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CMG-3TB
User Manual
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Guralp Systems CMG-3TB User Manual

CMG-3TB
Operator’s guide
Part MAN-BHO-0001
Designed and manufactured by Güralp Systems Limited 3 Midas House, Calleva Park Aldermaston RG7 8EA England
Issue C 2006-11-15
CMG-3TB
Table of Contents
1 Introduction............................................................................................................... 4
1.1 System configuration.......................................................................................... 5
1.2 Digital borehole installations............................................................................. 6
1.3 The hole lock system.......................................................................................... 7
The single-jaw hole lock...................................................................................... 7
The three-jaw hole lock........................................................................................ 9
2 Assembling the instrument.................................................................................... 11
2.1 Unpacking and packing.................................................................................... 11
2.2 Handling notes.................................................................................................. 11
2.3 Assembling the 3TB.......................................................................................... 12
2.4 Disassembling the instrument.......................................................................... 16
2.5 Control units..................................................................................................... 17
The breakout box................................................................................................ 17
Calibration...................................................................................................... 18
Mass locking and unlocking.......................................................................... 19
Centring.......................................................................................................... 19
The handheld control unit................................................................................. 19
Connections.................................................................................................... 20
Signal meter................................................................................................... 20
Calibration and control.................................................................................. 21
Banana plugs.................................................................................................. 21
The inclinometer monitor unit.......................................................................... 21
2.6 Operating the hole lock.................................................................................... 22
Engaging the hole lock....................................................................................... 23
Disengaging the hole lock.................................................................................. 24
Manual operation............................................................................................... 24
3 Installing the 3TB in a borehole............................................................................. 26
3.1 Installing a sensor with hole lock unit............................................................. 26
3.2 Installing a sensor using sand backfill............................................................. 31
Procedure............................................................................................................ 32
3.3 Assembling the winch...................................................................................... 36
3.4 Earthing a borehole sensor............................................................................... 40
Installations with AC power supplies............................................................... 40
Installations with DC power supplies................................................................ 44
External lightning protection............................................................................. 45
3.5 Levelling and centring...................................................................................... 46
3.6 Downhole orientation....................................................................................... 47
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Operator's guide
Installing the Scream! extension....................................................................... 47
Installing the reference instrument................................................................... 47
Measuring the orientation.................................................................................. 48
Applying automatic rotation.............................................................................. 53
4 Calibrating the 3TB................................................................................................. 55
4.1 The calibration pack......................................................................................... 55
Poles and zeroes................................................................................................. 56
Frequency response curves................................................................................ 57
Obtaining copies of the calibration pack........................................................... 57
4.2 Calibration methods......................................................................................... 58
4.3 Calibration with Scream! ................................................................................ 58
Sensor response codes........................................................................................ 62
4.4 Calibration with a handheld control unit........................................................ 62
4.5 The coil constant.............................................................................................. 63
5 Inside the 3TB......................................................................................................... 64
5.1 The sensors....................................................................................................... 64
5.2 The control system........................................................................................... 66
LOCK .................................................................................................................. 66
UNLOCK ............................................................................................................ 67
CENTRE ............................................................................................................. 69
5.3 The feedback system........................................................................................ 70
Hybrid feedback................................................................................................. 71
Conventional-response feedback....................................................................... 72
Comparisons....................................................................................................... 72
6 Connector pinouts................................................................................................... 74
7 Specifications.......................................................................................................... 76
8 Revision history...................................................................................................... 78
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CMG-3TB

1 Introduction

The CMG-3TB is a three-axis seismometer consisting of three sensors stacked vertically in a sealed borehole sonde, designed for use in cased boreholes with diameters between 5” / 89 mm and 9” / 229 mm.
The seismometer system is self-contained except for its 12 – 30 V power supply, which is provided through the same cable as the analogue data. Sensor functions such as levelling and mass locking are carried out through a surface control box.
The 3TB's sensors are sensitive to ground vibrations in the frequency range 0.0027 – 50 Hz. It outputs analogue voltage representing ground velocity on balanced differential lines. Each seismometer is delivered with a detailed calibration sheet showing its serial number, measured frequency response in both long and short period sections of the seismic spectrum, sensor DC calibration levels, and the transfer function in poles/zeros notation.
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Operator's guide

