Since the introduction of slimline borehole televiewers in the early 1990’s, there has been a considerable increase in the use of televiewers across all industries, including mining exploration and production, geotechnical engineering, university research, and groundwater exploration.
With current tools delivering much higher resolution images and faster collection rates, both acoustic and optical televiewers have become mainstream in subsurface investigations and provide valuable information about lithology, structure, fractures, and stress orientation within the borehole.
Below you will find information on how acoustic and optical televiewers work, features of the most technologically advanced Acoustic and Optical Televiewer probes (ATV and OTV), and the most common televiewer applications.
Televiewer Applications and Benefits:
- Detailed and oriented structural information
- Reference for core orientation
- Fracture detection and evaluation
- Breakout analysis
- Detection of thin beds
- Determination of bedding dip
- Orientation of stress tensor
- Lithology and mineralogical characterization
- Casing inspection
- Repeatable and precise
- Continuous high resolution in-situ measurement
- Easy to confirm accuracy by comparing ATV & OTV logs
Which Televiewer is Right for Me?
It is sometimes difficult to decide whether to run an acoustic or an optical televiewer. They offer slightly different wellbore information and work in different borehole conditions. The acoustic tool is an excellent descriptor of rock conditions whereas the optical version provides more lithological detail. The chart below provides summarizes the differences:
Requires fluid filled borehole- mud or water
|Dry or clear water filled borehole|
|Works well in water-based drilling mud. Does not work in oil-based mud. Limited use in polymer-based mud||Does not function in mud or dirty water|
|Borehole Diameter||5 to 76 cm (2 to 30 inches)||5 to 53 cm (2 to 21 inches)|
|Borehole Wall||Image quality is compromised by a very rough borehole wall or thick mud-cake||Image quality is compromised by mud-caked borehole wall|
|Resolution||High Resolution up to 2 mm||High Resolution up to 1 mm|
|Lithology||Some geological information is available related to rock hardness and coarse or fine texture||Full color digital 3D core image|
|Fractures||The log is very sensitive to fractures, especially in hard rocks||Small fractures can be lost in complex imagery|
|Weathered Zones||Describes weathered zones well due to rock hardness sensitivity and acoustic caliper measurement||Does not identify weathered zones very well|
|Breakout||The log describes, measures, and orients breakout caused by the stress regime||The log describes and orients breakout, but not clearly|
Acoustic Televiewer Overview
The acoustic probe generates an image of the borehole wall by transmitting ultrasonic pulses from a fixed transducer with rotating mirror and recording the amplitude and travel time of the signals reflected at the interface between borehole fluid and the formation (borehole wall). A harder borehole surface translates to higher amplitude recorded by the tool, whereas softer surfaces, fractures, and void spaces are recorded at a lower relative amplitude.
Acoustic televiewers work best in boreholes that are smooth, circular, and have a high acoustic impedance contrast between the borehole fluid and borehole wall. A rough borehole wall with significant breakouts and vugs or an oval shaped borehole scatter the acoustic energy weakening the signal recorded by the televiewer’s transducer. Particles in heavier drilling muds, such as oil and some polymer-based muds, both scatter and absorb acoustic energy and cause loss of image resolution. Poor tool centralization can also degrade image quality.
Many modern acoustic televiewers like the QL40-ABI-2G also feature multi-echo capability. This is achieved by digitally recording the reflected acoustic wave train which is analyzed in real-time by the digital signal processor. Algorithms allow the system to detect the reflection from the acoustic window and to separate and classify all subsequent echoes.
For example, if the tool is run in plastic PVC casing, it can record both the echo of the casing and that of the borehole wall. Multi-echoing processing extends the tool’s applications beyond identifying and classifying geology, structures, and fractures, and can be used to measure steel casing thickness and perform corrosion evaluation.
Modern televiewers also use built-in high precision orientation sensors incorporating a 3-axis fluxgate magnetometer and 3 accelerometers allows orientation of the images to a global reference and determination of the borehole’s azimuth and inclination.
For more information and technical specifications, please go to the QL40-ABI-2G product page.
Optical Televiewer Overview
The Optical Televiewer system consists of a completely redesigned optical assembly and electronics, and uses a high-resolution CMOS digital image sensor combined with a fisheye lens and LED lights.
The tool produces an extraordinarily sharp, continuous 360° unwrapped digital picture of the borehole wall, either in air or clear water. Resolution up to 1800 pixels over the borehole circumference can be achieved, making it ideal for lithological, mineralogical and structural analyses.
Optical televiewers function best in either dry or clear water filled boreholes, as cameras function by focusing light onto an image pickup device. If the borehole is fluid filled, it is recommended to run an optical televiewer before any other logging equipment to ensure no sediment is suspended in the borehole fluid. A mud-caked borehole wall also compromises image quality, and it may be necessary to clean the borehole wall or use a flocculant prior to logging depending on the extent and severity of the mud-cake.
A built-in high precision orientation sensor incorporating a 3-axis fluxgate magnetometer and 3 accelerometers allows orientation of the images to a global reference and determination of the borehole’s azimuth and inclination. Gyroscopic data can also be used in the processing of optical or acoustic image data if there is a strong magnetic influence while collecting the well log data (either due to steel casing or lithology).