In recent years, the advent of borehole imagery and the ability of computers to manage and process the much larger data sets involved, have revolutionised the mineral wireline logging industry.
A continuous, orientated, high-resolution representation of a borehole wall offers many advantages to the rock mechanics engineer. The data provides knowledge about geology, structure, fractures and stress orientation. Most of this knowledge might be gained by analysis of orientated drill-core, but that option has proved to be time consuming, imprecise and costly.
In this age of information technology, objective and precise data, captured in situ, is transferred directly from borehole to computer where it can be stored, processed, analysed and disseminated, literally at the touch of a button. This is good for density, gamma ray and resistivity logs ... the wiggly lines. It is fantastic for borehole imagery, which uses all the functionality and capacity of a modern computer. It brings the formation to the engineer's desk.
A Tadpole Plot
Twenty-five years ago, the resistivity Dipmeter was the only orientated measurement used in a borehole. It involved the correlation of three or four high-resolution resistance logs whose depth and position on the borehole wall were known. The result was a tadpole plot, a braille-like representation of bedding dip orientation. In mineral logging, the advent of borehole televiewers has made dipmeter logging a rare and specialised event; but the tadpole plot, as a means of displaying derived structures, has endured.
The Acoustic Televiewer logs the borehole wall in terms of hardness, measuring the amplitude of a high frequency, reflected sonic pulse at very high resolution. It describes the borehole skin rather than the formation beyond.
An acoustic televiewer log
Hard rocks reflect high amplitude signals, and soft rocks and fractures reflect low ones. Measurements of reflected amplitude are made continuously (send–receive cycles last a few hundred microseconds) by a rotating transducer or, more often in slim tools, a rotating sonic mirror aligned with a stationary transducer. The result is a map of the borehole wall with an individual resolution of about 2 millimetres in ideal conditions. The images above show what the log looks like. The borehole wall is unwrapped with the left edge (and right edge) of the resulting image aligned with magnetic north. Fractures and bedding planes appear as sinusoidal lines where the deepest point on the line is the direction of dip.
Reflection travel times (left image) for each cycle are measured and mapped in the same way as the reflected amplitudes (right image), resulting in a complete description of the borehole cross-section. Tool centralisation is important to ensure similar travel time and signal strength in all directions. Resolution is reduced in large boreholes and/or drilling mud where signal dispersal is a problem.
Because the acoustic televiewer is sensitive to rock hardness and can measure fracture orientations and apertures (lost in drill-core), it has become an important geotechnical tool in both sedimentary and hard-rock environments.
A limitation of acoustic tools is that they only function in fluid-filled holes. If the rock engineer requires data from dry boreholes, the Optical Televiewer should be employed. It measures the colour and shade of reflected light. The borehole wall is lit by a ring of light-emitting diodes (LEDs) on the tool, and reflections are directed to a light-sensitive sensor via a conical mirror. Resolution is very high, with pixel sizes down to well below 1 millimetre at HQ borehole diameter.
The optical device provides an orientated photograph of the borehole wall at high resolution and without perspective ... like virtual drill-core, but describing a larger sample diameter.
An optical televiewer image
The system does not offer a travel time image, and log quality is dependent on clean borehole fluid if it is run below the water table. In slim holes, optical televiewer images can be of such high quality and value that it is usually well worth cleaning the borehole wall and replacing dirty fluid before logging.
Data processing of all three image types, acoustic, optical and electrical, is normally performed in the same way. An empty structure log is placed over the image and populated by manually picked orientations ... sinusoids are fitted over selected events using a mouse. After picking and classifying (fracture, fault sedimentary bed, vein etc.) is completed, the structure log is orientated with respect to horizontal and true north, and displayed as a tadpole plot, rose diagram, polar plot, etc.
During data processing, the log analyst can derive the dip and direction of bedding, layering, joints and faults, as well as fracture condition and aperture. This is a major advantage if you are designing the foundations for a multi-billion dollar power station or dam wall. In fact, it is surely difficult to argue a case for not running the sonde if site investigation boreholes are to be drilled on that type of project. The system offers an in situ, objective, orientated description of the ground that may be analysed, stored, transmitted and shared easily. The user of televiewer data captures:
- The complete picture (no gaps)
- Fluid ingress points (in dry hole)
- Gas ingress points (in wet hole)
None of these benefits is guaranteed or consistently reliable from drill core.
There are limitations to the OTV system which can affect log quality.
Dirty water obscures the target.
In the big diameter hole scenario, it would be necessary to clean out the opaque borehole fluid, otherwise the advantage of the lighting power is lost completely. It is remarkably difficult, even with very bright lights in small diameter holes, to describe the borehole wall through dirty water.
Oily residue or water droplets often become attached to the lense cover.
If the OTV is required to log the dry section of a borehole, the logger should avoid dipping the sonde into the water before logging upwards. Any oily liquid will float to the top of the water and beads of oily water might damage the optical image.
The optical televiewer lighting plant
This usually has only an aesthetic effect on the data ... to be avoided, but not critical to the purpose of running the system.
Magnetic formations seriously affect image orientation.
If a borehole is drilled to intersect very magnetic formations and the borehole is close to vertical, then neither acoustic nor optical images may be orientated. You get a good image and events on it may be identified, classified and counted, but they cannot be orientated.
Orientation of the borehole trajectory depends on the televiewer sonde's navigation sub. The logged trajectory, as well as having value as a navigation log, is used to rotate picked events on an orientated image so as to be aligned with respect to true dip and azimuth.
- The measurement of borehole tilt with respect to the vertical, usually based on accelerometers, is unaffected by magnetism.
- The measurement of borehole azimuth, with respect to magnetic north and later corrected to be with respect to true north, will be damaged by magnetic formations.
- Orientation of the optical or acoustic images to magnetic north (left edge) will be damaged by magnetism confusing the magnetometer set in the navigation sub.
The solution is to orientate the image with respect to the high side of the bore, as determined by the accelerometers. Events will be orientated and may be rotated by reference to a repaired magnetometer-based navigation log, to a gyro-based navigation log or, if necessary, to the estimated trajectory of the borehole.
In magnetic formations, the logger needs:
- A borehole tilt that exceeds 1.5 degrees from vertical (so that it has a high side).
- A reliable navigation log from a gyro sonde or estimation.
Very often in these circumstances, the driller is required, by his client, to drill at an angle rather than nominally vertical. In most cases, where a borehole exceeds 100 metres in depth, the trajectory deviates from vertical anyway.
Questions to be addressed early in the programme:
- Which log is required, OTV, ATV or both?
- If OTV, can any dirty borehole fluid be replaced?
- If ATV, how deep is the fluid level (the sonde needs a water-filled environment)?
- Is the formation expected to be magnetic?
- Should the driller purposely drill non-vertical boreholes?
- Is the logger required to mobilise a gyro verticality system?
Each imaging tool has its advantages and disadvantages. Borehole conditions play a big part in image quality and that is where the driller should be involved. It is important that the borehole be prepared to offer the best conditions for the selected methodology. That might mean, where possible, flushing and cleaning for the optical televiewer. In all cases, fresh water within smooth borehole walls works best.
OTV logging can be frustrating if the logger and his client fail to plan properly and make resources, such as water tankers, available on site.
Wireline Workshop CC