1.1 System configuration

The CMG-3 series of seismic instruments share a number of features:
• a modular sensor sonde, which can be fitted with a single-jaw or
three-jaw holelock mechanism as required,
• a pit head installation including a breakout box, and
• a number of additional, optional control units which may be
connected to the breakout box to perform installation and maintenance tasks.
For example, a borehole or pit installation of a CMG-3TB or 3ESPB instrument with single-jaw hole lock has the following layout:
CMG 3-series instruments are also suitable for installing in boreholes with sand backfill. In this case no hole lock unit is necessary.
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CMG-3TB
The CMG-3V sensor is identical to the vertical-component module of the 3TB instrument, allowing you to build mixed arrays of 3V and 3TB sensors with identical response characteristics.

1.2 Digital borehole installations

The Güralp DM24 digitizer is available in a borehole sonde form. Connecting a Güralp borehole instrument to a downhole digitizer allows you to construct a true digital borehole installation. This has several advantages over a traditional borehole setup:
• Digital signals are not subject to attenuation as they travel up to
the surface, so signals received are stronger and more reliable.
• Digitizing the data at source allows you to ensure that its origin
can be reliably traced.
• The DM24 digitizer may also be combined with an
Authentication Module within the borehole sonde, allowing you to generate cryptographically-signed data at the point of origin.
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Operator's guide
A digital borehole installation can be provided with RS232, RS422 or fibre-optic links to the surface, depending on the depth of the borehole.
When a downhole digitizer is present, it takes the place of the strain relief unit in the borehole. The surface unit also takes a slightly different form, with a serial connector allowing you to attach a modem or other communications link. In this type of installation, instead of using the surface unit to pass control signals to the sensor, all functions can be accessed remotely
via
the digitizer.
If you prefer to install a stand-alone digitizer at the surface, it should be connected to the 19-pin
RECORDER
socket of the breakout box.

1.3 The hole lock system

The hole lock clamp unit in a 3TB instrument provides a stable platform for the sensor modules mounted above and below it. It is designed to maintain a positive pressure on the borehole casing over a prolonged period of time without attention, and to fix the sonde in place whilst avoiding transmitting any stresses.
Güralp Systems hole locks are constructed to order from accurate measurements of your borehole at the depth you wish to install the instrument. Either single-jaw or three-jaw hole lock units can be manufactured.
In installations with sand backfill, or where the instrument rests on the bottom of the borehole, a hole lock may be unnecessary.

The single-jaw hole lock

The single jaw hole lock is the standard option for triaxial borehole instruments. It consists of an active clamp arm and a number of skids or studs on the sonde body. The arm is attached to a compression spring, which forces it to swing out from the sonde and wedge the body against the borehole wall. A serrated steel jaw at the end of the arm provides maximum grip against the borehole casing. The skids or studs and the locking arm together form a multi-point clamp, which aligns the sonde body parallel to the axis of the borehole and holds it firmly in place so that it cannot twist or slip under the influence of ground vibrations.
There are several configurations of skids and studs which can provide a suitable clamp. Either
• the locking jaw pushes two steel skids against the side of the
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CMG-3TB
borehole, providing two line contacts;
• only the tips of the skids come into contact with the borehole,
providing three point contacts;
• a single skid is combined with a pad to provide one line and one
point contact; or
• three studs provide three point contacts.
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Operator's guide
Studs have the advantage of being smaller than skids, but the contact points are very close to each other. You should evaluate the various locking methods available to see which works best in your borehole.
The spring inside the lock provides around 60 kg of force at its locking position. A DC actuator retracts the arm into the body of the lock so that the sensor mechanism can be installed and removed. The actuator consists of a 14 W DC motor with a planetary reduction gearhead, which drives the nut of a ball lead screw through the helical drive gears. The thread of the lead screw is prevented from turning, and so moves linearly when the nut turns.
The motor has a power system separate from that of the sensor, and can be controlled from the surface using a hole lock control unit. Once the sonde is installed, the hole lock control unit may be removed. Without power, the hole lock will not be able to retract, and the sensor will be secured.

