Groundwater Training Course SOPAC, April Electromagnetic (EM) Induction method for Groundwater Investigations

Transcription

1 Groundwater Training Course SOPAC, April 2005 Electromagnetic (EM) Induction method for Groundwater Investigations

2 Electromagnetic (EM) Induction Method Basic principle: An AC electric current is applied to a transmitter coil This generates a primary electromagnetic (EM) field in the coil. This induces small electric currents in the ground, generating a secondary magnetic field. The secondary (ground) magnetic field depends on coil spacing, operating frequency ground conductivity Both magnetic fields are sensed by the receiver coil and a reading of apparent conductivity is given. The value of apparent conductivity depends on many factors: - porosity, - conductivity of pore fluid, - pore surface area, - degree of saturation of sub-surface sediments, - temperature, and - (if present) clay content.

3 Electromagnetic (EM) Equipment EM equipment is manufactured by a sole agent (Geonics a Canadian company) Two main types: EM31: single bar Spacing between coils: 3.7m Effective depth of penetration: 6m EM31-SH short version has coil spacing of 2m and effective depth of penetration of 4m EM34: 2 coils. Can use 3 spacings between coils: 10, 20 & 40m Effective depth of penetration varies with coil spacing & orientation

4 EM34 Equipment We will concentrate on EM34 (2 coil) equipment Exploration depths for EM34:

5 Use of EM34 equipment Transmitter coil Receiver coil

6 EM procedures for cross-island surveys (small islands) Transmitter coil should point back to receiver coil Start at one beach with person using receiver in front. Take GPS reading. Keep coil vertical and pointed at the other coil. Person with receiver (or 3 rd person) writes EM reading, GPS reading and comments. Person with transmitter makes mark on ground for next reading location. Both then walk at right angles to beach using compass for direction, taking EM and GPS readings at every 20m. Finish at other beach, and switch from 20m to 10m cable. Walk back over same locations as for 20m readings, and take 10m EM

7 Interpretation Methods of Interpretation: 1. Direct interpretation using multiple EM readings at selected locations Using (empirical) formulae Using EMIX34 computer software 2. Analysis of relative readings (shows area of saline water and fresher water) 3. Correlation of results with other more direct techniques: Salinity profiles from boreholes on same island Salinity profiles from islands with similar geology Electrical resistivity soundings

8 Relationships between EM readings & borehole salinity data Example from Pukapuka, Cook islands

9 Advantages & disadvantages EM is a very good method when used for rapid assessment together with other more accurate methods EM is a quicker method than ER but gives less information at a single location EM results can be interpreted in different ways leading to possible errors. It is necessary to: Calibrate the EM readings against known groundwater conditions (e.g. thickness to base of fresh groundwater from drilling or possibly ER soundings) Not accept any individual reading if it is significantly different from adjacent readings Build up a picture based on all the readings along am EM survey line and adjacent lines

10 Where to use it (and not use it)? EM works best where: The geology is not varying much The depth to water table is reasonably small (less than about 5m) The groundwater salinity changes from fresh to saline. Examples of islands / parts of islands where EM works well: Freshwater lenses on small coral islands Coastal sand aquifers on larger islands Examples where it does not work well (can be hard to interpret): Interiors of high volcanic islands (variable geological and hydrogeological properties). EM has been used in elevated areas of Fiji, however, for locating sites for boreholes. Interiors of high limestone islands especially where freshwater lens is thin (large limestone thickness above water table compared with freshwater lens thickness)

11 Case example of use of EM Abatao island, Tarawa atoll, Kiribati Major groundwater investigation in 2003 and adjacent island (Tabiteuea) for possible expansion of water supply to South Tarawa Investigations included: Drilling & Testing of 5 boreholes Installation of monitoring systems in the boreholes Water Quality analyses EM surveys Water level & EC logging at pits Mapping of fresh groundwater areas Estimation of Recharge Estimation of Sustainable Yields Preliminary design of Infiltration Galleries & pumping systems

12 Abatao island, Tarawa atoll, Kiribati Abatao Tarawa atoll Tarawa Lagoon Pacific Ocean

13 Abatao island photos & summary details Abatao Village Abatao Statistics: Length: 1.7 km Max. width: 800 m Min. width: 250 m Area: 75 ha Rainfall: approx. 1,950 mm/year Population: approx. 400

14 EM survey No 1 across island & past borehole EM1 Rig drilling borehole ABA1

15 EM surveys (20m spacing) across island, Abatao Transmitter coil Receiver coil Receiver box

16 Locations of 20 EM surveys and 5 boreholes, Abatao

17 EM Data Collection, EM survey No 7 (narrow part of island) Notes re EM conductivity: 1. is higher for 20m spacing than for 10m spacing (as more saline groundwater is included in the exploration depth) 2. increases with increasing groundwater salinity (e.g. closer to beach rather than in centre of island) 3. decreases with increasing ground level above water table 4. is abnormally high near buried metal objects (e.g. cables or pipes)

18 EM Data Collection, EM survey No 10 (wide part of island) Borehole ABA2 Borehole ABA3 Note: This set of data includes readings at two monitoring boreholes

19 Interpretation Interpretation: 1. Correlated EM results at boreholes with known thickness of fresh groundwater using: (a) (b) Salinity profiles from boreholes on Abatao and Tabiteuea As above and including data from nearby Bonriki boreholes 2. Estimation (Interpolation) of EM results using the lines of best fit see next 2 graphs

20 EM v freshwater thickness data from 11 boreholes on Abatao and Tabiteuea Note : this data is plotted in next graph

21 Abatao & Tabiteuea, Tarawa, Kiribati Relationships between EM results & borehole salinity data for 10m and 20m spacings

22 EM v freshwater thickness data from 7 extra boreholes on Bonriki Note : this data is added to the data in previous graph and is plotted in next graph

23 Relationships between EM results & borehole salinity data for 10m and 20m spacings All available data from Bonriki, Abatao & Tabiteuea, Tarawa, Kiribati

24 EM Data Interpretation, EM survey No 7 Estimated EM conductivity (based on lowest estimate of fresh groundwater from 10m & 20m spacings)

25 EM Data Interpretation, EM survey No 10 Estimated EM conductivity

26 Mapping of Abatao freshwater lens Tabiteuea Map of Abatao island, freshwater lens Tarawa thickness atoll, contours Kiribati using all borehole and EM data

27 Design of infiltration galleries based on Abatao freshwater lens thickness & sustainable yield estimates Map of Abatao showing layout of proposed infiltration galleries

28 Further information re use of EM in islands Anthony S.S. (1992). Electromagnetic method for mapping freshwater lenses on Micronesian atoll islands. Journal of Hydrology, 137 (99-111). Anthony, S.S. (1997). Hydrogeology of Selected Islands of the Federated States of Micronesia. Chapter 23, in Geology and Hydrogeology of Carbonate Islands, Developments in Sedimentology 54 (editors Vacher, H.L. and Quinn, T.M.), Elsevier, Amsterdam, pp Kauahikaua J. (1987). Description of a fresh water lens at Laura island, Majuro atoll, Republic of the Marshall Islands, using electromagnetic profiling. U.S. Geol. Survey Open-File Report, , 32pp. Stewart M.T. (1988). Electromagnetic mapping of fresh-water lenses on small oceanic islands. Ground Water, 26 (2): Stewart M.T. (1988). Electromagnetic mapping of fresh-water lenses on small oceanic islands. Ground Water, 26 (2):