Groundwater Training CourseGroundwater Training Course

SOPAC, April 2005SOPAC, April 2005

Electromagnetic (EM) Induction method Electromagnetic (EM) Induction method for for

Groundwater InvestigationsGroundwater Investigations

Electromagnetic (EM) Induction MethodElectromagnetic (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.

Electromagnetic (EM) EquipmentElectromagnetic (EM) Equipment

• EM equipment is manufactured by a sole agent (Geonics – a Canadian company)

• Two main types:

♦EM31: single bar Spacing between coils: 3.7mEffective depth of penetration: 6mEM31-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 & 40mEffective depth of penetration varies with coil spacing & orientation

EM34 EquipmentEM34 Equipment

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

Use of EM34 equipmentUse of EM34 equipment

Transmitter coil

Receiver coil

EM procedures for crossEM procedures for cross--island surveys (small islands)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 3rd 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

InterpretationInterpretation

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

Relationships between EM readings & borehole salinity dataRelationships between EM readings & borehole salinity data

Example from Pukapuka, Cook islands

Advantages & disadvantagesAdvantages & 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

Where to use it (and not use it)?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)

Case example of use of EMCase example of use of EM

AbataoAbatao island, Tarawa atoll, Kiribatiisland, 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

AbataoAbatao island, Tarawa atoll, Kiribatiisland, Tarawa atoll, Kiribati

Pacific Ocean

TarawaLagoon

AbataoAbatao

Tarawa atoll

AbataoAbatao island island –– photos & summary detailsphotos & summary details

AbataoVillage

AbataoAbatao

Statistics:Statistics:•• Length: 1.7 kmLength: 1.7 km•• Max. width: 800 mMax. width: 800 m•• Min. width: 250 mMin. width: 250 m•• Area: 75 haArea: 75 ha•• Rainfall: approx. 1,950 mm/yearRainfall: approx. 1,950 mm/year•• Population: approx. 400Population: approx. 400

EM survey No 1 across island & past borehole EM1EM survey No 1 across island & past borehole EM1

Rig drilling Rig drilling borehole ABA1borehole ABA1

EM surveys (20m spacing) across island,EM surveys (20m spacing) across island, AbataoAbataoTransmitter coilTransmitter coil

Receiver coilReceiver coil

Receiver boxReceiver box

Locations of 20 EM surveys and 5 boreholes,Locations of 20 EM surveys and 5 boreholes, AbataoAbatao

EM Data Collection, EM survey No 7 (narrow part of island)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 table4. is abnormally high near buried metal objects (e.g. cables or pipes)

EM Data Collection, EM survey No 10 (wide part of island)EM Data Collection, EM survey No 10 (wide part of island)

Borehole ABA2Borehole ABA2

Borehole ABA3Borehole ABA3

Note: This set of data includes readings at two monitoring boreholes

InterpretationInterpretation

Interpretation:1. Correlated EM results at boreholes with known

thickness of fresh groundwater using:(a) Salinity profiles from boreholes on Abatao and

Tabiteuea(b) As above and including data from nearby Bonriki

boreholes2. Estimation (Interpolation) of EM results using the

lines of best fit – see next 2 graphs

EM v freshwater thickness data EM v freshwater thickness data from from 11 boreholes11 boreholes onon AbataoAbatao andand Tabiteuea Tabiteuea

Note : this data is plotted in next graph

Relationships between EM results & borehole salinity dataRelationships between EM results & borehole salinity datafor 10m and 20m spacingsfor 10m and 20m spacings

Abatao & Tabiteuea, Tarawa, Kiribati

EM v freshwater thickness data EM v freshwater thickness data from from 7 extra boreholes7 extra boreholes onon Bonriki Bonriki

Note : this data is added to the data in previous graph and is plotted in next graph

Relationships between EM results & borehole salinity data for Relationships between EM results & borehole salinity data for 10m and 20m spacings10m and 20m spacings

All available data from Bonriki, Abatao & Tabiteuea, Tarawa, Kiribati

EM Data Interpretation, EM survey No 7EM Data Interpretation, EM survey No 7

Estimated EM conductivity (based on lowest estimate of fresh groundwater from 10m & 20m spacings)

EM Data Interpretation, EM survey No 10EM Data Interpretation, EM survey No 10

Estimated EM conductivity

Mapping ofMapping of AbataoAbatao freshwater lensfreshwater lens

TabiteueaTabiteueaisland, island, Tarawa Tarawa atoll, atoll, KiribatiKiribati

Map of Abataofreshwater lens

thickness contours using all borehole and EM

data

Design of infiltration galleries based onDesign of infiltration galleries based on AbataoAbatao freshwater freshwater lens thickness & sustainable yield estimateslens thickness & sustainable yield estimates

Map ofAbatao

showing layout of proposed infiltration galleries

Further information re use of EM in islandsFurther 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, pp693-706.

• 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, 87-0582, 32pp. Stewart M.T. (1988). Electromagnetic mapping of fresh-water lenses on small oceanic islands. Ground Water, 26 (2): 187-191.

• Stewart M.T. (1988). Electromagnetic mapping of fresh-water lenses on small oceanic islands. Ground Water, 26 (2): 187-191.