The three-jaw hole lock

A three-jaw hole lock is available which gives better grip on the borehole casing, but is bulkier and heavier than the single-arm lock. This is the standard option for uniaxial instruments; it can be installed in boreholes between 3.5” / 89 mm and 7” / 178 mm in diameter.
The three-jaw hole lock consists of a set of three active clamp arms attached to a compression spring, which forces them to swing out from the sonde and wedge themselves against the borehole wall. Serrated steel jaws at the end of each arm provides maximum grip against the borehole casing. This configuration ensures that the sonde body is
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CMG-3TB
held parallel to the axis of the borehole and prevented from twisting or slipping under the influence of ground vibrations.
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Operator's guide

2 Assembling the instrument

2.1 Unpacking and packing

The 3TB seismometer is delivered in a single transportation case, with the sensor system and hole lock mechanism (if ordered) packed separately. The packaging is specifically designed for the 3TB and should be reused whenever you need to transport the sensor. Please note any damage to the packaging when you receive the equipment, and unpack on a clean surface.
The package should contain:
• the seismometer, in sections;
• a cable to join the sensor to the breakout box;
• the breakout box;
• the hole lock control unit;
• a cable strain relief mechanism;
• a Handheld Control Unit (HCU) for monitoring sensor outputs
and calibration, if ordered; and
• a calibration data sheet.
The sensor is securely packed, and you will need to remove most of the foam packing before it can be removed.

2.2 Handling notes

The 3TB is a sensitive instrument, and is easily damaged if mishandled. It will not stand vertically upwards without support, and should not be operated until it has been securely installed in a borehole casing. If you are at all unsure about the handling or
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CMG-3TB
installation of the device, you should contact Güralp Systems for assistance.
• Do not bump or jolt any part of the sensor when handling or
unpacking.
• Keep the sonde sections vertical wherever possible. Carry them
by hand and store in a safe rack. Never drag or roll the sonde.
• Never lay the sonde horizontally whilst the sensors are
unlocked. If the sensor system topples over, you must inform Güralp Systems.
• Keep all the parts of the sensor system protected and clean so
that they can be joined together securely. Store in the original packaging if possible.
• Do not kink or walk on the data cable (especially on rough
surfaces such as gravel), nor allow it to bear the weight of the sensor.
• Do not connect the instrument to power sources except where
instructed.

2.3 Assembling the 3TB

The 3TB is delivered in separate sections, which need to be assembled before the instrument can be installed in a borehole. It is recommended that you perform these steps with the help of at least one other person.
Important: Make sure your environment is clean and dust free before assembling the unit. Stray fibres or particles cause damage to the “O”-ring seals between the components and may render the sensor unusable. Do not remove the protective caps on the ends of each unit until you are ready.
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Operator's guide
1. Ensure that the “O”-ring seals on the hole lock and sensor sections are clean and well greased.
2. Stand the horizontal sensor on the ground with the packing cap at the top, and support it to prevent it from falling over. This can be done either by using an assistant to hold the casing steady, or by strapping it to a support such as a bench leg.
3. Remove the packing caps from the top of the horizontal sensor and the bottom of the hole lock unit. Beneath the caps are connectors for the horizontal components.
4. Hold the hole lock unit above the horizontal sensor and join the
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CMG-3TB
connectors. Ensure that each is connected to its correct counterpart.
hole lock unit at an angle above the horizontal sensor and join the connectors. Ensure that each is connected to its correct counterpart. The wires are fairly short, so you will need a second person to hold the instrument whilst you connect them. Take care not to scratch the other components when attaching the connectors.
5. Align the hole lock unit with the
NORTH
mark on the horizontal sensor housing. Doing this will allow you to check the approximate orientation of the sensors at a glance.
The horizontal sensor consists of two distinct units (the north/south and east/west components), which are supplied already joined together with M3 × 8 cap screws. You should not need to undo this connection.
If you do separate the north/south and east/west components, make sure that both the signal cable and, if present, the pass­through to the key switch (red and green, held in the photograph below) are reconnected when you reassemble the instrument.
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Operator's guide
6. Push the hole lock unit into the horizontal sensor housing, twisting to align the holes.
7. Fit twelve M3×8 cap screws into the holes in the joint flange.
8. When all twelve screws are fitted, begin to tighten them with a ball-ended Allen screwdriver. Tighten evenly, working round the instrument in several passes, until the two sections are securely joined together.
9. The vertical sensor now needs to be attached to the other end of the hole lock. Remove the packing caps from the top of the hole lock–actuator section and the vertical sensor.
10.Hold the vertical sensor above the hole lock–actuator section and connect the two 15-way “D”-type connectors, as before. Ensure that each is connected to its correct counterpart.
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CMG-3TB
11.Push the vertical sensor housing into the hole lock–actuator section, twisting to align the holes.
12.Fit twelve M3×8 screws into the holes and tighten.
13.If you are using a single-jaw hole lock unit, attach skids or studs to the sonde as appropriate for your installation, using the fastenings provided.

