There are various imaging devices available to the geologist or engineer. These include the micro-scanner, based on electrical resistance, the acoustic televiewer, based on reflected sound, and two optical devices, the downhole camera and the optical televiewer.
The Mineral logger does not normally use the micro-scanner because it is really an oilfield tool or, at least, a tool used when the borehole is filled with drilling mud. It is a centralised tool but the measurement occurs at the borehole wall. There is no need for the signal to cross an annulus filled with mud. It also works well in water-filled mineral boreholes but its cost and size, at least for the moment, does not normally justify its deployment.
For borehole wall imaging, the choice for the mineral geologist is usually ATV or OTV; acoustical or optical televiewer. The OTV offers wonderful resolution, a true colour image and can cope with large rugged boreholes. In many cases, it is first prize. The problem with OTV is that the target borehole wall is often obscured, either by mud adhering to its surface or, below the water table, opaque borehole fluid. The latter can be flushed out and it is usually worth doing just that but cleaning the borehole wall represents a major challenge.
The ATV requires a water-filled environment, which, in shallow boreholes, is a significant limitation. Its image suffers from eccentric positioning of the sonde, a common problem in angled boreholes. The latest tools offer excellent resolution (about 2 millimetres in slim boreholes) but not as high as those produced by the optical version of the televiewer. Its image is blocky in big rough boreholes.
There are some big advantages though. The detector can see through dirty water and mud lining the borehole wall, it can even see through PVC casing. It offers a borehole radius image based on travel time so the log processor can generate an orientated cross-section of the borehole. The acoustic image is very sensitive to the presence of fractures, even in soft coal the contrast is remarkable. For this reason it is often regarded as the better geotechnical log whereas the OTV is the better geological log.
Generally, softer rocks produce a lower amplitude reflection, in this case represented by a darker image (normal practice in the industry). The important exception occurs where the log describes coarse sandstone (in the example below, the coal seam). The rough borehole wall results in some dispersal of the sonic signal and a darker image. This unfortunate exception disqualifies any attempt to create a rock hardness log based on the acoustic image unless, of course, there is no sandstone in the formation.
Quite often, the geologist is advised to run both optical and acoustic televiewers in his borehole. That should result in a more complete analysis as well as full coverage. Logging a 200 metre deep borehole with the OTV sonde will take about two hours. Processing the data captured, to a reasonable level, will take about six hours.
The camera sonde, now with both axial and orientated side-view, is a real-time measurement like OTV and ATV, but the user interacts with the logger during the job. He can be right there looking at the movie and asking the logger to go up, down, look left or right.
Verbal interaction can be recorded (you have to be careful what you say about the client when he is not there).
The camera image is not processed. That means no data processing burden, which makes life a lot simpler but means that there is no structure log (tadpole plot). Apart from its depth and orientation references, the tool merely observes, it does not measure. It has the advantage of being able to look downwards at the borehole into which it is being lowered.
The camera is not used to log structures, bedding direction and fracture frequency, for instance. It is used to check the borehole column for obstructions or the borehole wall for grouting failure or a mine shaft for water ingress and so on.
There is very little in terms of post log processing although the ability to take a still image from the movie, as done here, is always there.
The downhole camera is not directly related to the televiewer sondes but it falls within the mineral logger’s range of imaging services.
MC 2021
