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Geonics Limited EM31-SH Operating Manual

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Tel: (905) 670-9580
Fax: (905) 670-9204
GEONICS LIMITED
E-mail:geonics@geonics.com
1745 Meyerside Dr. Unit 8 Mississauga, Ontario Canada L5T 1C6
URL:http://www.geonics.com
EM31-SH
OPERATING MANUAL
GEONICS LIMITED
LEADERS IN ELCTROMAGNETICS
8-1745 Meyerside Drive, Mississauga, Ontario Canada L5T 1C6
Tel: (905) 670-9580 Fax: (905) 670-9204
E-mail:
geonics@geonics.com
Web site:
http://www.geonics.com
July 2007

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Summary of Contents for Geonics Limited EM31-SH

  • Page 1 Tel: (905) 670-9580 Fax: (905) 670-9204 GEONICS LIMITED E-mail:geonics@geonics.com 1745 Meyerside Dr. Unit 8 Mississauga, Ontario Canada L5T 1C6 URL:http://www.geonics.com EM31-SH OPERATING MANUAL GEONICS LIMITED LEADERS IN ELCTROMAGNETICS 8-1745 Meyerside Drive, Mississauga, Ontario Canada L5T 1C6 Tel: (905) 670-9580 Fax: (905) 670-9204 E-mail: geonics@geonics.com...
  • Page 2 Tel: (905) 670-9580 Fax: (905) 670-9204 GEONICS LIMITED E-mail:geonics@geonics.com 1745 Meyerside Dr. Unit 8 Mississauga, Ontario Canada L5T 1C6 URL:http://www.geonics.com NOTE TO EM31 AND EM34 USERS: Through the normal course of operation, it is possible that the battery contacts will become contaminated with dirt and grit.
  • Page 3 Tel: (905) 670-9580 Fax: (905) 670-9204 GEONICS LIMITED E-mail:geonics@geonics.com 1745 Meyerside Dr. Unit 8 Mississauga, Ontario Canada L5T 1C6 URL:http://www.geonics.com Section Subject Page Technical Specifications Introduction Operation Instructions Ground Conductivity Measurements 2.1.1 Initial Set-up Procedure 2.1.2 Equipment Functional Checks 2.1.3...