2.4 Disassembling the instrument

When the instrument is recovered, you may want to disassemble it. To do this, reverse the steps above, bearing in mind the following points:
• Make sure you only undo the screws that are necessary to
disassemble the instrument, and not the ones which hold each module together. Each joint has several sets of screws holding it together. Only one set from each joint needs to be undone—the set which was added during assembly. For the joint between the vertical sensor and the hole lock, this is the middle set of screws; for that between the hole lock and the horizontal sensors, it is the lower set. The joint between the two horizontal sensors should not be dismantled.
• When you detach one module from the next, do not yank them
apart, since doing this will damage the connectors inside. Insert flat-head screwdrivers either side of the seal, and carefully lever both sides up simultaneously so that the modules remain parallel. You will need someone to support the upper module as you do this.
When the two parts are separated, tilt the upper one to gain access to the connectors, and disconnect them without
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Operator's guide
scratching the other components.

2.5 Control units

The 3TB is operated from the surface through various control units. All the 3TB's functions can be accessed through one or other unit. Most can be removed from the site once the instrument is ready for use.
Some of these control units are optional and may not have been supplied with your installation. Their functions can be duplicated either by applying voltages directly to control lines (see appendixes for pinout information) or through a connected Güralp digitizer such as the CMG-DM24. The DM24 digitizer is able to pass commands to the instrument from a Data Communications Module (DCM) or a computer running Güralp Systems' Scream! software, allowing you to access all of the instrument's functions remotely.

