On 10th At British Geological Survey, Murchison House ... notes(EG… · Lecture Slides and Notes...

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Lecture given by Dr. James Tweedie

To the Edinburgh Geological Society and

the Mining Institute of Scotland

On 10th March 2010

At British Geological Survey, Murchison

House, Edinburgh.

Lecture Slides and Notes for publication

NOTE: An article on the early history of Borehole Survey instruments is being prepared for

publication in GeoDrilling International in Summer 2010 based on research carried out for

this lecture.

Dr. James Tweedie,

GeoMEM Ltd., Ralston Business Centre, Newtyle, Angus PH12 8TL.

Tel: +44 (0)1828 650618

Email: [email protected] Web: http://geomem.com

Professional Borehole Surveyors group on LinkedIn:

http://www.linkedin.com/groups?gid=2896260&trk=hb_side_g

Suppliers and developers of geological software; Software training; Borehole

surveying software systems design and expertise; Support and

maintenance.

mailto:[email protected]

http://geomem.com

http://www.linkedin.com/groups?gid=2896260&trk=hb_side_g

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The Slides and Notes:

Slide 2

Why drill?

�To access resources

�To obtain information

Why do we drill?

There are two main reasons:

Firstly: To access and extract obtain resources: This was the initial reason, with

drill holes used to access and extract sub-surface resources such as water, brine

(for salt) and gas/oil.

Secondly: To obtain information about the sub-surface environment. There is a

wide range of information can be gathered which enables us to take decisions on

further work.

These are not mutually exclusive and in many cases are combined. They are also a

simplification as there are a variety of specific requirements in different industries.

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Slide 3

Why Survey?

� To get borehole trajectory (path)

� Hence: Locate information in 3D

> Locational Borehole Surveys <

Why survey boreholes?

To obtain a borehole trajectory (or path) in 3 dimensions.

And so be able to locate ALL the other information obtained in the borehole to 3D

co-ordinates.

This talk is about locational or positional borehole surveys – a brief history, the

importance of surveying, the methods available, considerations about accuracy and

quality control and finally some thoughts for the future.

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Slide 4

Section:� A brief history (drilling and borehole surveying)

Why survey boreholes? Current surveying methods / instrumentsAccuracyTestingQuality ControlSummary and the future ?

Photo courtesy of Dag Billger, Imdex Technology Sweden.

However, first I’d like to deviate (pun intended) a bit from the main themes and go

through a brief and slightly selective history of drilling and borehole surveying.

This came about as I researched the history of surveying instruments – and

discovered a bit more about the older history of drilling.

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Slide 5

A brief history (China)

� 2000 years ago: First percussive drilling (bamboo rods)

� Circa 200 AD: Wells (for brine/salt) to 300m

� Circa 1050 AD: Flexible bamboo cable = greater drill depths

� 1700s: 300-400m wells are routine = industrial

� 1835: Shenghai well reaches 1000m = World first

� No record geological/geophysical investigation or surveying

What we recognise as drilling (as opposed to digging) appears to have started in

Sichuan province in China about 2000 years ago.

The first percussively drilled wells were to access and extract brine. The drills used

were of made of bamboo.

Around 200 AD indications are that wells were being drilled to 300m deep.

The around 1050 AD a major innovation, using thinner, flexible, bamboo “cable”

instead of solid bamboo pipe. This allowed greater depths to be reached as the

“drill string” was lighter with less resistance.

By the 1700s 300-400m deep wells are routine.

In 1835 the Shenghai well reaches 1000m. This is the first in the world to reach

this depth. In the US 500m was deepest well at that time.

There is no record of geological or geophysical investigation (or down hole

surveying) but drill rigs were located based on surface indications for gas and

brine.

Gas and brine transported via bamboo pipes, often 100s of kilometres long. This

represented a fairly large scale drilling and processing industry.

Data from: Ancient Chinese Drilling. Oliver Kuhn (Geo-X Systems), CSEG RECORDER, June

2004, pp39-43. Refers to: Drilling and Gas Recovery in Ancient China, Zhong Changyong and

Huang Jian. Wictle Offset Printing. 1997.

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Slide 6

Bamboo drills

A photo of Zigong in the early 1900s -derricks used to drill and produce brine and gas wells. Could be up to 100m high.

