Introduction
Borehole geophysics is the science of recording and analysing measurements of physical properties made in wells or test holes (Borehole Geophysics | U.S. Geological Survey, n.d.). Continuous or point data is obtained using probes and processed to produce geophysical logs. The geophysical responses from targets buried in the earth decrease with increasing depth of burial of the body (Gandhi & Sarkar, 2016). Borehole geophysics is used in groundwater and environmental investigations to obtain information on well construction, rock lithology and fractures, permeability and porosity, and water quality. In mineral exploration, these logs are very useful as they can provide valuable information, otherwise lost in zones of poor core recovery and also where boreholes narrowly miss an ore body (Gandhi & Sarkar, 2016). In this article, we give you more details on borehole geophysics.
Why log?
Geophysical logging provides essential information for understanding underground conditions. Geophysical logs provide unbiased continuous and in-situ data and generally sample a larger volume than drilling samples (Borehole Geophysics | U.S. Geological Survey, n.d.). Geophysical logging can be used in many fields:
Delineation of hydrogeologic units
The different hydrogeologic units found in the subsurface display a wide range of capabilities to store and transmit ground water and contaminants. Borehole-geophysical logging provides a highly efficient means to determine the character and thickness of the different geologic materials penetrated by wells and test holes. This information is essential for proper placement of casing and screens in water-supply wells and for characterizing and remediating ground-water contamination.
Definition of ground water quality
The quality of ground water is highly variable and ground-water contamination may be caused by man-made or natural sources. Integration of borehole-geophysics logging with water-quality sampling provides a more complete picture, whether the objective is to develop a water-supply well or remediate a contaminated aquifer.
Determination of well construction and conditions
Wells are the access points to the ground-water system, and knowledge of their construction and condition are important whether they are being used for ground-water supply, monitoring, or remediation. The location and condition of casing and screen can be rapidly evaluated with geophysical logging.
What are the common geophysical logs and how are they used?
Common geophysical logs include caliper, gamma, single-point resistance, spontaneous potential, normal resistivity, electromagnetic induction, fluid resistivity, temperature, flowmeter, television, and acoustic televiewer. A summary of geophysical logging techniques is given below (Gandhi & Sarkar, 2016):
| Types of logs | Parameter | Remarks |
| Gravity | Density | Accurate in situ determination of densities. |
| Magnetic | Magnetic field | Magnetic zonation. |
| ELECTRICAL | ||
| Resistance | Relative resistance | Simple measurement, qualitative analysis only true resistivities can be calculated; porosity, saturation, pore fluid resistivity can be determined. |
| Resistivity | Apparent resistivity | Resistive beds appear thinner. |
| SP | Natural potential | Shale, nonshale discrimination, qualitative. |
| IP | Chargeability | Can pick up disseminated ore mineralization, distinguish between clay zone and brackish water. |
| EM | conductivity | Used in dry holes, where resistivity fails; cannot pick up disseminated zones. |
| RADIOACTIVE | ||
| Natural gamma | Natural radioactive | Clay shale zones easily distinguished, used in dry and cased holes also. |
| Gamma gamma | Density | In situ density can be used only in dry holes. |
Conclusion
Borehole geophysics is a vital tool for understanding subsurface conditions, offering detailed and unbiased data that complements drilling. From delineating hydrogeologic units and assessing groundwater quality to evaluating well construction, geophysical logging plays a crucial role across multiple fields. Utilizing techniques such as electrical, radioactive, and magnetic logging provides valuable insights, especially in areas where traditional methods may fall short. This makes borehole geophysics an indispensable asset in groundwater studies, environmental investigations, and mineral exploration.
Reference
Borehole Geophysics | U.S. Geological Survey. (n.d.). Retrieved January 16, 2025, from https://www.usgs.gov/centers/new-york-water-science-center/science/borehole-geophysics
Gandhi, S. M., & Sarkar, B. C. (2016). Chapter 5—Geophysical Exploration. In S. M. Gandhi & B. C. Sarkar (Eds.), Essentials of Mineral Exploration and Evaluation (pp. 97–123). Elsevier. https://doi.org/10.1016/B978-0-12-805329-4.00012-0
