Ranger Survey Systems

Using Magnetometers & Accelerometers for Directional Drill–Hole Surveying

Technical Presentation

The Earth’s Gravitational Field• The earth’s gravitational field is defined by a force vector

that points directly down towards the earth’s core.• Every object that has mass will experience an acceleration

towards the core due to the earth’s gravitational field, which is measured in meters / second2 and given the units ‘g’.

• At the earth’s surface, the acceleration of an object due to gravity is equal to approximately 9.8 m/s2. This means that during every second of free–fall, the object’s speed will increase by 9.8 m/s.

• The intensity and direction of the gravitational field will change depending on an objects altitude above the earth or depth below the surface.

• The acceleration that is experienced by an object due to gravity can be measured using special sensors called accelerometers.

• Accelerometers can also be used to measure an objects acceleration in an arbitrary direction, if we have first determined which direction that gravity is pointing.

• So, if we have a survey instrument using accelerometers to determine which direction is straight down, then we can also determine which direction the instrument is pointing and how fast it is moving.

The Earth’s Magnetic Field

• The earth’s magnetic field is generated by processes deep within the earth as a result of electrically charged molten magma flowing around a solid iron core.

• The earth’s field is similar to the field generated around a very large bar magnet as if it were located at the centre of the earth.

• The lines of magnetic flux are drawn exiting from the north end of the bar magnet, looping around the earth, and entering at the south end of the magnet.

• The north end of the bar magnet points towards the earth’s Magnetic South Pole (Antarctic) and the south end of the bar magnet points towards the earth’s Magnetic North Pole (Arctic).

• The earth’s Magnetic North Pole is offset from True North (i.e. the axis of earth’s rotation) by approximately 11 degrees.

• If we know which horizontal direction the field lines are pointing, in relation to where we are on the earth, then we know which direction magnetic north is. This is how a magnetic compass works.

Modelling the Magnetic Lines of Flux

The Earth’s Magnetic Field

• The magnetic poles are defined as the points on the earth where the magnetic field lines have an Inclination of 90 degrees.

• The magnetic field intensity is the strength of the magnetic field at particular point on the earth’s surface.

• The field intensity and pole locations change with both geographical location and time, however these changes happen over many years.

• The magnetic field lines do not run exactly parallel to the planet’s surface, they also follow a vertical angle. This angle is known as Magnetic Inclination (or Magnetic Dip) which varies with latitude.

Magnetic Pole Locations & Field Intensity

• Changes in the magnetic field are monitored at many locations around the world and are updated in real–time on various websites.

***** INSERT WEBSITE LOCATIONS ******

Properties of the Magnetic Field• To measure the Earth's magnetism at a specific location, we must measure both the direction and intensity

of the magnetic field.

• The Earth's magnetic field is described by the following properties:

• Declination (DEC) – The horizontal angle between magnetic north and true north.

• Magnetic Dip (MDI) – The angle between the surface of the earth and the magnetic field lines at a

particular location on the earth. Also called Magnetic Inclination.

• Deviation (DEV) – The error in a compass reading caused by nearby metallic objects.

• Magnetic Intensity (MGI) – The strength of the magnetic field at a particular geographic location

measured in nanoTesla (nT).

• The strength or magnitude of the magnetic field is actually composed of 3 components, a Horizontal North

(X) component, a Horizontal East (Y) component, and a Vertical component (Z).

• All 3 of these components are used to calculate the Total Magnetic Intensity (MGI).

• The parameters describing the direction of the magnetic field are declination (DEC) and Magnetic Dip

(MDI), which are usually measured in units of degrees.

• Azimuth (AZI) is the horizontal angle between the direction in which an object is pointing and the direction

of magnetic north (i.e. compass bearing).

Note: All magnetic readings from Ranger products are related to Magnetic North and it is up to the

Geologist to decide how those readings relate to True North, based upon their location.

The Magnetic Compass• A magnetic compass consists of a small, lightweight magnet, called a

needle, balanced on a nearly frictionless pivot point. One end of the needle is often marked "N" for north, or coloured in some way to indicate that it points toward the earth’s Magnetic North Pole.

• The north end of the needle is magnetised in such a way so that it is attracted to the earth’s Magnetic North Pole (which is actually the south end of our bar magnet in the northern hemisphere).

• Because the needle can only rotate in the horizontal plane, it will line up with the horizontal component of the magnetic field (i.e. parallel to the direction of the magnetic field lines).

Disadvantages of the Magnetic Compass

• The compass relies on the movement of a mechanical object (i.e. the needle) within a very weak magnetic field, so the needle is prone to erratic movements, which means that it is not very stable and will take some time to settle.

• Movement of the needle becomes more erratic the closer the compass moves towards the magnetic poles.

• The magnetic compass is susceptible to vibrations felt through the housing, such as an unsteady hand.

• The value that is read from the compass may be subject to human error.

Fluxgate Magnetometers

• A fluxgate magnetometer consists of a small, magnetically susceptible core wrapped by two coils of wire.

• An alternating electrical current is passed through the input (drive) winding, driving the core through an alternating cycle of magnetic saturation (i.e. magnetised, un–magnetised, inversely magnetised, un–magnetised, magnetised).

