guarantee that the borehole will be productive. Geology is variable, fractures are irregular, and no surface-based method can fully resolve what is at depth. The survey significantly improves the probability of a successful outcome — which is exactly its purpose — but it cannot eliminate the inherent uncertainty of drilling into the subsurface.

Misunderstanding 3: The survey is only needed once, regardless of site area. If a large property has multiple buildings to supply, or if the primary borehole underperforms and a second is being considered in a different location, a new survey traverse over the new area is the correct approach. The survey's data applies to the traverses that were actually measured — extrapolating the interpretation to unsurveyed areas of the property introduces significant uncertainty.

Misunderstanding 4: Depth in the report is exact. The interpreted depth of a target feature has an uncertainty range. The geophysicist will typically express this as a range rather than a single number. When the driller plans the programme, the recommended drill depth accounts for this uncertainty by targeting below the expected horizon — so that if the feature is a few metres deeper than estimated, the borehole still intersects it.

How Everest Drilling Uses the Survey Report to Plan the Drill Programme

When Everest Drilling receives a geophysical survey report prior to mobilising a rig, the report is reviewed to extract the following key information that directly informs the drill programme:

• Target location on the ground: The site plan and the geophysicist's ground markings are used to confirm the drill point. If the marked target location is inaccessible for the rig (proximity to structures, overhead obstacles, slope, or surface type), the driller contacts the geophysicist before mobilising to agree an adjusted location that still captures the target anomaly.
• Recommended drill depth: The quoted drill depth is set at or beyond the geophysicist's recommended depth, ensuring the target horizon is fully penetrated.
• Expected geology and casing programme: The interpreted section informs what the drill is likely to encounter at each depth interval — how deep the weathered zone extends, where hard rock begins, and at what depth the target fracture zone is expected. This allows the casing programme to be planned appropriately for the site.
• Any noted anomalies or risk factors: If the report flags cautions — shallow clay, potential dolerite intrusions that may deviate the borehole, or areas of fill — these are factored into the drilling approach and discussed with the client before work commences.

A geophysical survey report is a technical advisory document, not a guarantee. Everest Drilling treats it as a serious planning input — reading the full report, querying the geophysicist where needed, and designing the drill programme to give the target the best possible chance of delivering the intended outcome.

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