Left and centre image from: Zhong and Huang, 1997 as reproduced in Ancient Chinese Drilling, Oliver Kuhn, CSEG RECORDER, June 2004, pp39-43. Right image from Kuhn, 0. 2004.

“Fish tail” drill bit and bamboo rod

Model of the flexible bamboo

“cable” rods.

These are some pictures from Oliver Kuhn’s article on Ancient Chinese Drilling.

The left hand picture shows the production derricks, known as “Heaven Carts”, in

the Sichuan province in the early 1900’s. You can get some idea of the scale of

these from the single story building in the central middle ground. These could be

up to 100m or 300 feet high. To put this in perspective – this is about 50% higher

than the highest of the pylons proposed for the Beauly-Denny power line.

The middle diagram shows one of many different and specialised bamboo drill

bits. The heavy “fish tail” created the first section of large diameter hole from the

surface. Hollowed logs were used as casing.

The right hand picture shows a model, in the Salt Museum in Zigong, of the

storage/operation wheel for the flexible bamboo cable.

It appears that many of the drilling tool designs we use today were in existence in

China centuries ago.

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Slide 7

A brief history (contd.) “Western”

� 1800s: Percussive drilling / cable tool rig

� 1858/59: Drake: Steam driven cable tool rig, casing, oil!

� 1860s-1880s: Rotary drill developed/introduced

� 1870 onwards: Need to survey recognised, many attempts - Acid etch methods popular

� 1928: Alexander Anderson – research showed extensive deviation at end of holes

� 1925-1930: Sperry-Sun (Pew): SURWEL gyroscope

� 1940s – present: Rapid development of technologies

A Brief history (“Western”): In the West, percussion drilling, in the form of the cable tool rig, was in use

throughout the 1800’s – one wonders if the ideas were originally brought from

China.

Most boreholes would have been for water, however, the major breakthrough for

the drilling industry came in 1859 when Col. Edwin Drake drilled for Oil using a

steam driven cable tool rig, he also used iron casing for the first time. Despite

many problems – both technical and financial, he hit oil at a depth of just under 70

feet or 23 metres. This was a very shallow hole by todays standards but it sparked

a new and very profitable industry. It also provided the impetus for further

advances in drilling technology and operation.

The rotary drill was developed (in France) in the 1860s and was introduced to

drilling industry world-wide over the next two decades.

It was in the 1870s that the need to survey holes was recognised – which suggests

that drilling companies and their clients began to recognise that boreholes deviated.

There were very many survey methods and instruments developed – though the

main one that is remembered and was widely used is the Acid Etch method. Acid

Etch is recorded as being first used by a G. Nolten of Dortmund.

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In 1928 one Alexander Anderson of Fullerton, California, made a systematic study

of borehole deviation and published his evidence. This drove the research into

more practical and safer (than acid etch) methods of surveying. He also designed

orientation apparatus for borehole surveying systems.

Between 1925 and 1930 the Sperry-Sun team was created and developed a

gyroscopic borehole survey instrument upon which the new company was

launched in 1930. And the rest, as they say, is history.

Over the intervening decades to the present the development of technology

continued apace – being, in the main, driven by the petroleum industry. The

appearance of computers and the decreasing size and increasing power of these

systems allowed smaller survey instruments to be made with large data storage and

processing capabilities. The growth of development of down-hole survey

instrumentation has been almost exponential over recent years.

However, many of the ideas and concepts have been in existence much longer….

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Slide 8

“Deep borehole surveys and problems”M H Haddock (Mining & Technical Institute, Coalville, Leicester). (McGraw-

Hill, 1931)“That all deep boreholes deviate—and by deep boreholes we imply all those over 1,000

ft. in extent—is established beyond any doubt, and indeed much shallower boreholes deviate in more or less degree.”

Detailed book: borehole deviation; Surveying methods(10*)/devices; models & de-survey formulae.

* 10 methods: Fluid; Plummet & magnetic needle; Electrical; Pendulum; Photographic; Gyrostatic; Plastic cast; Pricker; Inertia and Seismographic.

“Deep borehole surveys and problems” by M H Haddock was published by

McGraw-Hill in 1931 and gives a great insight into the inventiveness that was

brought to bear to solve the problem of surveying boreholes and provide other

down hole information from the 1870s up to the 1930s.