  • Page 4: Measured Quantities

    EM31-SH SPECIFICATIONS MEASURED (1) Apparent conductivity of the ground in QUANTITIES millisiemens per meter (mS/m)* (2) Inphase component in parts per thousand (ppt) of the ratio of the secondary to primary magnetic field PRIMARY FIELD SOURCE Self-contained dipole transmitter SENSOR...
  • Page 5 EM31-SH MANUAL Page INTRODUCTION Measurement of ground resistivity is one of the oldest geophysical techniques. Table taken directly from Heiland*, lists typical values of resistivity for a variety of geological materials (pages 4-8). The values given are in ohm-centimeters and must be divided by one hundred to give ohm-meters.
  • Page 6 EM31-SH MANUAL Page Figure 1: Induced Current Flow in Ground This instrument is calibrated to read the correct conductivity when the earth is uniform. In the event that the earth is layered, with each layer of different conductivity, the instrument will read an intermediate value as discussed in more detail in Section 5.2.
  • Page 7 EM31-SH MANUAL Page INTRODUCTION (Cont’d) Theoretical calculations show, as will be quickly evident to the operator, that the reading obtained is essentially independent orientation instrument with respect to the earth. There is, however, a small dependence on the height above the ground; lifting the instrument from the surface of a uniform earth to the height of about 0.7...
  • Page 8 EM31-SH MANUAL Page Table 1A: Resistivities of Igneous and Metamorphic Rocks...
  • Page 9 EM31-SH MANUAL Page Table 1B: Resistivities of Consolidated Sediments...
  • Page 10 EM31-SH MANUAL Page Table 1B (cont’d): Resistivities Consolidated Sediments...
  • Page 11 EM31-SH MANUAL Page Table 1B (cont’d): Resistivities Consolidated Sediments...
  • Page 12 EM31-SH MANUAL Page Table 1C: Resistivities of Unconsolidated Formations...
  • Page 13 EM31-SH MANUAL Page EM31-SH FRONT PANEL FEATURES...
  • Page 14 EM31-SH MANUAL Page OPERATING INSTRUCTIONS The EM31 can be used both to measure the electrical conductivity of the ground and to detect buried metal objects. Section describes procedure measuring ground conductivity and Section 2.2 for buried metal detection. GROUND CONDUCTIVITY MEASUREMENTS 2.1.1...
  • Page 15 EM31-SH MANUAL Page long life “non-replaceable” battery will maintain the Polycorder’s memory for at least five years. It can be replaced if necessary, but that must be done at the factory. Battery Life Ni-cad battery life with the Polycorder on the shelf is about 18 months.
  • Page 16 EM31-SH MANUAL Page CHARGE BATTERIES CHARGE BATTERIES CHARGE BATTERIES CHARGE BATTERIES Then it turns itself off and begins drawing from the backup. The polycorder “knows” not to operate on the backup battery alone. If you try to turn it on again, the polycorder immediately displays CHARGE BATTERIES and shuts down again.
  • Page 17 EM31-SH MANUAL Page Replacing Batteries Ni-cad batteries can be recharged several hundred times, effective working life continually decreases. Eventually the batteries need to be replaced. It is a good practice to replace the battery pack annually. You can change batteries without losing memory.
  • Page 18 EM31-SH MANUAL Page Turn the instrument OFF using the MODE switch, before connecting the receiver coil, then align and connect the receiver coil tube main frame tube. instrument is now ready to proceed with the functional checks. 2.1.2 Equipment Functional Checks The Range switch should be set at 100 mS/m position for all the following tests.
  • Page 19 EM31-SH MANUAL Page If, on the other hand, the difference in readings has increased, the PHASE potentiometer should be rotated in a counter clockwise direction instead and the procedure described above repeated until there is no change in the readings. Always remember to set the COARSE control back to its original position.
  • Page 20 EM31-SH MANUAL Page 2.1.3 Operating Procedure a) Positioning the instrument with the should strap adjusted so that the instrument rests comfortably on the hip as shown, turn the Mode switch to the OPER position and rotate the Range switch so that the conductivity reads in the upper two-thirds of the full range.
  • Page 21 The orange button on the transmitter boom is used only in conjunction with the data recording system. It is also possible to collect data with the use of computer, by connecting the computer directly to the RS- 232 output port on the EM31-SH front panel (with optional RS-232 interconnect cable).
  • Page 22 EM31-SH MANUAL Page BURIED METAL DETECTION 2.2.1 Set-up and Operation Procedure The inphase component of the induced magnetic field is significantly more sensitive to large metallic objects thant the quadrature phase (quad-phase) component used for ground conductivity measurements. Typically, the EM31 inphase component will detect a single 55 gallon drum to depths of about 2 meters to the top of the drum.
  • Page 23 EM31-SH MANUAL Page increasing or decreasing, and possibly negative values or some combination of each. NOTE: It is always advisable, when surveying for buried metal to record both the inphase and quad-phase components. While the inphase, in general, is a better detector of metal, the quad-phase is more sensitive to long, extended targets (eg.
