Normally, soil and rocks are examined visually, during drilling, on the basis of detritus or, later, on the basis of a drill core. However, detritus can give a distorted picture of the actual rocks and geological boundaries. Thus, geophysical sondes are highly useful tools for a detailed investigation of geological cross sections, lithologic boundaries, the locations of fractures, and hydrogeological parameters, as well as for assessing the technical state and conformity of a bored well.
Further analysing the logging data opens up a number of possibilities, for example, for stratigraphic studies, correlation of cross sections, assessment of the movement of water in the rocks (the well), and for the calculation of porosity and geotechnical parameters. The measurements are always referenced as accurately as possible to ground level, and continuous data logging ensures an uninterrupted log of the entire cross section.
Geophysical surveys are conducted using various physical methods, the most common ones being electrical, radiometric, seismic, acoustic, and optical methods. Electrical measurements are based on resistivity, apparent resistivity, and electromagnetic induction. Radiometric methods include measurements of naturally occurring gamma radiation (gamma ray logging) and of artificially produced gamma radiation (gamma–gamma logging). Seismic and acoustic methods are used to record the travel time and amplitude of high- or low-frequency sound waves emitted by the utilised sonde. Optical or camera methods are employed to capture a 360° colour image. Borehole inclination and azimuth are measured by means of an accelerometer array and a magnetometer.
The Geological Survey of Estonia (EGT) uses sondes from Robertson-Geo Ltd, which are built and designed specifically for geophysical borehole and well surveying. The manufacturer is a British company of international renown that has been operating for 40 years and whose equipment is manufactured in accordance with ISO 9001 standards. The EGT’s existing equipment has been procured in partnership with the Environmental Investment Centre through their government grant and LIFE IP CleanEST (Development of an integrated water management and its modern tools in Estonia – strategic choices for future) projects.
To achieve the best results, it is necessary to employ a combination of multiple sondes depending on the purpose of the survey. The maximum surveyable well depth for us is 1,000 m.
Geophysical measurement methods and sondes:
Measures continuous changes in the diameter of a borehole by probing the walls of the borehole by means of three sensors. The sonde enables us to inspect the condition of well casings and to identify damaged areas. In uncased sections of a borehole or well, the method is employed to record fractured intervals and caverns in the rocks, including their depths. The measurement accuracy of the sonde is in the centimetre range.
Enables the possibility to determine the lithologic variability of the rocks surrounding a borehole. As different types of rocks and sediments put out different levels of gamma radiation, the radiation can be used to distinguish different rock layers in the cross section, primarily based on the content of clay minerals (235U, Th, 40K), which produce higher levels of gamma radiation.
Gamma radiation from a radiation source excites rocks within a radius of about 15 cm from the wall of the well. Two receivers (LSD at a longer distance and HRD at a shorter distance) measure gamma rays emitted by the radiation source as they are scattered back from the surface of rocks. The denser the rock, the stronger the backscattered gamma radiation signal. Based on the measurement results, it is then possible to determine the geology and lithologic boundaries of the area, as well as to assess cementation and gaps behind the casing pipe. The measurement results are displayed in g/cm³. The density logging sonde uses a removable, shielded Cs-137 gamma radiation source.
Both apparent resistivity and electrical potential measurements allow us to determine the geology and lithologic boundaries of an area by means of an electric current. The propagation of electric current in rocks depends on their porosity and water content (as well as chemical composition). This method can only be employed in uncased boreholes and fluid-filled environments.
Involves recording changes in the above indicators across the water column of a well. Measurements taken in a casing pipe normally yield stable values. Damaged areas where water from another layer flows into the well show up as a clearly distinguishable change in the curve, i.e. a gradient. The first measurement (made in a static water column) is compared with a second one performed after pumping, and the reasons for any changes are analysed.
Allows us to delimit water-rich and water-poor intervals in a borehole or well, i.e. to distinguish aquifers. Measurements are mostly performed under dynamic conditions, i.e. while pumping water out of the well, but also under static conditions in the case of self-flowing groundwater. The sonde enables the possibility to verify the integrity of the casing pipe of a bored well, or to determine areas where water is flowing in or out.
Bathometer with a capacity of 1 litre, which can be used to collect water samples from a depth of up to 1,000 m. Water samples can be collected, for example, from different intervals.
This method is essentially echolocation. The sonde emits a low-frequency (0.5–1.5 MHz) ultrasound signal in a 360° arc and measures the amplitude and time of travel of the backscattered signal. The result is a qualitative picture of the sides of the borehole, showing both the texture of and fractures in the rock. Simultaneously, the inclination and azimuth of the sonde are recorded, allowing us to deduce the dip angle and direction of fractures.
The method offers a side-view of the casing pipe of a bored well for visual inspection of its condition across 360 degrees. Well suited for detecting fractures and caverns in cloudy well water. Detailed analysis allows us to identify the joints between and damaged areas on casing pipes. Measurements can only be performed in fluid-filled environments.
Vertically captures a 360° coloured high-resolution image of the sides of a bored well. The recorded data can be processed to create a 3D image of the borehole and to determine the dip angle and direction (strike) of layers and fractures. The camera sonde is well suited for use in dry wells and in clear water. Not suitable for wells where the water is cloudy or high in suspended solids.
The measured dip angle and azimuth can be used to calculate the true depth of the borehole relative to ground level. The corresponding sensors are used in acoustic and optical camera sondes. Magnetometer-based azimuth measurement cannot be performed in a metal casing pipe.
The arrival of a 20 kHz sound wave is measured by three receivers (at different distances from the transmitter) in a borehole, and the amplitude and time of arrival of P-waves and S-waves are recorded over about 2,000 microseconds. The wave propagation speeds calculated based on the logs can then be used to determine lithology, porosity, degree of fracture, and rock strength and elasticity.
The sonde also allows us to assess the quality of cementation behind casing pipes (CBL – cement bond log). Cementation quality can be assessed based on amplitude: the lower the amplitude, the denser and more filled the area behind the casing pipe.
Geophysical borehole and well surveys mainly yield numerical values associated with an exact depth, which are presented as lines (curves) or images in a graph.
Last updated: 19.05.2023
