⚡ electrical engineering

Preliminary Subsurface Characterization Using Pseudo-3D ERT and SPT

This study demonstrates that combining standard penetration testing (SPT) with pseudo-3D electrical resistivity tomography (ERT) provides a more reliable subsurface characterization method for difficult terrain, successfully identifying critical features like water-filled voids that SPT alone would have missed.

Original authors: Gerry "Bo" Jr. B, Applegen Joyo

Published 2026-08-06

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Gerry "Bo" Jr. B, Applegen Joyo

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the ground beneath our feet as a giant, invisible puzzle. For centuries, engineers have tried to solve this puzzle using a method called the Standard Penetration Test (SPT). Think of SPT like poking a giant, heavy straw into the dirt to see how hard it is to push through. It's a bit like trying to guess what's inside a wrapped gift by poking it with a stick; you get a good idea of the texture right where you poke, but you might miss a whole empty space or a hidden rock just a few inches away. This works great on flat, easy-to-reach fields, but in mountainous areas, dragging the heavy machinery needed for this "poke" is a nightmare. Plus, if there's a hidden cave or a floating rock right next to your straw, the test might miss it completely, leading to shaky foundations for bridges or roads.

Enter a newer, lighter tool called Electrical Resistivity Tomography (ERT). If SPT is poking with a stick, ERT is like shining a giant, invisible flashlight through the earth. It sends a tiny electric current into the ground and measures how much the soil resists it. Different materials, like wet clay, dry sand, or a water-filled cave, resist electricity differently. By mapping these resistances, scientists can build a 3D picture of what's underground without digging a single hole. The big question scientists have been asking is: Can we trust this "electric flashlight" on its own, or do we need to combine it with the trusty "poke stick" to get the full picture? This is the story of how researchers in the Philippines tried to mix these two methods to solve a tricky road problem.

The Story of the Wobbly Road and the Electric Flashlight

In the mountainous province of Bukidnon, Philippines, there is a stretch of road connecting Bukidnon, Davao City, and Cotabato that has been acting up. It's a "problematic section," meaning the ground underneath is behaving badly, likely causing the road to sink or settle too much. To fix it, engineers needed to know exactly what was hiding beneath the asphalt.

The researchers, Gerry "Bo" Bagtas Jr. and Applegen Joyo, decided to play detective using two different tools. First, they used the old-school method: the Standard Penetration Test (SPT). They drilled a single hole, about 10.5 meters deep (roughly the height of a three-story building), and dropped a heavy 63.5-kilogram hammer from a height of 75 centimeters to drive a sampler into the soil. They counted how many blows it took to push the sampler down. This gave them a detailed look at the soil right next to their hole. They found that the top layers were mostly silt, while the deeper layers were clay. They also found that the groundwater was sitting at a depth of 5.0 meters.

But here's the catch: SPT is like looking through a keyhole. It only tells you what's in that one tiny spot. If there was a giant, water-filled cave right next to the hole, the SPT might have missed it entirely. The hammer might have just hit a hard rock (called a "floater") and stopped, making the engineers think they hit solid bedrock, when in reality, there was a void nearby.

To get a wider view, the team brought out the "electric flashlight": Electrical Resistivity Tomography (ERT). Instead of one hole, they laid out five long lines across the site, each 150.0 meters long. They planted electrodes (metal stakes) every 5.0 meters along these lines. Using a device called the WTS Geophysical Solutions WDA-1, they sent electric currents into the ground and measured how the soil fought back. They used a specific pattern called the "Wenner-Schlumberger array," which is a clever way of arranging the stakes to get a good mix of detail and depth.

The magic happened when they combined the data. They took the five separate 2D slices of data and mashed them together using special software (Res2DInv and Res3DInv) to create a "pseudo-3D" model. It's like taking five flat maps and stacking them to see a 3D landscape.

What They Found

When the researchers compared the "poke stick" (SPT) results with the "electric flashlight" (ERT) results, they found something fascinating. The two methods agreed on the general soil types: the shallow parts were silt, and the deep parts were clay. This gave the team confidence that the electric method was working correctly.

However, the ERT revealed a secret that the SPT completely missed. The 3D model showed a strange, low-resistance area—a "red spot" in their visual map—suggesting a potential water-filled void or cavity. This is a big deal because, in the geology of that area (which includes limestone), caves and cavities are common. If a road is built over a hidden cave, it can collapse or sink.

The SPT results had shown a weird drop in the number of blows needed at the very bottom of the hole, which hinted that the soil was getting weak. But without the ERT, an engineer might have just shrugged and thought, "Oh, the soil is just soft there." They might have missed the fact that the weakness was actually caused by a hidden hole nearby that the drill never even touched. The ERT showed that the drill hole was just next to the cavity, which explains why the drilling didn't lose water (a common sign of hitting a cave) but the soil was still unstable.

