OMER ALON

Petrophysical and electromagnetic properties of carbonate rock units:  Example from the Eastern-Upper Galilee region 

Understanding the petrophysical parameters governing the subsurface is crucial for comprehending the myriad processes influencing its structure. This knowledge is pivotal for addressing complex geological models relevant, among many subjects, to geoengineering and civil projects. This study emphasizes the integration of both borehole and core-log data, ensuring robust parameter calculations and data validation. Moreover, it also aims to provide insights of magnetic field propagation anomalies, particularly in a homogenous and consolidated carbonate substratum in light of the subsurface natural and artificial voids.

For the purposes of the current study, a site in Upper Galilee in northeast Israel was selected, which includes an Eocene limestone sequence, highly fractured. 9 boreholes were drilled in two morphological terraces, into ~22 m and 30 depth, with the VLF-EM measurements carried out in the boreholes to characterize the anomaly of the magnetic field amplitude and phase, while direct petrophysical analysis (magnetic susceptibility, density, Rock-quality designation (RQD), and seismic velocity) measured in core sample from 38 m depth. The drillings were conducted with knowledge of the presence of an excavated tunnel, to examine artificial cavity's impact.

The current study reveals that a high negative correlation (R=-0.7) is observed between RQD to the magnetic susceptibility values. Also, in the comparison made between borehole and core-log data, it was observed that in fair rock a moderate decay is shown in the magnetic amplitude (~-0.1 dBV/m), while in a massive and continuous limestone rock a sharp decay is shown (~-0.7  dBV/m). Indeed, a strong negative correlation (R=-0.78) was found between RQD and the amplitude derivative. Conversely, a positive medium (R=0.5- 0.55) relationship was identified between magnetic amplitude derivative and magnetic susceptibility.

Following VLF-EM measurements, anomalies in the amplitude derivative of magnetic field amplitude emerge, categorizing the subsurface into three stratigraphic sections characterized by repeated anomalies at depths of ~5 m in both terrace and ~12 m and ~18 m in the lower and upper terrace, respectively. Anomalies beyond 18 m suggest a potential localized conductivity variation, creating a secondary magnetic field (72 dBV), which distorts the image of the magnetic anomaly from the lithology variation in same depth (52 dBV), when the hypothesis is that this is a phenomenon created due to the excavated tunnel

These findings underscore the complexity of utilizing vertical magnetic field measurements to assess spatial lithographic structures, and study of the rock conductivity and mineralogical composition should be carried out. However, integrating geophysical surveys with the approach utilized in the current study provides a comprehensive understanding of subsurface conditions, crucial for supporting engineering and geotechnical decision-making.