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Guralp Systems CMG-3V Operator's Manual

Triaxial borehole seismometer
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CMG-3V
Triaxial Borehole Seismometer
Part MAN-BHO-0004
Operator's guide
Designed and manufactured by
Güralp Systems
1 – 3 Midas House
Calleva Park
Aldermaston RG7 8EA
U.K.
Proprietary Notice: The information in this manual is
proprietary to Güralp Systems Limited and may not be
copied or distributed outside the approved recipient's
organisation without the approval of Güralp Systems
Limited. Güralp Systems Limited shall not be liable for
technical or editorial errors or omissions made herein,
nor for incidental or consequential damages resulting
from the furnishing, performance, or usage of this
material.
Issued
2004-12-23

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  • Page 1 CMG-3V Triaxial Borehole Seismometer Part MAN-BHO-0004 Operator's guide Designed and manufactured by Güralp Systems 1 – 3 Midas House Calleva Park Aldermaston RG7 8EA U.K. Proprietary Notice: The information in this manual is proprietary to Güralp Systems Limited and may not be copied or distributed outside the approved recipient's organisation without the approval of Güralp Systems...

  • Page 2: Table Of Contents

    CMG-3V Contents 1 Introduction...........................4 1.1 System confguration....................5 1.2 Digital borehole installations..................6 1.3 The hole lock system..................... 7 The three-jaw hole lock....................7 The single-jaw hole lock....................8 2 First encounters........................10 2.1 Unpacking and packing....................10 2.2 Handling notes......................10 2.3 Assembling the 3V.......................11 2.4 Control units........................
  • Page 3 Operator's guide CENTRE ........................53 5.3 The feedback system....................55 Hybrid feedback......................56 Conventional-response feedback................58 Comparisons........................59 Appendix A Connector pinouts..................60 Appendix B Specifcations....................62 December 2004...

  • Page 4: Introduction

    CMG-3V 1 Introduction The CMG-3V is a single-axis seismometer contained in a sealed borehole sonde. 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.

  • Page 5: System Confguration

    For example, a borehole or pit installation of a CMG-3V or 3ESPV instrument with a three-jaw hole lock has the following layout: CMG 3-series instruments are also suitable for installing in boreholes with sand backfll.

  • Page 6: Digital Borehole Installations

    CMG-3V 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.

  • Page 7: The Hole Lock System

    Operator's guide 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. A digital borehole installation can be provided with RS232, RS422 or fbre-optic links to the surface, depending on the depth of the borehole.

  • Page 8: The Single-Jaw Hole Lock

    CMG-3V borehole casing. This confguration ensures that the sonde body is held parallel to the axis of the borehole and prevented from twisting or slipping under the infuence of ground vibrations. 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.
  • Page 9 Operator's guide 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.

  • Page 10: First Encounters

    CMG-3V 2 First encounters 2.1 Unpacking and packing The 3V seismometer is delivered in a single transportation case, with the sensor system and hole lock mechanism (if ordered) packed separately. The packaging is specifcally designed for the 3V and should be reused whenever you need to transport the sensor.

  • Page 11: Assembling The 3V

    Operator's guide 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.

  • Page 12: Control Units

    CMG-3V 2.4 Control units The 3V is operated from the surface through various control units. All the 3V's functions can be accessed through one or other unit. Most can be removed from the site once the instrument is ready for use.
  • Page 13 Operator's guide The SENSOR 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. The HOLELOCK POWER connector is a 10-way mil-spec plug, •...
  • Page 14 CMG-3V Surface interface units In a standard surface control unit, control commands are processed by a microcontroller within the unit, which then drives motors within the 3V sensor. The 3VIC is a variant of the 3V which has an integrated controller located in its own module within the sensor assembly.
  • Page 15 Operator's guide To unlock the instrument, hold down the UNLOCK switch for at least six seconds. The sensor's microcontroller will free the mass and ready it for use. Once this is done, the controller automatically starts a centring cycle (see below). If you issue an UNLOCK command when the mass is already free, the instrument will aiempt t o l ock t he m ass f rst, a n d then unlock it in sequence as normal.

  • Page 16: The Handheld Control Unit

    CMG-3V 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. Connections The HCU provides two identical 26-pin connectors for aiaching t o t he HCU or •...