This book covers the subject in great detail and most designers and developers of

borehole survey instruments today would recognise at least one or two forerunners

of the present range of survey instruments.

Haddock describes ten general types of instrument: Fluid; Plummet & magnetic

needle, Electrical; Pendulum; Photographic; Gyrostatic (gyroscope); Plastic cast;

Pricker; Inertia and Seismographic. I was not aware of most of these – in spite of

being involved in borehole surveying for 30 years !

I have included a couple of illustrations from the book.

The one on the left is a combined Acid Etch and Compass device to measure both

dip and direction. Although I have heard of Acid Etch (though I have never seen or

used it) I had never seen this method of obtaining both borehole angles. The

gelatine was warmed up then allowed to cool and “lock” the compass in place. The

thermos casing allowed slower cooling for deep boreholes. Hydrofluoric acid was

used to etch the glass of the test tube.

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The right hand drawing second illustrates a more complex mechanical device

indicating the detail of the technical diagrams in the book. Other diagrams and

descriptions of operating methods show why the acid etch may have been popular

– simplicity and ease of use – despite being somewhat of a health and safety issue !

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Slide 9

Comments across the decades:“The amount of trouble, litigation and random speculation that could be

avoided by a correct knowledge of the course of deep boreholes is immeasurably great.” M.H, Haddock 1931.

“Generally speaking the present geological engineer does not seem to be enamoured of the highly ingenious and exact suite of post-war instruments, being in many cases content to sacrifice precision to rapidity, ease and cheapness.” M.H, Haddock 1931.

"The amount of drilling for exploration is vast, and this drilling is expensive. Notwithstanding this large investment, the quality of the information obtained from these drilling programmes is relatively poor." and " ... poor detailed knowledge of the orebody location..." Prof. Michael Hood, CTME, 1999.

“we are still accepting borehole surveys with inaccuracies of the order of 10% of the distance down hole.” Anton Wolmarans (De Beers), 2005.

I find it interesting to compare some comments in Haddocks book with more

recent ones which indicate that, perhaps, not a great deal has changed in some

quarters over seven decades.

The first comment from Haddock may ring true for those that remember one of the

reasons given for the invasion of Kuwait by Iraq. Cross border, slant or inclined

drilling !

The last comment by Anton Wolmarans is pretty damning for the mining industry.

If we accepted a 10% inaccuracy in GPS for our cars or walking we’d be up to one

kilometre off target for any 10 kilometre stretch.

This level of inaccuracy would certainly not be accepted by engineers for road

building or most other aspects of construction.

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Slide 10

Section:A brief history (drilling and borehole surveying)

� Why survey boreholes?Current surveying methods / instrumentsAccuracyTestingQuality ControlSummary and the future ?


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Slide 11

Why survey boreholes?

� Drilling is expensive� Borehole data gathering is expensive� All boreholes deviate

Therefore:� Maximise value of data obtained� Must know 3D location of that data

Why do we need to survey boreholes?

Because drilling and the associated data gathering (lithological logging,

geophysics, geotechnical, core orientation, structure, photographic) are expensive

and therefore the value must be maximised. All boreholes deviate – despite what

drillers may claim!

To make use of all other borehole data we must know the location of that data in

3D.

We probably all know “horror” stories of boreholes that haven’t been surveyed and

assumptions made about where they ended up.

One I remember is of the steeply inclined hole that intersected an “infinite”

thickness of coal. An assumption was made that is was still crossing the coal seam

when it had, in fact, deviated and was travelling along the seam. I suspect it may be

apocryphal but it serves as an illustration of assumptions made with un-surveyed

holes.

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Slide 12

Why survey boreholes? (contd.)

“Downstream” operations rely on location of data:� Directional drilling / orientation of wedges

� Planning hole extensions / branches

� Lithological logs and sections / 3D models

� Core orientation / structural analysis

� Geophysical data location

� Extraction planning (reserves, resources)

� Financial planning / Investment

� Engineering – tunnels, grouting

� Blasting – essential for explosive planning

� Geothermal holes / environmental monitoring

“Any non-trivial borehole that is not surveyed is incomplete”

All “downstream” operations need to know the position of the borehole and the

location of data associated with it.

