Groundwater Training Course
SOPAC, April 2005
Electromagnetic (EM) Induction method
for
Groundwater Investigations
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.
Electromagnetic (EM) Equipment
• EM equipment is manufactured by a sole
agent (Geonics – a Canadian company)
• Two main types:
♦ EM31:
¾ single bar
¾ Spacing between coils: 3.7m
¾ Effective depth of penetration: 6m
¾ EM31-SH ‘short version’ has coil
spacing of 2m and effective depth of
penetration of 4m
♦ EM34:
¾ 2 coils.
¾ Can use 3 spacings between coils:
10, 20 & 40m
¾ Effective depth of penetration varies
with coil spacing & orientation
EM34 Equipment
♦ We will concentrate on EM34 (2 coil) equipment
♦ Exploration depths for EM34:
Use of EM34 equipment
Transmitter coil
Receiver coil
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 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
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
Relationships between EM readings & borehole salinity data
Example from Pukapuka, Cook islands
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
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 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
Abatao island, Tarawa atoll, Kiribati
Abatao
Pacific
Ocean
Tarawa atoll
Tarawa
Lagoon
Abatao island – photos & summary details
Abatao
Village
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
Abatao
EM survey No 1 across island & past borehole EM1
Rig drilling
borehole ABA1
EM surveys (20m spacing) across island, Abatao
Transmitter coil
Receiver coil
Receiver box
Locations of 20 EM surveys and 5 boreholes, Abatao
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)
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
Interpretation
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
boreholes
2.
Estimation (Interpolation) of EM results using the
lines of best fit – see next 2 graphs
EM v freshwater thickness data
from 11 boreholes on Abatao and Tabiteuea
Note : this data is plotted in next graph
Relationships between EM results & borehole salinity data
for 10m and 20m spacings
Abatao & Tabiteuea, Tarawa, Kiribati
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
Relationships between EM results & borehole salinity data for
10m and 20m spacings
All available data from Bonriki, Abatao & Tabiteuea, Tarawa, Kiribati
EM 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 10
Estimated
EM conductivity
Mapping of Abatao freshwater lens
Tabiteuea
Map of Abatao
island,
freshwater lens
Tarawa
thickness
atoll,
contours
using all
Kiribati
borehole and EM
data
Design of infiltration galleries based on Abatao freshwater
lens thickness & sustainable yield estimates
Map of
Abatao
showing
layout of
proposed
infiltration
galleries
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, 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.