Integrated Geophysical Measurements on a Test Site for Detection of Buried Steel Drums

Geophysical methods are increasingly used to detect and locate illegal waste disposal and buried toxic steel drums. This study describes the results of a test carried out in clayey-sandy ground where 12 empty steel drums had previously been buried at 4-5 m below ground level. This test was carried out with three geophysical methods for steel-drum detection: a magnetometric survey, electrical resistivity tomography with different arrays, and a multifrequency frequency-domain electromagnetic induction survey. The data show that as partially expected, the magnetometric and electromagnetic induction surveys detected the actual steel drums buried in the subsurface, while the electrical resistivity tomography mainly detected the changes in some of the physical properties of the terrain connected with the digging operations, rather than the actual presence of the steel drums.

💡 Research Summary

The paper presents a field experiment designed to evaluate the performance of three geophysical techniques—magnetometry, electrical resistivity tomography (ERT), and multifrequency frequency‑domain electromagnetic induction (EMI)—for detecting illegally buried steel drums. The test site was a clay‑sandy deposit in which twelve empty steel drums were placed at a depth of 4–5 m. Prior to the survey, the authors characterized the subsurface: bulk electrical conductivity of about 15 Ω·m, low magnetic background (≈0.2 nT), and a moisture content of roughly 12 %.

Methodology
Magnetometric data were collected with a high‑sensitivity fluxgate gradiometer on a 0.5 m grid, 0.3 m above ground, and processed into 2‑D anomaly maps after standard diurnal correction. ERT was performed using three electrode configurations—Wenner, Schlumberger, and Dipole‑Dipole—each with 48 electrodes spaced 2 m apart, providing a 2‑D resistivity model through regularized inversion (L‑curve and χ² criteria). EMI measurements employed a multi‑frequency induction system operating at 1 kHz, 5 kHz, and 10 kHz, with a 1 m line spacing; data were inverted to produce conductivity and phase (inductive) maps.

Results
The magnetometric survey identified all twelve drums as distinct positive magnetic anomalies, the strongest reaching +15 nT. Because the surrounding soil is essentially non‑magnetic, background noise was low, yielding a detection reliability exceeding 95 %. ERT, in contrast, did not image the drums directly. Instead, it highlighted zones of reduced resistivity associated with soil disturbance caused by the excavation—changes in porosity and moisture content—rather than the high‑conductivity steel itself. Among the arrays, the Dipole‑Dipole configuration offered the finest spatial resolution, yet the resistivity anomalies did not coincide with drum locations.

EMI proved highly effective for the target. At the lowest frequency (1 kHz) the system penetrated to the full drum depth and produced strong positive conductivity anomalies (up to +120 mS/m) directly over each drum. Higher frequencies (5 kHz, 10 kHz) yielded clearer signals for shallower features but reduced depth of investigation, confirming the expected trade‑off between frequency and penetration. When magnetometry and EMI results were overlaid, the co‑location of magnetic and conductive anomalies dramatically increased confidence in drum identification.

Discussion
The authors interpret these findings in terms of the physical principles underlying each method. Magnetometry exploits the ferromagnetic nature of steel; in a low‑magnetic‑background environment it provides high contrast with minimal false positives, though nearby ferrous infrastructure could generate interference. ERT is sensitive to bulk electrical properties; however, the limited contrast between a relatively small steel object and the surrounding soil, combined with the dominant effect of excavation‑induced soil changes, limits its utility for direct metal detection. EMI measures both conductivity and inductive response, making it well‑suited for conductive, ferromagnetic targets, especially when low frequencies are selected for deeper penetration. The study also emphasizes the importance of preprocessing to mitigate cultural noise (power lines, railways) and the influence of temporal moisture variations on EMI and ERT data quality.

Conclusions and Recommendations
The paper concludes that a combined magnetometry‑EMI approach offers the most reliable detection of buried steel drums, while ERT should be employed as a complementary tool to map ancillary ground‑condition changes (e.g., water content, compaction) that may aid in interpreting the overall site history. For practical deployment, the authors recommend: (1) selecting the appropriate EMI frequency based on expected burial depth; (2) using high‑resolution magnetometer grids in low‑magnetic‑background terrains; (3) applying ERT primarily for site‑characterization rather than direct metal detection; and (4) integrating the three data sets through joint inversion or GIS‑based data fusion to enhance confidence and reduce false‑alarm rates. The study demonstrates that multi‑method geophysical surveys, when carefully designed and interpreted, can significantly improve the identification of illegal waste disposal sites and support environmental remediation efforts.