The most immediate is Directional Drilling where the drill head may be steered

based on immediate knowledge of its location. Wedges can be set using survey

instruments.

The ability to plan extensions to a hole or branch from it. The start of a branch

must have known angles and co-ordinates.

Lithological data provide data for cross sections and 3D models of a site and

allows for correctly placed core orientation and hence structure within the model.

The location of anomalies / bodies detected by geophysical and related surveys

will be correctly located.

The 3D model built up then provides the best basis for reserve estimates and

extraction planning which in turn allows for financial and investment planning

based on the best available data.

Other areas are: Engineering – for planning the line of tunnels and in creating

effective grout screens and in blasting where knowledge of the blasthole location is

essential for explosive design and is a requirement under Health and Safety

legislation in many countries.

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Currently geothermal or ground source energy is a rapidly growing technology.

May use vertical holes of about 25 to over 100m and these will deviate. If there are

numbers of holes in any given area then it is essential to know their paths.

Longer environmental monitoring holes would also benefit from being surveyed.

I believe that any non-trivial borehole that is not surveyed is incomplete

– there is a qualifier to this statement which I will add later.

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Slide 13

Section:A brief history (drilling and borehole surveying)Why survey boreholes?

� Current surveying methods / instrumentsAccuracyTestingQuality ControlSummary and the future ?


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Slide 14

Current surveying methods / instrumentsMain methods include:� Electronic multishot (EMS) systems [magnetic]� Gyroscope based (mechanical, laser, MEMS)� Offset / optical (linked stations)� Mechanical (e.g. Tropari / Pajari) [magnetic]

Almost all have data storage on board or transmit data to surface. Storage: Downloaded & processed when tool at surface.

Acid etch : still being used by at least one exploration company within the past decade.

The main surveying methods in current use are:

Electronic multi-shot systems - usually abbreviated to EMS. These have sensors

to measure the earths Gravity and Magnetic field to provide the Inclination and

Direction or Azimuth of the instrument at specific station depths in the hole.

Because they measure the magnetic field they are referred to as Magnetic

instruments and should not be used in areas of continuous or strong magnetic

disturbance.

The measurements are usually taken at discrete points down the hole (say 5 or 10

metres).

Gyroscope systems or inertial navigation. These can use mechanical, optical and

MEMS gyro systems. MEMS stands for Miniature Electro Mechanical System and

is the latest generation of solid state sensor.

It provides continuous measurement as it navigates down the hole and is not

affected by magnetism.

North seeking gyro systems allows surveys to be independent of external data as

they find their own reference direction.

Offset type surveys are usually optical. Measurements are taken at intervals which

need to be interlinked so they are at fixed distances (e.g. 3m).

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They work a bit like a land surveyor sighting from one position to another to create

a survey chain and are not affected by magnetism.

The Fotobor and Maxibor tools are examples of offset optical systems.

Mechanical systems use compass and inclinometer systems. They are generally

single shot (e.g. Tropari / Pajari). The single points can be joined to create a

“multi-shot” survey.

All, apart from mechanical, have onboard computers and data storage or an ability

to transmit data to surface devices.

With onboard storage the data are downloaded and processed when the tool has

returned to the surface and removed from any protective barrels.

To my knowledge Acid Etch was still being used by one UK based mineral

exploration company within the last decade.

There are electronic equivalents available which addresses the safety issue – but at

an initial cost outlay.

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Slide 15

Survey instrumentsAvailable from a number of established companies.

� Electronic Multishot Systems (EMS) (accelerometer / magnetometer) now easy and low cost to manufacture

� Gyroscope systems are current development area: Miniaturisation of both mechanical / optical systems and development of MEMS gyros

� Optical instruments (linked survey) – still popular - low cost, non-magnetic.

Each industry or sector of industry has it’s preferred survey methods – oil & gas usually leaders in development. But now innovative small diameter systems from mining / exploration.

Survey instruments are available from a number of established instrument

developers and manufacturers around the world. These tend to be split into two

sectors: Oil and Gas and Mining/Quarrying/Mineral Exploration.

In particular, in the mining, mineral exploration and related sectors there are

several very similar EMS systems. Mostly because the instruments are relatively

easy and cheap to manufacture and the technology is tried and tested and familiar

to most users.