  • Page 17: The Inclinometer Monitor Unit

    Operator's guide Signal meter The upper section of the HCU contains a simple voltmeter for monitoring various signals from the instrument. To monitor the velocity output, switch the dial to V VEL. • To monitor the mass position output, switch the dial to V MASS •...
  • Page 18 CMG-3V To measure the aiitude of a 3V instrument: 1. Connect the inclinometer monitor unit to the CONTROL connector of the surface control unit. 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;...

  • Page 19: Operating The Hole Lock

    Operator's guide 2.5 Operating the hole lock The hole lock, if fied, can be ex tended a nd retracted u sing th e hole lock control unit: 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 surface control unit, and to a mains power supply.

  • Page 20: Engaging The Hole Lock

    CMG-3V 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 unit, there will be an additional dial compared to the unit pictured; turn this until the built-in ammeter reads around 0.1 A.

  • Page 21: Disengaging The Hole Lock

    Operator's guide 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.

  • Page 22: Installing The 3V In A Borehole

    CMG-3V 3 Installing the 3V in a borehole Before installing any instrument in a borehole, it is recommended that you prepare the installation site so there is clear access all around the hole. Keep the borehole capped at all times except when inserting or •...
  • Page 23 Operator's guide 2. If applicable, you should test the hole lock mechanism before installing the sensor. For safety reasons, the hole lock is normally supplied with the arm extended. To test the mechanism, connect the signal cable to a surface Control Unit and Holelock Control Unit, and aiempt to retract the hole lock arm (see Section 2.5, “Operating the hole...
  • Page 24 CMG-3V 4. Aiach t he l ifing l o op t o t h e s e nsor u s ing f o ur M 5 M16 s c rews (provided).
  • Page 25 Operator's guide 5. Join the loop to the boiom of the strain relief mechanism using the linking cable provided. 6. Using a small winch, hoist up the sensor package and strain relief mechanism until both are hanging by the lifing cable, with the strain relief mechanism extended.
  • Page 26 CMG-3V Instead, you should either move the instrument to a narrower section of the borehole and try again, or contact Güralp Systems to ft a longer hole lock, quoting accurate measurements of your borehole. 12.Power up the instrument from a suitable power supply.

  • Page 27: Installing A Sensor Using Sand Backfll

    Operator's guide the skids or studs on the sonde keep the sensor package vertical within the borehole. Do not drag too far, or you will damage the contact points. 19.Lower the load bearing cable by around 30 cm to engage the strain relief unit inside the borehole casing, and to provide some slack in the cables.

  • Page 28: Procedure

    CMG-3V The following photographs show the steps involved in backflling with sand: Procedure To install a sensor at the boiom of a borehole of known depth using sand backflling: 1. Measure or calculate the physical volume of the unit which is to be installed in the borehole.
  • Page 29 Operator's guide 4. Fix the main lifing cable to the shackle on top of the strain relief mechanism, and run the signal cable through the mechanism using the built-in clamps (without tightening them.) Do not allow the signal cable to bear any of the sensor's weight. 5.
  • Page 30 CMG-3V 6. Join the loop to the boiom of the strain relief mechanism using the linking cable provided. 7. Hoist up the sensor package and strain relief mechanism until both are hanging by the lifing c able, w ith t he s train r elief mechanism extended.
  • Page 31 Operator's guide 9. Position the assembly over the top of the borehole. Do not allow it to drag across the ground. 10.Lower the sensor so that its base is level with the borehole mouth. Set the depth gauge on the winch to zero. 11.Calculate how much lifing c able m ust b e l owered i nto t he borehole, taking into account the length of the sensor and the strain relief assembly or digitizer.

  • Page 32: Assembling The Winch

    CMG-3V 15.When you have reached the boiom, u se t he w inch t o l if t h e package slightly, taking the slack of the cable. This ensures that the sensor is hanging vertically within the borehole, and is no longer in contact with the sand bed.
  • Page 33 Operator's guide There are two sections for each leg of the tripod. The upper sections are pre-aiached to the head of the tripod; the lower sections are supplied detached. 1. Slide the lower sections all the way into the head with the retaining tape loops facing outwards.
  • Page 34 CMG-3V 4. The lifing cable is supplied with a loop at one end. Run this over one of the pulleys at the top of the tripod, so that the loop hangs down between the legs. If the loop is not provided, you can make...
  • Page 35 Operator's guide 6. Extend each of the three legs in turn to the height you require, fnishing at the leg with the winch aiached. 7. Take the end of the load-bearing cable without the loop, and screw it to the axle inside the winch using a 4 mm Allen key (provided) as shown.
  • Page 36 CMG-3V 8. Aiach t he h andle t o t he s ide o f t he w inch opposite the ratchet mechanism, and fasten it in place with a collar, washer and screw, using the larger Allen key. 9. Wind the cable onto the winch by rotating the handle. Ensure that the cable builds up neatly across the drum.
  • Page 37 Operator's guide If the ratchet prevents you from winding the cable on, twist the metal boss in the DOWN direction to free the cable. 10.Remove the handle, and screw it onto the metal spool of the ratchet mechanism. 11.Hang the strain relief unit and instrument(s) from the loop at the other end of the cable.

