Question 1

Where does Digital Surveying work?

Accepted Answer

Digital Surveying's footprint covers primarily the African region and extends beyond to the Middle East and Asia.

Question 2

Why should I be using downhole logging on my projects?

Accepted Answer

The cost of drilling boreholes and recovering core is a significant investment. Maximizing the potential of that investment would make the most sense. Downhole logging has the ability to acquire an expanded in-situ physical properties, structural and hydrological data sets from these holes at only a fraction of the initial drilling costs.

Question 3

What does downhole logging data give me that I cannot get from core logs?

Accepted Answer

Geologists love core and for a good reason. Core, however, is not without its short comings:
Its sample volume in relation to the area of interest is small.
In “bad” ground, core is sometimes difficult to log.
Core can be mixed in the core tray.
Core depths can be inaccurate in holes with core loss.
Orientated core required for any orientation of structural features.

Downhole logging can overcome most, if not all, of these short comings in most cases:
Downhole logs generally sample a larger volume of the wall rocks.
A range of in-situ physical property measurements is not always possible from core.
Data can be acquired in zones of broken formation.
Data is acquired as a single run over of the entire borehole length with a calibrated depth measurement.
Orientated structural information without the need for orientated core can be obtained.

Question 4

Are the tools calibrated?

Accepted Answer

Not all of the logging tools require calibration but where required, the calibrations are conducted as per OEM specifications with approved calibration equipment. Calibrations are conducted pre-job and at regular intervals during multi-hole logging operations. All tools also have regular function tests done to assure optimal operability while in the field.

Question 5

How does in-situ physical properties differ from laboratory measurements?

Accepted Answer

When core is removed from the ground it is no longer under the same temperature, pressure or saturation constrains as it was in the ground. This will have an impact on any laboratory measurements conducted. In some cases these differences can be small while in others these can have a marked effect on downstream interpretations. By acquiring in-situ physical property measurements these differences can be quantified and any interpretations validated or discredited. 
Downhole logging in-situ measurements capture data over a larger sample volume over the entire borehole length where laboratory measurements are conducted on core samples (small volume) at discrete depths or intervals.

Question 6

How accurate are the log depths?

Accepted Answer

The industry accepted depth accuracy is 1m per 1000m of borehole depth. The logging system depth is calibrated regularly to account for any wear and tear to keep the depth accuracy within this limit.

Question 7

The log data depths, are they TVD (True Vertical Depth) or along the hole?

Accepted Answer

Downhole logging data is generally presented as depth (distance) along the borehole path.

Question 8

How long does it take to log a borehole?

Accepted Answer

The length of time required to complete the logging of a multi-tool tool suite in any borehole is dependent on numerous factors such as:
The number of tools in the tool suite.
The speed that the individual tool can run at.
In hole conditions or problems.
Time constraints on operations like project mandated work hours.
Access to the borehole.

Tool logging speeds are around 4-6m/min for most tools but tools like the televiewers need to be run at about 2m/min, so it can take a significant amount of time.

Question 9

Does drilling method affect data quality?

Accepted Answer

For majority of the tools deployed there is little to no difference in data quality due to drilling method. The televiewers are one of the tools most affected by changes in rugosity of the borehole wall. Therefore, a smooth diamond drilled (cored) hole will yield significantly better images than a percussion drilled hole with a rough wall. All tools will be affected to some degree by cavities and washouts in the borehole, but these effects are usually localized.

Question 10

What borehole diameters do the logging tools work in?

Accepted Answer

The downhole logging tools deployed by Digital Surveying are designed to work primarily in the hole diameters encountered in the minerals exploration and mining sector, so 60mm to about 160mm. They do however work in larger diameters but do require some changes to deployment procedures to maximize the data quality in the larger diameter holes.

Question 11

Do the logging tools work in cased holes?

Accepted Answer

This is a bit of a yes and no answer and will depend on the tool type being run and the type of casing in the hole. Generally, the tools work best in an open hole, but limited data can be acquired in cased off sections of the borehole.

Question 12

Do logging tools map geology directly?

Accepted Answer

No, downhole logging tools measure different physical properties of the wall rocks, Like density or resistivity. These physical properties can be correlated with geology but are not a direct mapping of the rock type.

Question 13

How is downhole logging data correlated with geology?

Accepted Answer

The downhole logging tools measure physical properties of the formation like density or sonic velocities. These will need to be correlated with the geology as a post log process either as a single or multi-parameter correlation. A simple cross-plot of the physical property versus geology might suffice in some situations while more complex machine learning correlations will be required in others.