Gyroscopic systems have been around for nearly 80 years and developments

continue apace with miniaturisation of components and improvements in data

processing techniques.

Over the past 4 years a number of MEMS and similar small gyro systems have

appeared. These cost considerably less than the traditional systems.

There are a number of optical systems available – the Fotobor / Maxibor family

from Reflex being one. These are still popular as easier to use and lower cost, non-

magnetic instruments when compared to gyro systems.

Mechanical single shot are still available but appear to be losing ground to the

multi-shot systems.

The non-magnetic instruments can be run inside casing and drill rods. Magnetic

systems must be run through the bit or in open hole with no magnetic disturbances.

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The Oil and Gas industry has traditionally led the way in surveying developments

and these have usually been adapted for use in mining, mineral exploration,

engineering and similar industries.

There have been a number of instruments developed specifically for the mining

area but did not transfer to petroleum use. The Fotobor family is one example.

However, now some of the companies that were traditionally in the mining sector

are developing slim-line tools for use in Oil and Gas….

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Slide 16

Survey instruments

Transfer of technology from Mining to

Oil & Gas industry !


MEMS gyro systems may represent a lower cost and easier method of surveying

for the Oil and Gas industry.

There is also at least one adapted mining EMS system being used in Oil & Gas

exploration at present.

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Slide 17

Section:A brief history (drilling and borehole surveying)Why survey boreholes? Current surveying methods / instruments

� AccuracyTestingQuality ControlSummary and the future ?


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Slide 18

Accuracy:

“Any non-trivial borehole that is not accuratelysurveyed is incomplete”

Accuracy of survey depends on:� Accuracy of instrument itself – (sensors,

electronics, tolerances)� Operational accuracy: during survey� Calculation process� Location on planet / environment

I will now add the qualifier to my earlier statement:

Any non-trivial borehole that is not accurately surveyed is incomplete

An inaccurate survey may be worse than no survey at all. Without a survey you

know that you do not really know where the hole went. With an inaccurate survey

you may assume that it is accurate with potentially serious consequences.

The accuracy of a borehole survey depends on several factors:

The accuracy of the instrument – that is, its components. These consist of the

sensors and electronics and is defined by the tolerances / limitations of these parts.

There is the operational accuracy (or rather the errors that can be introduced when

running the survey).

The calculation processes themselves may introduce errors or interpolate data.

The location on the planet affects magnetic survey instruments. There are also

other aspects of the borehole environment that can cause loss of accuracy.

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Slide 19

Accuracy:

Instrumental accuracy:� Depends on sensitivity of sensors, electronic

noise, magnetism (magnetometers). Drift in Gyro systems / compensation. Design!

� Usually quoted as result accuracy for the sensor:

Some published accuracy values from instrument

manufacturers.

ND+/-1.0+/-0.2Magnetic

NDN/A+/-0.25Non-magnetic



ND+/-0.3+/-0.2Non-magnetic


0.1%NDNDNon-magnetic


ND+/-0.01+/-0.01Non-magnetic


% hole lengthDirectionInclinationTool type

The theoretical instrumental accuracy can usually be calculated from the known

tolerances and limits of the components. The practical accuracy can be found by

running laboratory tests with the complete unit.

Because the Inclination (or dip) and Direction (or Azimuth) angles are the required

end product the accuracy is usually quoted in these terms.

This table shows some manufacturers quoted accuracies for the angles for a range

of instruments (includes Optical, EMS and Gyro systems).

Sometimes the accuracy is quoted as percentage of hole length or ratio – for

example 1 in 1000.

I’ve deliberately left these tools anonymous.

It must be remembered that these values are probably going to be the best case in

field use - unless the manufacturer has specifically carried out operational field

trials.

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Slide 20

Accuracy:

Operational accuracy depends on several factors:

� Skill, knowledge & experience of operators� Aspects of the borehole (open hole)� Mechanical accuracy (centralisers)� Environmental effects (temperature,

magnetism)� Spacing of measurements

The operation of the survey is where a number of serious inaccuracies can be

introduced.

During operation the accuracy of a survey depends on:

The skill, knowledge and experience of the operators. They should know the

instrument they are using, know how it works and be aware of its strengths and

weaknesses.