  • Page 24 EM31-SH MANUAL Page INSTRUMENT CALIBRATION Prior to shipping, the instrument is calibrated in the factory to read properly. If necessary, calibration procedures are easily carried out as described below. IMPORTANT – The most critical adjustment is the QF (quadrature fine) potentiometer which has been precisely adjusted at the factory.
  • Page 25 EM31-SH MANUAL Page Figure 2: Conductivity Correction: Instrument on the ground surface For 2 m unit...
  • Page 26 EM31-SH MANUAL Page SURVEY TECHNIQUE Surveying with the EM31 is straightforward. As pointed out in 2.1.3. measurements may be made either continuously or on a station-by-station basis. In either case it is always recommended, as for any other geophysical survey, that survey...
  • Page 27 EM31-SH MANUAL Page SURVEY TECHNIQUE (cont’d) at each measurement station. Thus if the lines are in a north-south direction the operator would normally walk along the line with the instrument pointing in a north-south direction; at each measurement station he can also take a reading with the instrument pointing east-west to check that this is essentially the same as the north-south reading.
  • Page 28 EM31-SH MANUAL Page DISTANCE D IS OF THE ORDER Figure 3: Typical Response over a pipe SURVEY TECHNIQUE (cont’d) It should be remembered that the EM31 is an electromagnetic tool and care should be taken near obvious conductors until the operator has satisfied himself as to their possible effect.
  • Page 29 EM31-SH MANUAL Page SURVEYING TECHNIQUE (cont’d) In general the conductivity readings obtained with the EM31 will vary smoothly from one region to another. In some cases, however, as for example where a well defined vertical contact separates a poor conductor from a very...
  • Page 30 EM31-SH MANUAL Page DATA INTERPRETATION Uniform Halfspace The EM31 has been designed to operate over a range of resistivities from a few ohm-meters to a thousand ohm-meters. For higher conductivities the instrumental response departs from linearity as shown in Figure 2, where it will be seen σ...
  • Page 31 EM31-SH MANUAL Page Multi-Layered Earth A geophysical model that is of particular importance is the horizontally layered earth, and the EM31 allows a very simple interpretive technique for this model. In order to utilize the model the terrain layering must be well defined and constant over a lateral distance of at least five meters in any direction from the instrument.
  • Page 32 EM31-SH MANUAL Page Figure 4: Multi-Layer Response Diagram...
  • Page 33 EM31-SH MANUAL Page σ = 1.000 - R(Z ) + k σ σ σ σ = 1m, = 8(1.00 – 0.71 + (5 x 0.71)) = 30.7 mmho/m σ = 2m, = 8(1.00 – 0.45 + (5 x 0.45)) = 22.4 mmho/m σ...
  • Page 34 EM31-SH MANUAL Page Multi-Layered Earth (cont’d) The table accompanying the figure shows the calculations which yield the value of apparent conductivity at various thicknesses of the upper layer. Thus as we traverse such a structure we would find that the apparent conductivity fell from 30.7 millsiemens per meter at a larger distance from the...
  • Page 35 The instruments employed were the Geonics Limited EM31 and a prototype version of the Geonics Limited EM34. Both instruments were calibrated to read terrain conductivity in millimhos per meter directly;...
  • Page 36 EM31-SH MANUAL Page Introduction (cont’d) effective depth penetration of the system is determined by the intercoil spacing and is independent of skin depth and thus of ground conductivity. This feature greatly simplifies interpretation of survey results. In order to vary the depth...
  • Page 37 EM31-SH MANUAL Page Example A: Heart Lake, Ontario (EM31) Measurement Interval: 100 feet over till, 10 feet over sand/gravel This survey line compares results obtained with conventional resistivity equipment (Wenner array with “a” spacing of 1 foot and 20 feet) and the Geonics EM31. It is...
  • Page 38 EM31-SH MANUAL Page Example C: Cavendish Test Site (EM31) Measurement Interval: 50 feet This survey, which was carried out over line C to establish the overburden resistivity shows that with the exception of the swamp area the overburden is extremely resistive. The value of resistivity obtained over the swamp is in good agreement with that from other measurements.
  • Page 39 EM31-SH MANUAL Page Example E: Cooksville/Mississauga, Ontario (EM31) Measurement Interval: 25 feet This example illustrates a survey carries out with the EM31 to search for a buried river channel. The channel, which is filled with glacial till, has been cut into the Dundas shale which, as seen from the example, has a resistivity of the order of 25 to 30 ohm-meters.
  • Page 40 EM31-SH MANUAL Page Example G: Pre-glacial River Valley (EM31, EM34) Measurement Interval: 100 feet This survey was performed to outline a pre-glacial river valley whose existence had been suggested from water-well data. The survey was performed with the EM34 at a station interval of 100 feet and with intercoil spacing of both 50 and 100 feet.
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