The breakout box

The breakout box is normally placed where the signal cable emerges from the borehole. It provides connectors for attaching the various other control units, supplies power to the instrument and relays output signals to a recorder or digitizer.
• The
SENSOR
connector is a 32-way mil-spec plug, and should be connected to the borehole instrument with the cable provided.
• The
RECORDER
connector is a 26-way mil-spec plug. This
should be connected to an analogue data recorder or stand-alone
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CMG-3TB
digitizer. In systems using downhole digitizers, this is replaced by a 10-way mil-spec serial connector for attaching to a Data Communications Module (DCM), modem or other communications link.
• The
CONTROL
connector is a 26-way mil-spec plug intended for connecting to an external controller or Handheld Control Unit, with the same pin out as the
RECORDER
connector.
• The
POWER
connector is a 10-way mil-spec plug, which should be connected to a source of 12 – 30 V DC power, for supplying to the borehole instrumentation. When operating the hole lock, you should connect the Holelock Control Unit to this connector. Because of the high voltages employed, the hole lock circuitry is entirely isolated from the rest of the electrical systems in the sensor and surface unit; it is not usual to power the sensor whilst using the hole lock.
For deep-borehole installations (over 50 m) we recommend that you use a breakout box with internal line drivers, to ensure that logic signals are reliably transmitted to the sensor. Contact Güralp Systems for advice.
Note:
The breakout box looks very similar to other Güralp breakout boxes. However, its internal wiring is different from that used for some other instruments. For this reason, if you are using several instrument types, you should mark each breakout box clearly so that it is always used with the correct instrument.
Calibration
To calibrate the instrument, the
Calibration enable
line must be activated. This operates a relay which allows a calibration signal to flow through the transducer feedback coil. This provides an extra force acting on the sensor masses, producing a corresponding deflection in the output signal, which can be analysed by a control computer to extract the seismometer's response characteristics.
Most Güralp instruments are manufactured with active-low
Calibration enable
lines. However, instruments with active-high
calibration can be manufactured on request.
Mass locking and unlocking
The 3TB is delivered with its sensor masses locked, so that they will not be damaged in transit. You should lock the masses whenever you
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Operator's guide
need to move the instrument.
To unlock the instrument, hold down the
ENABLE
and
UNLOCK
buttons (or the
UNLOCK
switch on a breakout box) for at least six seconds. The sensor's microcontroller will free the vertical, N/S and E/W sensor masses in turn and ready them for use. Once this is done, the controller automatically starts a centring cycle. If you issue an
UNLOCK
command when the masses are already free, the instrument will attempt to lock the masses first, and then unlock them in sequence as normal.
To lock the instrument, hold down the
ENABLE
and
LOCK
buttons (or
the
LOCK
switch) for at least six seconds. The sensor's microcontroller will lock the vertical sensor mass, followed by the N/S and E/W sensor masses in turn. After this, the controller locks the base of the horizontal instrument, tilting it until it is held against its end stop. The instrument is now protected against accelerations up to 10g, and is ready for transportation.
Centring
To centre the instrument, hold down
ENABLE
and
CENTRE
buttons
(or the
CENTRE
switch) for at least six seconds. If the masses are locked, the microcontroller will do nothing. Otherwise, it attempts to zero the output of the vertical, E/W, and N/S sensors in sequence by exerting a small extra force on the boom. For the vertical sensor, a motor-driven adjuster presses a small spring lever against the boom until the mass position output indicates an offset close to zero. In the case of the horizontal sensors, the sensor frame is tilted on its base plate. Again, the controller monitors the mass position sensor and stops the centring process once it reaches its lowest offset.
After successful centring, the mass position outputs should be in the range 0.1 – 0.8 V. If the centring process leaves the mass position outputs above ±1.1 V, start another centring cycle. You will probably need to perform several rounds of centring before the masses are ready.

The handheld control unit

This portable control unit provides easy access to the seismometer's control commands, as well as displaying the output velocity and mass position (
i.e.
acceleration) on an analogue meter.
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CMG-3TB
Connections
The HCU provides
• two identical 26-pin connectors for attaching to the
HCU
or
RECORDER
connectors of the breakout box, and
• a 10-pin connector through which you can power the
instrument, if desired. The power pins on this connector are directly connected to those on the
SENSOR POWER
connector of the breakout box. When using this alternative power connection, you should ensure you do not inadvertently connect two power supplies together.
Signal meter
The upper section of the HCU contains a simple voltmeter for monitoring various signals from the instrument.
• To monitor the velocity outputs, switch the dial to
V, N/S
or
E/W VEL
according to the component you want to monitor.
• To monitor the mass position outputs, switch the dial to
V, N/S
or
E/W MASS POS
according to the component you want to
monitor.
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Operator's guide
• You can set the range of the meter with the
RANGE
switch. When switched to 10 V, the meter ranges from –10 to +10 V (as marked.) When switched to 1 V, the range is –1 to +1 V.
Calibration and control
You can calibrate a 3TB sensor through the HCU by connecting a signal generator across the yellow and green
CALIBRATION SIGNAL
inputs and turning the
CAL ENABLE
dial to the component you wish to calibrate. The sensor's response can now be monitored or recorded, and calibration calculations carried out.
The section of the HCU below the calibration lines controls the instrument's mass control system. To initiate locking, unlocking, or centring, hold down the
ENABLE
switch on the HCU
together with
the appropriate switch for the command you want to issue for at least six seconds.
Banana plugs
The remainder of the HCU provides useful connections for each of the signal lines from the instrument, for attaching to your own equipment as necessary.