[Gyro system introduction to EMS users story ]

Running tools in open holes where debris may cause the instrument to lie

diagonally across the hole, or where vibration against rough walls may cause the

sensors to take longer to “settle” than expected.

Eccentric or misaligned centralisers can cause quite dramatic false deviations.

Instruments may have a permanent bend. (Can be introduced by driving over a tool

with a Landrover !).

Both rapid temperature changes and magnetic disturbances can cause errors. For

example, if an instrument is left in the sun for 30 minutes before being lowered

into a hole 10 or more degrees lower then the rapid temperature change may be too

fast for the tool calibration process to compensate.

The spacing of measurements: A survey at 5m intervals in a highly deviated hole

will calculate more accurately than one at 50 or 100m.

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Slide 21

Accuracy:

Calculation Accuracy:Pre-processing: Usually initial calculations to

obtain Direction / Inclination at measurement points.

Gyroscope (Navigation): Filtering (Kalman) processing for drift, temperature compensation, etc.

Final: De-surveying calculations (point to point):� Tangential� Average angle� Minimum curvature� Least squares

There are usually at least two sections of calculation in surveying:

1) The initial pre-processing which converts the raw sensor data into the angles at

the various down hole stations. This can be more or less complex depending on the

source and quantity of the data. EMS system produce little data (a few kilobytes)

compared with Gyro systems that can produce several 10s or 100s of Megabytes

for a survey.

Each will carry out compensation and filtering of some sort or another and every

calculation will involve some rounding errors in computer maths. However,

generally these are several orders of magnitude less than sensor noise !

However, it may be possible that the manufacturers’ implementation of processes

could be flawed – (early versions of software or new system in development).

2) The final calculations convert the angles into a 3D borehole path – known as de-

surveying there are several methods to do this. Minimum curvature or least squares

are more accurate than Tangential and Average angle as they allow for borehole

curvature rather than straight line interpolation. This is usually partly under end-

user control as it is available in both survey and exploration / mine planning

software. But slightly differing results because of different de-surveying may cast

doubt on the survey. Here knowledge of the methods used is useful.

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Slide 22

Accuracy:

Location / “Attitude”:

� High latitudes: Steep magnetic field = loss of accuracy for Azimuth

� Horizontal surveys: Possible induced magnetic fields in steel rods

These are perhaps less usual sources of inaccuracy for magnetic instruments but

worth considering.

In high latitudes the magnetic field dips steeply. This makes the magnetometer

readings more variable and so less reliable.

The attitude of the survey can cause unexpected effects. A horizontal survey that

cuts the earths magnetic field, east to west, may generate enough of a genetic field

in steel rods that it’s influence is felt by the sensors several metres away. I know of

reports of this from South Africa and Ireland.

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Slide 23

Accuracy:

So, Accuracy relies on three main areas:� Accuracy of instrument, sensors, pre-processing

controlled by manufacturer� Accuracy of final de-surveying method used

(software package)� Accuracy of operation controlled by

surveyors

How do we test accuracy of operation?

In summary:

So, how do we test accuracy of survey operation..

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Slide 24

Section:A brief history (drilling and borehole surveying)Why survey boreholes? Current surveying methods / instrumentsAccuracy

� TestingQuality ControlSummary and the future ?


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Slide 25

Testing:

Testing Survey Instruments (in test holes)� Few or no standard test facilities� Tests either at manufacturers or user sites� Usually in-ground, and tests compared to other instruments.

Good / informative test by Anton Wolmarans(2005): Voorspoed (surface pipe) / De Beers.

The only way to operationally test survey instruments is in test boreholes. These

could be actual drilled holes or artificial constructed holes – usually some form of

fixed pipe.

To my knowledge there are few, if any, standard borehole test facilities around the

world for mining, mineral exploration and engineering purposes.

Test boreholes do exist either at the instrument manufacturers premises or at some

major users sites.

The test holes are usually in ground and tests are comparative with other survey

instruments – usually gyros. It is better if the entire length of the hole could be

independently surveyed.

Such a test was carried out by Anton Wolmarans of De Beers in 2005 at their

Voorspoed site. This was using an artificial borehole had been surveyed along its

entire length using conventional surface surveying.

