Televiewers for borehole logging in mineral exploration Developments in Borehole Imaging Technology

MARCUS CHATFIELD, from Wireline Workshop, illustrates the incredible transformation that acoustic and optical televiewers have brought to borehole wall imaging in mineral explorations projects.

In recent years, the advent of borehole imagery, captured by acoustic and optical televiewers, and the ability of computers to manage the very large data sets involved, have revolutionised the wireline logging industry. The continuous, orientated, high-resolution representation of the borehole wall it enables offers many advantages to the geologist.

Perhaps this is more comprehensible when the significance of data is considered.

Shortcomings of ‘conventional’ analysis

Typically, data provides knowledge about geology, structure, fractures and stress orientation. Some of this knowledge might be gained by conventional analysis of orientated drill core. However, this option has proved to be time-consuming, imprecise and costly.

The Wireline log as an alternative

In contrast, using the wireline log as an alternative, there is no lost core, the log can be repeated, and processing is straightforward. Consequently, the cost of televiewer imagery per log metre is relatively low.

Incredible convenience

There is no doubt, that televiewer imagery offers incredible convenience.

In this age of information technology, with televiewers objective and precise data, captured in-situ, is transferred directly from the borehole to the 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 measurements but it is fantastic for borehole imagery, which uses all the functionality and capacity of a fast computer. It brings the formation to the geologist’s desk. Geologists love their rocks and have always been a little suspicious of those wiggly lines, but they have embraced the use of image data as a source of important and sometimes critical information.

Dipmeter logging

Borehole televiewers have also emerged as a reliable alternative to the resistivity Dipmeter used in producing tadpole plot.

Usually, most logging technology, in mineral and oilfield exploration, targets sedimentary formations. Thus, there has always been a need to measure dip and direction of strata.

Conveniently, thirty years ago, the resistivity Dipmeter was the answer. The Dipmeter logging involved the correlation of three or four high-resolution resistance logs whose depth and position on the borehole wall was known. The result was a tadpole plot, a braille-like representation of bedding dip orientation.

In mineral exploration, the tadpole plot, as a means of displaying derived structures, has endured. However, the advent of borehole televiewers has made dipmeter logging a rare and specialised event. Essentially, borehole televiewers are used to produce tadpole plot.

The role of borehole televiewers

1. The Acoustic Televiewer

The Acoustic Televiewer describes the borehole wall in terms of hardness, measuring the amplitude of a high-frequency reflected sonic pulse at a very high resolution. It describes the borehole skin rather than the formation beyond. Hard rocks reflect high amplitude signals and soft rocks and fractures reflect low ones. The individual measurements of reflected amplitude are made continuously (send-receive cycles last a few hundred microseconds) by a rotating sonic mirror aligned with a stationary transducer. The result is a map of the borehole wall with a resolution of about 2 millimetres.

• An acoustic televiewer log

The amplitude image on the right shows what the log looks like. The borehole wall is unwrapped with the left 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 for each cycle are measured and mapped in the same way as the reflected amplitudes, resulting in a complete description of 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.

2. The Optical Televiewer

A limitation of acoustic tools is that they only function in fluid-filled boreholes. If the geologist 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 lightsensitive sensor via a conical mirror. Resolution is very high, with pixel sizes down to well below 1 millimetre at HQ borehole diameter.

• An optical televiewer image

The optical televiewer provides an orientated photograph of the borehole wall at high resolution and without perspective...like a virtual drill-core. It is an excellent geological tool. 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 worth cleaning the borehole wall and replacing dirty fluid before logging.

Data processing

Data processing of both acoustical (ATV) and optical (OTV) images is 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. It can be a protracted process, usually taking much longer than data capture. The user might consider contracting a consultant to perform post-processing analysis. After picking and classifying (fracture, fault sedimentary bed, vein etc) is completed, the structure log is orientated in line with the horizontal and true north and displayed as a tadpole plot.

In many cases, both ATV and OTV tools are run which increases the knowledge gained, because the two methods do not measure the same thing, and provide quality assurance. High-level interaction between the geologist and processor/consultant is required to get the best value from the log data.

Each imaging tool has its advantages and disadvantages. For instance, the OTV copes with large-diameter boreholes with rough surfaces far better than the ATV. Borehole conditions play a big part in image quality and that is where the driller should be involved.

And so, the borehole must be prepared to offer the best conditions for the selected methodology. That might mean, where possible, flushing and cleaning the optical televiewer. Using ATV in smooth water-filled diamondcored boreholes results in excellent data and independence from the driller. In all cases, fresh water within smooth borehole walls works best.

The transformation of borehole imaging

Generally, there is no question that the advent of borehole televiewers has transformed how borehole wall imaging tasks are carried out in mineral exploration projects. They bring the convenience of delivering reliable data continuously and as and when needed, costeffectively.

For more information on televiewers for borehole logging, contact Marcus Chatfield on +27 (079) 181 2076