The inclinometer monitor unit

The borehole sensor system can operate successfully in boreholes with a tilt angle up to 3.5 °. To check that the instrument is installed suitably close to the vertical, a two-axis inclinometer is installed within the sensor housing. The inclinometer monitor unit is used as a visual guide to the sensor's tilt only, and should not be used if precise attitude information is required. You should check the inclination of the instrument
before
unlocking the sensor masses, since too great a
tilt can damage the components.
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CMG-3TB
To measure the attitude of a 3TB instrument:
1. Connect the inclinometer monitor unit to the
CONTROL
connector of the breakout box.
2. Switch the
ON/OFF
switch on the monitor unit to the
ON
position. The inclinometer is powered separately from other parts of the system; this switch provides power to the downhole circuitry as well as to the monitor unit. The inclinometer should not normally be powered up whilst the sensor is in use.
3. Read off the X and Y components of the tilt from the analogue meters.
4. If both tilts are within the green shaded region, the instrument is close enough to vertical that it can be levelled and centred successfully. If either output is in the red shaded region, you should
not
attempt to unlock or centre the sensor masses. Instead, if possible, you should move the instrument within the borehole to a place where it can lie closer to vertical.
If you need to use the outputs of the inclinometer for some other purpose, you can also connect a multimeter to the banana sockets on the inclinometer monitor unit.

2.6 Operating the hole lock

The hole lock, if fitted, can be extended and retracted using the hole lock control unit:
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Operator's guide
Caution: The hole lock may be using high-voltage mains (outlet) power.
1. Connect the hole lock control unit to the
HOLELOCK POWER
connector of the breakout box, and to a mains power supply.
Alternatively
, connect a 12 – 24 V DC power supply across the
input terminals of the hole lock control unit.
Do not connect
both DC and mains power at the same time.
The hole lock control unit supplied in regions with 220 V AC mains power differs from that supplied for 110 V AC mains power. You should ensure that you provide the correct voltage to the hole lock control unit, otherwise damage may result to the sensor.
2. Open the
MASS LOCK / UNLOCK – POWER ON / OFF
flap on
the breakout box, and set the sensor power switch to the
off
position. The hole lock will
only
function whilst the power is off, to avoid injecting current transients from the mains power supply into the sensor electronics.
3. If you are using a deep-borehole hole lock control unit, set the dial to zero.

Engaging the hole lock

To extend the jaw of the hole lock:
1. Hold the switch on the hole lock control unit in the
EXTEND
JAW
or + position. If you are using a deep-borehole control
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CMG-3TB
unit, there will be an additional dial compared to the unit pictured; turn this until the built-in ammeter reads around 0.1 A.
2. When the arm makes contact with the borehole casing, the current will drop slightly. Continue holding the switch in the
EXTEND JAW
position.
3. When the lock arm reaches its fully extended position, the motor will automatically stop and the current will drop to 0 A. If using a deep-borehole unit, return the dial to zero.
4. If the current has not dropped quite to zero after 30 – 40 s of operation, release the switch, wait a few seconds, and push it back to the
EXTEND JAW
position briefly. If the arm is not completely extended, you will see a surge of current. If the current remains constant, the jaw is at its maximum reach.
5. Once the sensor is locked in place, it is recommended that you remove the hole lock power cable and control unit from the site. Without power, the hole lock will not be able to retract, and the sensor will be secure.

Disengaging the hole lock

To retract the jaw of the hole lock:
1. Tension the load bearing cable, to take up any slack.
2. Hold the switch on the hole lock control unit in the
RETRACT
JAW
or – position. If using a deep-borehole control unit, also turn the dial until the built-in ammeter indicates 0.3 – 0.5 A. More current is drawn retracting the arm, because the motor is now working against the spring.
3. When the lock arm reaches its fully retracted position, the motors will automatically stop and the current will drop to 0 A. If using a deep-borehole unit, return the dial to zero.

Manual operation

If you prefer, you can operate the hole lock by applying voltages directly to the sensor.
• To extend the jaw, connect the
Hole Lock Motor
pin on the
sensor (or on the breakout box's
HCU
or
RECORDER
connectors)
to a +12 V power source, and the
Hole Lock Motor Return
pin to
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