When we talk about borehole geophysics, we actually refer to a science that records and analyzes physical property measurements made in test holes and wells. When learning how to do geophysical borehole logging, probes capable of measuring different important properties are lowered inside a borehole. The goal is to collect point or continuous data that is then displayed as geophysical log. Many logs are usually collected because getting data from multiple sources becomes effective, as opposed to relying on a single analysis.
Borehole geophysics are commonly utilized in environmental and ground-water investigations, with the goal of gathering information on rock lithology, fractures, well construction, water quality, porosity, and permeability.
The common geophysical logging system is made out of drawworks, probes, cables, processing modules, power modules, and specific data recording units. The modern systems are nowadays controlled by an advanced computer. It is capable of collecting multiple logs by using a single probe and one pass.
With the use of borehole geophysical logging, we gain much information. We use that information to better understand subsurface conditions that are mandatory for conducting environmental and ground-water studies. The big advantage is that we obtain completely unbiased data that comes from the site surveyed. Generally, the information that we gather is larger and more accurate than when using other drilling samples.
The Most Common Geophysical Logs Utilized
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Caliper logs - These record borehole diameter, which are important for well construction, like drilling-bit or casing size. It is also important for caving or fracturing statistics. Borehole diameter often affects the log response so caliper logs are used to analyze things like flowmeter logs interpretation.
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Gamma logs - These record natural gamma radiation levels emitted by rocks around the borehole. Shale- and clay-bearing rocks emit high gamma radiation.
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Spontaneous-potential logs - These record voltages or potentials developed between surrounding fluids or rocks and borehole fluid. The logs are used to determine water quality and lithology and you will only see them applied in mud-filled or water-filled open holes.
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Single-point resistance logs - These record electrical resistance noticed at various borehole points. Generally, when grain size increases, resistance increases. They can also decrease when borehole diameter increases, and when we see increasing fracture density or dissolved-solids water concentration.
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Normal-resistivity logs - These record electrical resistivity when analyzing the surrounding rocks, water, and the actual borehole environment. Typical electrode spacing is 16 inches for the short-normal resistivity. For long-normal resistivity, we see a spacing of 64 inches used between electrodes.
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Fluid-resistivity logs - These record the water's electric resistivity when looking inside the borehole. Fluid resistivity modifications reflect differences noticed in water concentration levels for dissolved solids. The logs are always useful when delineating zones that are water bearing. Also, we see the system used when the borehole has a vertical flow.
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Temperature logs - Pretty explanatory, the logs record borehole water temperature. They are needed to identify vertical flow and delineating the water-bearing areas. Temperature gradients show borehole flow present between zones. Usually, we see a difference of around 1 degree Fahrenheit for every single 100 depth feet.
On the whole, the systems used are very complex but they do help with understanding countless geophysics data.
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