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Slide 26

Testing

Voorspoed test (2005) – approx 370mScatter of surveyed paths in plan around the Voorspoed test pipe. Largest scatter distance is approximately 75m. Colours show in and out runs of same instrument. Most show good precision (repeatability) without accuracy. Redrawn from Wolmarans (2005)

PVC pipe(test hole)

100 metres

Accuracy varies from 0.1% to 14% of length for different instruments.

PLAN

This is a summary diagram of the Voorspoed trials of different instruments,

including: Gyroscope, Magnetic (EMS) and Optical.

The tools have been rather over-anonymised in this diagram !

Two paths of the same colour show IN and OUT surveys for the same instrument.

There are a number of points raised by these tests:

The precision of most IN/OUT pairs is good – that is, they show agreement.

And most show good agreement with the profile of the test pipe.

One Gyro path is closest to the actual path (so most accurate)

The orange paths are the optical instrument. Whilst the precision is pretty good and

the actual path profile is similar, it is the farthest away from the actual end point.

A rotation clockwise would probably overlay the paths. This is possibly a simple

operator error – this survey type is totally reliant on correct initial (collar) angles

for the survey. If the instrument was not aligned to the collar angles then this error

is possible.

The remaining EMS instrument surveys show some differences – In/Out on one

(green) is widely spaced – possibly due to rapid temperature changes on the In

survey.

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The other is offset by about 20 metres. One EMS instrument was found to be out

of calibration. These are OPERATOR errors (operational and maintenance). There

is also a possibility that the magnetic variation (declination) has not been allowed

for. This happens more often that you’d expect.

So, the results of this test could not be used to indicate that one instrument

performed better than the others but showed clearly that operational and

maintenance issues play a major role in determining survey accuracy.

This was a good test site and a great initiative by Anton Wolmarans and his

associates at De Beers. Unfortunately, the test pipe was removed as mining

operations moved into the area.

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Slide 27

Testing: Present status

� Tests in “Standard” boreholes are useful� Best are “breakthrough” or surface

(Voorspoed). Known end point.� Most existing use known “good” instruments

(gyro) as control (assumption of accuracy)?� Most at instrument manufacturer, or larger

user sites (cost)� But no internationally recognised standard

test sites for various industry sectors.

At present the process of testing depends on a number of “standard” boreholes

provided by manufacturers and some users.

The best tests are “breakthrough” type where both start and end point can be

independently surveyed. Voorspoed allowed a survey of the entire test pipe.

Most existing test holes use known good instruments (usually Gyros) as the control

or reference surveys. This assumes accuracy of these instruments.

Test boreholes cost money to drill – only some manufacturers and larger users can

afford to do this.

However, there are no internationally recognised standard test sites for the

different industry sectors.

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Slide 28

Section:A brief history (drilling and borehole surveying)Why survey boreholes? Current surveying methods / instrumentsAccuracyTesting

� Quality ControlSummary and the future ?


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Slide 29

Quality Control: Typical survey data ...

This screen capture shows a typical set of results for a down hole location survey.

These results are from an EMS survey – using accelerometers and magnetometers.

This has the primary results: Station depth, Dip (Inclination) and Azimuth

(Direction) and the de-surveyed results: Easting, Northing, Elevation.

UpDown and LeftRight are distances relative to a straight line projection from

collar – which can be used to generate a “bullseye” target plot.

The Dip values appear to be pretty reasonable. However, there is something

obviously wrong with the Azimuth to a depth of 62, possibly 65, metres. This

effect shows magnetic disturbance from either casing or magnetic rock. Here the

survey has started from 11m so it seems they surveyor has tried to avoid casing

and the remainder of the effect may be due to magnetic rock.

In this form the survey data is almost useless and results in plots like ….

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Slide 30

Quality Control: A picture is worth ....

The effects ofmagnetic

disturbance

...a thousand numbers!

.. this.

Here are the plots of several boreholes in plan showing the effects of magnetic

disturbance in the upper sections. The lower sections are consistent and probably

reasonably good representations of the path. Though there are one or two “kicks”

lower down which need to be accounted for.

The two NE trending holes show the same paths in the magnetic zone – in a way

this could be a magnetic log !

In this form these, approximately, 100m hole surveys are probably 5-10 m away

from true location and, therefore, cannot be used in this form.

