The same techniques used in geophysical measurements on the surface can also be used for measurements in boreholes. The applications are also the same, including mineral exploration, feasibility studies for construction in bedrock, environmental monitoring and groundwater investigations.
There are many types of equipment and suppliers. NGU has uses equipment from Robertson Geologging Ltd. This equipment consists of a data logger, Robertson Micro Logger, two cable winches and several probes that can log one or more parameters. Data are stored continuously on the PC that is connected to the data logger.
NGU has two winches:
- RG 500 m, standard 4-core cable, can log up to 500 m, 12V DC motor, 150 kg
- RG 2000 m, standard 4-core cable, can log up to 2000 m, 220 V, 1500 W motor, 350 kg
Where the site is accessible by car, the winches can stay in the vehicle. Otherwise, the equipment is transported by tractor, ATV or helicopter.
NGU uses the following probes for logging geophysical parameters in boreholes:
- Temperature conductivity gamma, TCDS. The probe is also referred to as a TCN probe. This probe measures the temperature (°C), electrical conductivity of water (μS/cm) and total natural gamma radiation (API standard). The probe diameter is 38 mm and it can be used in old boreholes with a hole diameter of 46 mm. Logging speed is 3 m/min. Measurement accuracy is +/- 0.5 °C for temperature and +/- 2.5% at 500 μS/cm for conductivity.
- Water Quality Probe, SWQP. (Slim Water Quality Probe). This probe has sensors for temperature (accuracy 0.02 ºC), electrical conductivity in water, water pressure, 0.25% (0-1500 dbar), pH and Eh. It also has sensors for O2 and NO3, but these are difficult to measure and the data is very uncertain. The probe diameter is 50 mm, the measuring speed is 3 m/min and minimum hole diameter is 56-60 mm.
- Resistivity, ELXG, electric log. This probe measures the resistivity (ohmm) in rock. It uses a pole-pole electrode configuration with electrode distances of 0.4 m (SIC) and 1.6 m (LN). Measurements can be made only in water-filled holes. During processing, corrections are made for water conductivity and hole diameter. The diameter of the probe is 45 mm and it can be used in holes of at least 56 mm. Logging speed is 5 m/min. The probe also has a Gamma sensor.
- Self Potential, SP. Self Potential is measured by an integrated part of the resistivity probe.
- Induced polarization, INSP, Induced Polarization probe. This probe sends out pulses of 110 mV (+/-) and the voltage (IP voltage) is measured in time windows after the power is turned off. The relationship between IP voltage and voltage in current time is the IP effect and is measured in %. It can be used to survey electrically conductive minerals (sulfides, graphite) and clay. Is particularly well suited for mapping impregnation ores. This probe can only be used in water. The probe is 44 mm and the minimum hole diameter is 56 mm. Logging speed is 5 m/min.
- Seismic velocity, TRSG (Three Receiver Sonic Gamma). This probe measures the seismic P- and S-wave velocity (m/s) in rock. The measurements require a water filled hole and the probe must be centralized in the hole with centralization springs. Logging speed is 4 m/min and the hole should be logged upward. The probe diameter is 45 mm. The minimum hole diameter when using springs is 76 mm, but it can be used in holes between 56 to 60 mm without springs or with the spring blades taped to the probe.
- Deviation Measurement, BDVS (Bore Hole deviation verticality probe). This probe is used to measure borehole course, direction angle (azimuth) and dip angle (measured from the vertical). Direction angle is measured with a 3-axial magnetometer, 0-360º (+/- 1.5º). The probe cannot be used inside steel casing. The dip angle is measured by three accelerometers, 0-180º (+/- 0.5º), and is not affected by magnetic material. The probe diameter is 48 mm and the minimum bore diameter is 56 mm. Logging speed is 5 m/min. The probe also has gamma sensor. The method is similar to that described under Optical TeleViewer.
- Gamma spectrometry, SGAM. This probe measures levels of Potassium (%), Uranium (ppm) and Thorium (ppm). Total natural gamma radiation is also measured (API standard). A continuous log can be made with a logging speed 1 m/min, or point measurements can be made for 5-6 minutes to map the entire energy spectrum. The contents of K, U and Th in this point are calculated. The diameter of the probe is 60 mm and the minimum bore diameter is 70 mm. The probe can be used in both air and water.
- Magnetic susceptibility, BMSG (Bartington Magnetic Susceptibility Gamma). This probe measures ability of a material to be magnetized, called magnetic susceptibility. This is an undimensioned parametershowing the relationship between the induced magnetic fields and applied external magnetic field in a material. The size is stated in SI units or SI 10-5. The probe diameter is 43 mm and the minimum bore diameter is 56 mm. The probe has gamma sensor and can be used in both air and water.
In addition, NGU following probes:
- Optical TeleViewer, DOPTV (Digital Optical TeleViewer). This probe continuously films every mm of the the walls of the borehole. A light source (LED) illuminates the borehole wall. The resulting image is an unwrapped and oriented image of the internal borehole wall where one can see cracks, rock types and geological structures. The strike, dip and degree of opening can be measured for each crack. Fracture frequency, stereogram, rose charts and borehole deviation are calculated and plotted. The image can also be used to calculate foliation direction. The TeleViewer is used with springs to centralize the camera within the borehole. Minimum borehole diameter is 76 mm. It can be used in air and water, but best pictures are obtained in water. Logging speed is 1 m/min, and the resolution is 360 or 720 pixels around the borehole wall.
- Acoustic TeleViewer, HRAT (High Resolution Acoustic TeleViewer). The acoustic televiewer uses sound instead of light. An ultrasonic pulse is sent towards the borehole wall. The travel time and reflected amplitude of the sound pulse is detected. If a sound pulse reaches a crack, the travel time increases and the amplitude decreases. An "image" of the borehole wall is created from these parameters. As for the optical televiewer, this is a straightened and oriented image where you can see the cracks and geological structures. Fracture frequency, stereogram, rose charts and borehole deviation are calculated and plotted. It is not so easy to see rock boundaries with this televiewer, but cracks are quite visible, often more visible than in the optical televiewer image. A borehole breakout and deformation log can be made from the measuremenst that provides information about the stress conditions in the borehole. The direction of the greatest principal stress can be calculated. It is a prerequisite that the hole is filled with water, but the water does not need to be clear.
- Flowmeter, HRFM (High Resolution Impeller Flow Meter Probe). This probe measures the vertical water flow in boreholes. It measures the rotations per minute (RPM) of a propeller which is lowered or raised in the wellbore, dynamic logging. By measuring both upwards and downwards in a borehole it is possible to map the water flow by detecting changes in rpm. Logging speed is 5 m/min. Water flow can be quantified to l/hour. A static measurement can also be made, such as with pumping over and under a crack.
- Water Sampler, SLWS (Slimline Water Sampler). With this probe water samples can be taken in boreholes, up to 2000 m deep. The sample volume is 0.5 l and the minimum hole diameter is 46 mm. The probe is filled with water as it is lowered in the hole. At the desired depth, the sampler is closed and 0.5 l is retrieved.
Sample log from Arnestad school, Asker.
Geophysical logging was performed in a 675 m deep borehole by Arnestad school. The hole is located within the Oslo graben, and penetrates through the cambrosilurian slates, alum shale and down into the underlying granite. One sees that the alum shale has high gamma radiation and low apparent resistance. The transition to the granite at about 485 m appears clearly on all logs. Calculated porosity is done using Archie's law and will be wrong when resistance is low (alum shale).