Question 14

How difficult is it to work with downhole logging data?

Accepted Answer

Downhole logging data is highly structured data consisting of a depth and a measured value or values for that depth step. These data sets are provided as simple CSV or LAS files and can be imported into all of the major well log processing software or 3D modelling packages used in the mining industry. If data conditioning is required, this can be done in an excel workbook.

Question 15

Can downhole logging data be used for machine learning?

Accepted Answer

A carefully selected tool suite will give a multi-parameter physical properties data set, covering the entire length of the borehole in each borehole. This will give a significant body of data to work with and train a machine learning model. Outputs like geology prediction, grade estimation, identify equipment failures and so forth are achievable. Data quality and consistency are key to any machine learning modelling exercise and should be taken into account when scoping a downhole logging program.

Question 16

Why is a caliper log so important?

Accepted Answer

The 3-arm caliper tool is included in every logging job and runs as the first tool of the tool suite. The caliper measures the internal diameter of the borehole and is considered as a borehole integrity check, to make sure the hole conditions are good enough to risk running the rest of the tool suite. It is also required for processing of the televiewer image data for accurate feature dip calculations.

Question 17

What is the resolution of a televiewer image?

Accepted Answer

Much like a digital camera the televiewer images have a native resolution. In the case of the televiewers the pixels at the borehole wall will vary depending on borehole diameter. The larger the hole the larger the pixel dimensions. In a 76mm hole the pixel size at the borehole wall will be about 2mm X 2mm.

Question 18

What features in the borehole wall can the televiewers map?

Accepted Answer

Acoustic and optical televiewers are reliant on contrast existing between features present in the borehole wall. In the acoustic televiewer, these contrasts are acoustic amplitude contrasts in the reflected signal and in the optical televiewer these would be colour contrasts. Any feature with a significant contrast would be visible. These features could include geological contacts, fractures, joints and borehole breakout.

Question 19

What is the difference between an acoustic televiewer and optical televiewer?

Accepted Answer

The acoustic televiewer uses reflected acoustic pulses to image the borehole wall while the optical televiewer is more like a digital camera with a RGB colour sensor. The acoustic televiewer requires a fluid filled borehole to assist in the acoustic coupling between the tool and the borehole wall while the optical televiewer works best in dry holes.

Question 20

How do televiewers orientate the image?

Accepted Answer

The televiewers have an on-board magnetometer navigation system allowing for the orientation of the acquired images. As this system is magnetometer based, any magnetic affects due to geology, or steel casing, will affect the image orientation over that zone. The navigation system also has accelerometers to measure the tilt of the tool away from vertical.
In magnetically disturbed environments it is advisable to combine the televiewer image data with a gyroscopic deviation survey to remove the effects of the magnetic disturbance.

Question 21

What in-hole conditions can affect televiewer data quality?

Accepted Answer

The televiewers are remarkable tools and cope well with changes in in-hole conditions while logging. There are a few circumstances when these changes will result in degraded image quality.

Acoustic televiewer:
Increased borehole wall rugosity due to drilling method (percussion holes) or washouts will negatively impact the image quality.
Tool eccentricity in the hole due to washouts or cavities will degrade the image quality.
Magnetic disturbances or steel casing will affect image orientation.
Lack of borehole fluids will result in no image data being acquired.

Optical televiewer:
Increased borehole wall rugosity due to drilling method (percussion holes) or washouts will negatively impact the image quality, but not as severely as the acoustic televiewer.
Tool eccentricity in the hole due to washouts or cavities will degrade the image quality.
Magnetic disturbances or steel casing will affect image orientation.
Mudcake on the borehole wall will impact image coverage.
Water turbidity, if logged below water level, will degrade the image.

Question 22

What is the value of core orientation?

Accepted Answer

Core orientation is beneficial in determining an orebody reserve’s estimate. It also improves mine planning, geotechnical mine design, and safety issues. This method provides data for effective discovery. It is also used for planning and extraction. Each orebody structure has its own complexities. The core orientation process affects estimates of the ore body.  The data produced from this process makes core orientation indispensable.

Using core orientation makes exploration drilling more cost effective and accurate. The minerals inside the earth that have formed over many thousands of years have developed at a certain angle. Core orientation is key to understanding this unseen, underground position.

Question 23

How do north seeking gyros orient themselves without a magnetic compass?