  • Page 38: Earthing A Borehole Sensor

    CMG-3V 3.4 Earthing a borehole sensor To achieve the best performance from any borehole instrument, you must make sure that the sensor electronics, its casing and the power supply share a common, local ground, and that all power and data lines are adequately protected against lightning and other transients.
  • Page 39 Operator's guide The best local earth point in many installations is the borehole itself. For this to work, the borehole must have a conductive casing and be situated close (<30 m) to the surface installation. In such an installation you need only connect a cable (green wire in the photograph below) from the local ground plate to the borehole casing.
  • Page 40 CMG-3V An earth strap can be used to ensure a good connection. If the lower borehole is flled with salt water, the instrument will be adequately grounded without any further action. Fresh water is an inferior conductor. In a dry or sand-flled borehole, or one with a non-conducting casing, you will need to ensure the sonde is grounded by some other means.
  • Page 41 Operator's guide The sensor's load bearing cable is suitable for this purpose, provided it is secured to the sensor's lifing l oop w ith a m etallic c lamp a s s hown below. This provides an additional frm contact between the sonde and the load-bearing cable.

  • Page 42: Installations With Dc Power Supplies

    CMG-3V For boreholes with a metallic casing at the boiom and plastic above, we recommend connecting a cable between the sensor housing and the ground plate so that the lower borehole casing acts as the earthing point. Again, the If there is a signifcant distance ((30 m) between the borehole and the surface installation, the resistance of the earth cable may make it impractical to use the borehole as an earthing point.

  • Page 43: External Lightning Protection

    Operator's guide power supply will couple to the sensor. As with AC installations, if the borehole is more than around 30 m from the surface enclosure, you will need to provide a second earthing point for the local ground plate. DC power is most commonly available at self-contained installations with power supplied from baieries, solar panels, or a wind generator.

  • Page 44: Levelling And Centring

    CMG-3V If you are using two earthing points, for example in the DC installation shown above, it may be convenient to connect the lightning conductor to the supply-side earthing point. In any case, the lightning earth must be well separated from the borehole (and its earth, if it needs one.) 3.5 Levelling and centring...
  • Page 45 Operator's guide zero the outputs from the vertical, N/S and E/W sensors in sequence. Afer s uccessful c entring, t he m ass p osition o utputs should be in the range 0.1 – 0.8 V. 9. If the centring process leaves the mass position outputs above ±1.1 V, repeat steps 4 and 5.

  • Page 46: Calibrating The 3V

    CMG-3V 4 Calibrating the 3V 4.1 Calibration methods Velocity sensors such as the 3V are not sensitive to constant DC levels, either as a result of their design or because of an interposed high-pass flter. Instead, three common calibration techniques are used.

  • Page 47: Calibration With Scream

    Operator's guide 4.2 Calibration with Scream! Güralp digitizers provide a calibration signal generator to help you set up your sensors. Calibration is most easily done through a PC running Güralp's Scream! sofware. 1. In Scream!'s main window, right-click on the digitizer's icon and select Control..

  • Page 48: Calibration With A Handheld Control Unit

    CMG-3V 9. Convert to volts using the µV/Bit value given on the digitizer's calibration sheet for the input port. 10.A calibration resistor is installed within the calibration loop, allowing you to convert this value to a calibration acceleration, which is analogous to the ground acceleration under normal working conditions.

  • Page 49: The Coil Constant

    Operator's guide signal frequency, R is the magnitude of the calibration resistor and K is the feedback coil constant. R and K are both given on the calibration sheet supplied with the 3V. The calibration resistor is placed in series with the transducer. Depending on the calibration signal source, and the sensitivity of your recording equipment, you may need to increase R by adding further resistors to the circuit.