The Verdict

The paper concludes that while SPT is still the gold standard for getting detailed, specific data, it has blind spots. ERT is the perfect partner to fill those gaps. By combining them, the team got a much clearer picture of the underground world.

However, the authors are careful not to say this is a magic bullet. They found that while ERT could guess the soil type and strength, the numbers it gave for how strong the soil was (shear strength) were very "conservative." In engineering speak, "conservative" means "super safe but maybe a bit too cautious." If you design a building based only on the ERT numbers, you might build a fortress when a regular house would do, which costs more money. The paper suggests that we need more research to fine-tune the math that turns electric resistance into strength numbers.

So, the final takeaway is simple: In tricky, mountainous, or cave-prone areas, don't just poke the ground. Shine a light on it, too. Using both the heavy hammer and the electric flashlight together gives engineers the best chance to build safe, stable roads without getting surprised by a hidden cave.

Technical Summary: Preliminary Subsurface Characterization Using Pseudo-3D ERT and SPT

Problem Statement
Standard Penetration Testing (SPT) remains the global standard for geotechnical site characterization, mandated by codes such as the National Structural Code of the Philippines (NSCP). However, SPT possesses inherent limitations that compromise its efficacy in specific environments. It relies on heavy, difficult-to-mobilize machinery, making it problematic in mountainous terrains. Furthermore, SPT offers a "narrow" view of the subsurface, limited to the immediate vicinity of the borehole. This narrow resolution creates two critical risks: the failure to detect discrete geologic features like cavities (unless the borehole accidentally intercepts them) and the potential misidentification of "floaters" (large rocks) as bedrock, leading to incorrect depth assessments. While Electrical Resistivity Tomography (ERT) offers a non-destructive, portable alternative capable of generating 2D or 3D subsurface profiles, its application as a standalone characterization method is hindered by inconsistent literature and a lack of established correlations for precise engineering properties.

Methodology
To address these gaps, the authors conducted a geotechnical investigation on a problematic road section in Barangay Omonay, Bukidnon, Philippines—a region characterized by uneven, sloping terrain and Pliocene–Pleistocene lithology (conglomerate, sandstone, mudstone, and limestone). The study employed a hybrid approach combining invasive and non-invasive techniques:

Standard Penetration Test (SPT): A single 10.5-meter-deep borehole (BH-1) was drilled. A 63.5-kg hammer drove a sampler at 1-meter intervals (down to 6.0 m) and 1.5-meter intervals (below 6.0 m). Recovered soil samples underwent laboratory testing for moisture content, particle size distribution, and Atterberg limits to classify soils according to the Unified Soil Classification System (USCS). Empirical relationships were used to correlate SPT N-values with unit weight and shear strength parameters.
Electrical Resistivity Tomography (ERT): Five 150-meter survey lines were established using a Wenner-Schlumberger array with 5-meter electrode spacing. The WTS Geophysical Solutions WDA-1 system was used to measure apparent resistivity.
Data Integration: The five 2D ERT lines were collated into a pseudo-3D dataset using Res2DInv and subsequently inverted using Res3DInv (Aarhaus GeoSoftware) to generate a 3D resistivity model. This model was compared against the SPT data to validate soil classifications and identify anomalies.

Key Results

Soil Classification Consistency: Both methods yielded consistent soil classifications. SPT identified silt (ML) in shallow layers and clay (CH) in deeper sections. The inverted 3D ERT model corroborated this, showing the absence of silt (indicated by red-colored spots) at greater depths.
Cavity Detection: The most significant finding was the detection of a potential water-filled void by the ERT method, which was not intercepted by the single SPT borehole. The ERT model revealed a distinct anomaly (bounded by red broken lines in the 3D slices) that explained the sudden drop in SPT N-values observed at the bottom of the borehole. Without the ERT data, this drop might have been dismissed as weak soil or a measurement error, rather than a structural discontinuity.
Shear Strength Parameters: The study calculated shear strength parameters (cohesion $c'$ and friction angle $\phi'$ ) using both SPT empirical correlations and novel resistivity-based correlations. The resistivity-derived parameters were found to be "highly conservative," particularly for deep-seated clays. While this ensures safety in design, the authors note it may lead to uneconomical engineering solutions.

Significance and Conclusions
The paper concludes that while SPT remains the more detailed and established method for geotechnical investigation, its limitations in inaccessible or cavity-prone (karstic) regions can be effectively mitigated by integrating ERT. The study demonstrates that ERT provides a "wider" view of the subsurface, capable of detecting anomalies that narrow-borehole methods miss.

The authors modestly assert that ERT cannot yet replace SPT as a standalone method due to the need for further verification of void attributes and the lack of robust, site-specific resistivity correlations for engineering properties. However, the combination of both methods yields a more comprehensive subsurface interpretation. The primary recommendation arising from this work is the necessity of follow-up SPT investigations to confirm the extent of discontinuities detected by ERT, ensuring that infrastructure design accounts for potential settlement causes like undetected cavities.

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