Accepted Answer

It utilises a Gyrocompass which is self-calibrated (it has an internal gyro so it does an internal calibration).  It is a non-magnetic compass which is based on a fast-spinning disc and the rotation of the Earth which is used to find the geographical direction automatically. Furthermore, a gyroscope is an important component of a gyrocompass.

Advantages of a Gyro:
It finds the true north which is more useful than the magnetic north.
It is not affected by the ferromagnetic materials which distort the magnetic field.

Question 24

Does a mine survey expert need to be on site to directionally survey boreholes?

Accepted Answer

It all depends on the Mining House and Geology department but in most cases, a mine survey expert is not needed. Instead, drilling contractors in charge of the directional surveys and orientations are on site. The contractors share data with their client after surveys are done.

Question 25

Can survey data be transmitted from the drill site to the data warehouse?

Accepted Answer

Yes, it can. On the product rental side, we make use of the Axis onSite data management system which clients use to transfer data from the tablets after surveys onto the onSite cloud.

Question 26

Can directional survey be conducted during drilling?

Accepted Answer

Surveying can be performed while drilling occurs or after it has been completed. Regardless of which conventional directional survey method is used (Single-shot, Multi-Shot, Gyro tool), the following information is known at the end of a successful directional survey:
Survey Measured Depth
Borehole Inclination
Borehole Azimuth (corrected to relevant North)
Magnetic Field
Magnetic Dip
Northing
Easting
Elevation
Gravity Tool Face
DLS
Latitude
Longitude

DIRECTIONAL SURVEY . DRILLING PRODUCTS
CORE & BOREHOLE LOGGING . GEOSCIENCE . GEOTECH
HYDRO . PETROPHYSICS . PREDICTIVE ASSAY

FREQUENTLY ASKED QUESTIONS

Where does Digital Surveying work?

Why should I be using downhole logging on my projects?

What does downhole logging data give me that I cannot get from core logs?

Are the tools calibrated?

How does in-situ physical properties differ from laboratory measurements?

How accurate are the log depths?

The log data depths, are they TVD (True Vertical Depth) or along the hole?

How long does it take to log a borehole?

Does drilling method affect data quality?

What borehole diameters do the logging tools work in?

Do the logging tools work in cased holes?

Do logging tools map geology directly?

How is downhole logging data correlated with geology?

How difficult is it to work with downhole logging data?

Can downhole logging data be used for machine learning?

Why is a caliper log so important?

What is the resolution of a televiewer image?

What features in the borehole wall can the televiewers map?

What is the difference between an acoustic televiewer and optical televiewer?

How do televiewers orientate the image?

What in-hole conditions can affect televiewer data quality?

What is the value of core orientation?

How do north seeking gyros orient themselves without a magnetic compass?

Does a mine survey expert need to be on site to directionally survey boreholes?

Can survey data be transmitted from the drill site to the data warehouse?

Can directional survey be conducted during drilling?

DIRECTIONAL SURVEY . DRILLING PRODUCTS CORE & BOREHOLE LOGGING . GEOSCIENCE . GEOTECH HYDRO . PETROPHYSICS . PREDICTIVE ASSAY

FREQUENTLY ASKED QUESTIONS

Where does Digital Surveying work?

Why should I be using downhole logging on my projects?

What does downhole logging data give me that I cannot get from core logs?

Are the tools calibrated?

How does in-situ physical properties differ from laboratory measurements?

How accurate are the log depths?

The log data depths, are they TVD (True Vertical Depth) or along the hole?

How long does it take to log a borehole?

Does drilling method affect data quality?

What borehole diameters do the logging tools work in?

Do the logging tools work in cased holes?

Do logging tools map geology directly?

How is downhole logging data correlated with geology?

How difficult is it to work with downhole logging data?

Can downhole logging data be used for machine learning?

Why is a caliper log so important?

What is the resolution of a televiewer image?

What features in the borehole wall can the televiewers map?

What is the difference between an acoustic televiewer and optical televiewer?

How do televiewers orientate the image?

What in-hole conditions can affect televiewer data quality?

What is the value of core orientation?

How do north seeking gyros orient themselves without a magnetic compass?

Does a mine survey expert need to be on site to directionally survey boreholes?

Can survey data be transmitted from the drill site to the data warehouse?

Can directional survey be conducted during drilling?

DIRECTIONAL SURVEY . DRILLING PRODUCTS
CORE & BOREHOLE LOGGING . GEOSCIENCE . GEOTECH
HYDRO . PETROPHYSICS . PREDICTIVE ASSAY