  • Page 50: Inside The 3V

    A triangular spring supports the weight of the mass; this spring is pre- stressed, with a natural period around 0.5 s. CMG-3V sensors have no spurious resonances below 140 Hz, and weigh around 180 g.
  • Page 51 Operator's guide The signal voltages output by the 3V are proportional to ground velocity, and are transmiied f rom t he i nstrument o n b alanced diferential lines. In addition, mass position signals are sent as single- ended circuits referred to analogue ground on the output plug. The 3V also receives control signals, which are used to clamp and unclamp the masses, and to run the motors which level and centre the instrument once in position.

  • Page 52: The Control System

    CMG-3V 5.2 The control system The internal operations of the 3V are supervised by a control microprocessor, which drives the mass clamping and centring adjustment motors. It responds to commands sent on three input lines by grounding for 0.2 – 7 seconds.

  • Page 53: Unlock

    Operator's guide UNLOCK This command unlocks the sensor mass and prepares the instrument to begin operating. If UNLOCK is activated when the mass is already unlocked, the processor will lock it and aiempt to unlock again. This is useful if you suspect that the locking procedure has failed.
  • Page 54 CMG-3V This graph shows the entire process of unlocking and centring a triaxial instrument: 1 – 5. The masses are checked, as described above, and unlocked, each in turn. 6. The BUSY LED pulses to indicate that it is centring the Z component.

  • Page 55: The Feedback System

    Operator's guide Afer s uccessful c entring, t he m ass p osition o utput s hould b e i n t he range 0.1 – 0.8 V. If the centring process leaves the mass position output above ±1.1 V, you should start another centring cycle by activating the CENTRE command again.

  • Page 56: Hybrid Feedback

    CMG-3V displacement transducer is aiached t o t he s ensor i nertial m ass. T he sensor should always be operated with the displacement transducer centred or nulled, so that the response to input acceleration is linear. There are two types of feedback system which can be used in a 3V instrument, known as hybrid and conventional-response feedback.
  • Page 57 Operator's guide The hybrid feedback circuit contains a single capacitor in parallel with a resistor, resulting in a single dominant pole at 0.033 Hz (30 s). Below this frequency, the response of the seismometer is fat t o g round acceleration;...

  • Page 58: Conventional-Response Feedback

    CMG-3V Conventional-response feedback The conventional-response feedback system has an additional parallel feedback circuit, consisting of a non-inverting integrator in series with a resistor. This arrangement results in two poles at specifed frequencies. The velocity response of a conventional-response system is defned by a transfer function identical to that of a conventional long-period sensor with a damping constant ζ...

  • Page 59: Comparisons

    Operator's guide active low-pass flter p rovides a h igh-frequency c utof p o int a t a frequency you specify. Comparisons The fgures b elow p lot t he c omparative r esponse o f a c onventional velocity output broadband sensor and a hybrid output broadband sensor.

  • Page 60: Appendix A Connector Pinouts

    CMG-3V Appendix A Connector pinouts This table combines the pinouts for all connectors on the borehole sensor and surface control unit. Control signals are normally active low, but active high versions can be supplied on request. Column (a): The single connector on the 3V sensor, a 32-way mil-spec waterproof plug (02E-19-32P).
  • Page 61 Operator's guide Lock motor Velocity / acceleration mode switch (see Note) Signal ground Control signal ground Motor return (centring and lock motors) Case ground Note: The “velocity / acceleration mode switch” pin is used by the automatic centring process to switch the instrument temporarily into a mode where its response at normal frequencies is fat to acceleration, and thus in linear proportion to the mass position.

  • Page 62: Appendix B Specifcations

    CMG-3V Appendix B Specifications Hybrid sensors Velocity output bandwidth 0.1 – 50 Hz High pass flter output fat 0.01 Hz – spec* to acceleration High pass flter output fat spec – 50 Hz* to velocity Mass position output DC – 0.1 Hz...
  • Page 63 Operator's guide Current at 24 V DC during 115 mA† centring (average) Current at 24 V DC during 200 mA† locking and unlocking *spec refers to the quoted frequency response value, e.g., for a “30 s” sensor, the value of spec would be 30 s = 0.033 Hz. **Sensors are available with a range of sensitivities between 2 M 750 and 2 M 10,000 V/m/s.

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