Electronically Measuring the Earth’s Magnetic Field

A magnetometer is an electrical device that is used to easily, and accurately, measure the direction and intensity of a magnetic field.

• The constantly changing magnetic field induces an electrical current in the output (sense) winding which can be measured by a detector.

• The extent to which the induced output current, is out of step with the input current will depend on the strength of the background magnetic field.

• The current in the output coil is then converted to an analogue output voltage, which is proportional to the magnetic field strength.

• This voltage can be easily measured by an electronic circuit, converted to the correct format by a microprocessor, and displayed to the user in way that they can understand.

Fluxgate Magnetometers

• In a surveying instrument, the coils are mounted within the casing with one of the fluxgate coils orientated along the axis of the tool.

• The total magnitude (i.e. Intensity) of the magnetic field can be determined by combining the voltage readings from each of the 3 coils.

• Magnetic North is determined by sensing which direction the field is the strongest.• The Azimuth (i.e. Compass Bearing) of the survey instrument can be calculated by

determining the horizontal angle between the axis of the tool and the direction of magnetic north.

Obtaining a Compass Bearing (Azimuth)

• In order to measure the magnetic field vector in 3–dimensional space, a tri–axial fluxgate magnetometer is used, which consists of three fluxgate coils, orientated at right–angles (perpendicular) to each other, with one fluxgate for each axis (i.e. X, Y & Z).

Y

ZX

MA

G 2

MAG 1 MAG 3

MEMS Accelerometers

• An accelerometer is an electromechanical device that will measure acceleration forces that are either static (i.e. gravity) or dynamic (i.e. motion or vibration).

• A Micro–Electro–Mechanical System (MEMS) accelerometer is just a miniaturised version of this device that can be attached to a circuit board.

• An accelerometer behaves as a damped mass on a spring. When the accelerometer experiences an external force such as gravity, the mass is displaced until the external force is balanced by the spring force. The displacement is translated into acceleration.

• In a capacitive type MEMS accelerometer the displacement is converted to a voltage which can be extremely accurate in representing the amount of acceleration if the device is calibrated properly.

• An accelerometer at rest on the Earth's surface will actually indicate 1g upwards along the vertical axis. To obtain the inertial acceleration (i.e. due to motion of the accelerometer alone), this gravity offset must be taken into account.

1. Electrode Anchor

2. Fixed Electrode

3. Intermediate Electrode

4. Displacement Gap & Capacitive Sensor

5. Moving Mass

6. Damped Spring

Measuring Acceleration (Gravitational Intensity)

MEMS Accelerometers

• At each survey point in the drill hole, the direction of the tool is assumed to be unknown because it cannot be determined from any previous reference. This means that the gravitational force vector may be pointing in an unknown direction.

• To determine the direction of gravity in 3–Dimensional space, three accelerometers are needed, one for each directional axis (i.e. X, Y & Z).

• Once the direction of gravity is found using the accelerometers, the values can be used in conjunction with the magnetometer readings to accurately determine the tool’s Azimuth.

• Accelerometer readings are needed when determining Azimuth because in order to extract the horizontal component of the magnetic field, a vertical reference direction is needed.

Measuring Acceleration in 3 Dimensions

+X–X

+Y

–Y

+Z

–Z

• When an accelerometer is rotated within its reference plane, the gravity acceleration in relation to the tilt angle produces a sinusoidal output signal.

• The output signals from all three accelerometers are used to calculate the tool’s inclination.

• The angle of gravitational roll can also be calculated by determining how much the sensitive axis has rotated in relation to the gravity axis.

MEMS AccelerometersInclination & Gravitational Roll

• Because the accelerometers are fixed within the survey instrument, when the tool rotates, the accelerometers rotate by exactly the same amount.

Source : http://www.colibrys.com

• The Inclination angle that the tool makes in relation to the surface of the earth can also be determined, by using the accelerometers as tilt sensors.

Source : http://www.colibrys.com

Mapping the Path of a Drill–Hole• If the change in depth from one survey point to another is known (usually read from the

winch when lowering the tool down the drill–hole), then the path of the drill–hole can be mapped by recording the Azimuth & Inclination readings at each location.

• Before the survey instrument is sent down the drill–hole, the direction of magnetic north, the intensity and dip of the magnetic field, and the inclination of the tool, is determined for a depth of zero.

• A value for the depth interval is stored within the device which tells the tool exactly how far it will be lowered by the surveyor before a new survey shot is taken.Note: The Depth Interval must be entered correctly by the surveyor when conducting the survey for this method to work properly.

• The smaller the depth interval, and hence the larger the number of survey points that are taken, the more accurate the representation of the drill–hole will be. However, the survey will obviously take longer to complete.

• The magnetic field measurements can be used to determine if there are any magnetic materials present, that may introduce errors when conducting the survey. These anomalies will show up as large changes in the magnetic field intensity and dip angle.

Displaying The Results• Once the shots are taken, and the

survey instrument is removed from the drill–hole, the results can be downloaded, and then viewed on a PDA device or personal computer.

• The raw data can also be converted into a 3–dimensional representation of the drill–hole, and displayed on the computer in a way that is not only visually appealing, but easy to understand by people with differing levels of expertise.

Mobile PDA Device

Desktop Computer or Laptop