This is a serious quality issue for Magnetic tools in areas with intermittent

magnetic disturbances (they would not be used in areas of known continuous

magnetic disturbance).

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Slide 31

Section view:Paths distorted

because of azimutherrors

These badly incorrect azimuths seriously affect the apparent dips in section.

This gives a good indication of the actual offsets from true end-positions.

Can we do anything about this?

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Slide 32

Azimuth corrected (effects of magnetism removed)

Yes!

This shows one of the previous boreholes with the borehole path corrected by

using an accurate collar azimuth and the first good down hole azimuth to

interpolate the azimuth angles between. Of course, this makes a number of

assumptions and must be based on the surveyors experience. In this case the

accuracy of the survey is still doubtful and must be flagged as a possible quality

issue.

Normally, magnetic disturbances over 2-3 stations can be interpolated fairly safely

to produce good surveys. However, in all cases any change to the data must be

flagged for future reference.

The user is usually alerted to this situation by the processing software and should

be allowed to over-ride the Azimuth calculation.

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Slide 33

Quality Control: Inclination data

Accelerometers / Inclinometers (Inclination / Dip)� In a wide range of Instruments: primary or

supplementary (e.g. in Gyro)� Present generation are pretty accurate –

improving all the time.However: If moving or vibrating then incorrect readings. Easy to spot (high/low gravity).

Again: An operationally induced error.

The present generation of Accelerometers and Inclinometers are pretty accurate

and, when used correctly, will give good results for Inclination.

Because of this and their relatively low cost they are present in a wide range of

instruments either as part of the primary sensor system (such as the Electronic

Multi-shots) or as a supplementary sensors in Gyro and Optical systems.

However, errors can be introduced if the instrument is moving when the reading is

taken. It may not have stopped on station or is vibrating. This is easy to spot in the

data – the gravity should be 1 plus or minus 0.02 (2%). If it is outside these limits

then there is a gravity error. Magnetic calculations depend on this in EMS system

so it affects the Azimuth results as well.

This, again, is an operational error and is, often, the result of impatient drillers.

There is a good paper by Tim Sindle and others** on the subject of borehole

survey accuracy which was presented at the Australian Mining Technology

Conference in September 2006. ** Adding Value to Exploration Boreholes by Improving Trajectory Survey Accuracy. T G

Sindle, I M Mason, J E Hargreaves and J H Cloete. Australian Mining Technology Conference

26 - 27 September 2006

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Slide 34

Section:A brief history (drilling and borehole surveying)Why survey boreholes? Current surveying methods / instrumentsAccuracyTestingQuality ControlSummary and the future ?


- 41 -

Slide 35

Summary: Sources of inaccuracy

� Noise and tolerance of electronics/sensors.

� Pre-processing/filtering processes

� De-surveying calculations

But mainly: � Survey operation and location /

environment

NO NOTES

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Slide 36

Summary: Survey operators

Therefore operators should be:

� Knowledgeable about the survey system(s)

� Recognise system strengths & limitations

� Skilled in operation of instrument

That is: Fully trained, experienced.

NO NOTES

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Slide 37

The future ?� Survey instruments becoming smaller and more

powerful� Gyro systems with EMS sensors. Dual / multi-

purpose systems?� High speed data links for MWD/Directional

drilling?� Professional qualifications for borehole

surveyors?� Professional status / recognition for borehole

surveyors?� International standard borehole survey

instrument test sites (for each industry sector)?

What about the future?

We’re already seeing the component parts of survey instruments becoming smaller

and more powerful.

We may see gyro systems with complete sets of EMS sensors – allowing dual /

multipurpose systems. This could allow continuous magnetic logging as part of a

borehole survey. This is already happening in Cone Penetrometer Test equipment.

Some form of high speed data link between instrument and the surface for MWD

to allow surface controlled directional drilling and realtime surveying.

Perhaps we need professional qualifications for borehole surveyors – drillers

already need qualifications so it seems reasonable to extend the requirements to

surveying.

Extending that idea perhaps there’s scope for a professional organisation for

borehole surveyors – at an international level.

Finally, perhaps it would be useful to set up independent international borehole

survey test sites where objective operating tests could be carried out on

instruments.

-- END --

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