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APPLICATION OF MAGNETIC METHOD IN GROUNDWATER PROSPECTING AND
ENVIRONMENTAL STUDIESGROUP 9 SEMINAR PRESENTATIONON ABDULMALIK
TIJANI 100813001CHUKWUEMEKA THEOPHILUS 100813019AFFIAH RAPHAEL
100813007OUTLINEINTRODUCTIONLITERATURE REVIEWMETHODOLOGYCASE
STUDIESCONCLUSIONREFERENCEINTRODUCTIONGroundwateris the water found
underground in the cracks and spaces in soil, sand and rock. It is
stored in and moves slowly through geologic formations of soil,
sand and rocks called aquifers.Anaquiferis an underground layer of
water-bearing permeable rock or unconsolidated materials (gravel,
sand, or silt) from which groundwater can be extracted using a
water well.
INTRODUCTIONEnvironmental Pollution is the undesirable state of
natural environment being contaminated with harmful substances as a
consequence of human activities or natural phenomenon
INTRODUCTION
Magnetic techniques measure the remnant magnetic field
associated with a material or the change in the Earth's magnetic
field associated with a geologic structure or man-made object. They
have been used for regional surveys since the early 1900s in the
hydrocarbon industry and for longer in mineral prospecting however
little use has been made directly for groundwater studies. This is
mainly because groundwater cannot generate a magnetic anomaly. In
groundwater prospecting, magnetic method is mainly used for
regional reconnaissance survey to delineate subsurface structures
such as faults in basement environment. Electrical and gravity
method can be used for a detailed studies on the selected area(s).
Results of magnetic surveys are usually presented as line profiles
or magnetic anomaly maps. An example of this airborne use of
magnetic surveys is given by Combrinck et al. (2001). In
sedimentary region, particularly where the basement depth exceed
1.5 km, magnetic contour anomalies are normally smooth and
variation are small, reflecting the basement feature rather those
of the near-surface.(Telford et al., 1990).
LITERATURE REVIEWDobrin and Savit, (1988) used magnetic survey
to determine the magnetic relief over sedimentary basin areas,
which almost reflect more than the lithology of the basement rather
than its topography .Babu et al. (1991) describe the use of
magnetic survey in mapping bedrock topography, and in particular
possible groundwater reservoirs in hard-rock (igneous and
metamorphic) terrains. Sultan S.A et al. (2007), evaluated the
groundwater potential of Al Qantara East, North Western Sinai,
Egypt using Magnetic and Electrical Resistivity Method. El Sayed
Selim and Essam Aboud (2010) determine the sedimentary cover and
structural trend in central sinai using gravity and magnetic data
analysisOther use of the magnetic technique together with
resistivity surveys in volcanic terrain has been described by
Aubert et al. (1984).
BASIC PRINCIPLES OF MAGNETICMETHODThe Earth possesses a magnetic
field caused primarily by sources in the core. The form of the
field is roughly the same as would be caused by a dipole or bar
magnet located near the Earth's centre and aligned sub parallel to
the geographic axis. The intensity of the Earth's field is
customarily expressed in S.I. units as nanoteslas (nT) or in an
older unit, gamma (): 1 = 1 nT = 10-3 T., From coulombs law Where,
F = Force of attraction or repulsion between two magnetic poles
(Newton).B = Magnetic field (nT). = Magnetic permeability.M =
magnetic moment.V = Magnetic potential
ContdControlling propertyThe main controlling physical property
in magnetic method is magnetic susceptibility.PrincipleThe magnetic
methods are based on the fact that the magnetic bodies present in
the earths surface contribute to the magnetic field of the earth.In
general, when the magnetic field of the earth or one of its
components is measured on the surface, bodies possessing magnetic
moments different from those of the surrounding rocks contribute to
the deviations in the measured quantities. From the magnetic
anomalies, it is possible to locate anomalous objects.
The different parameters measured during magnetic investigations
are total magnetic field (intensity and direction) and different
space componentsMagnetic surveys have a certain inherit
limitations. Hence for unique and accurate solutions, magnetic
prospecting is often carried out along with the gravity or other
methodsAPPLICATIONFinding buried steel tanks and waste
drumsDetecting iron and steel obstructionsLocating unmarked
mineshaftsAccurately mapping archaeological featuresMapping basic
igneous intrusive & faultsEvaluating the size and shape of ore
bodies
DATA AQUISITION Magnetic readings are taken on the stations
along the traverses using the magnetometer with the pointer facing
the geographic north. Base station, which is a station that is at
the centre of all the traverses, is chosen. Using one magnetometer
requires the person(s) carrying out the survey to return to the
base station at regular interval of time to take measurements.
Magnetic readings, time, elevation and the coordinates of the each
station are taken. The base station reading is plotted against the
station, as well as other stations, and used to correct for diurnal
variation. Correction for regional variation is carried out to
generate residual anomaly which then interpreted accordingly.
CONTD
Examples of the use of Proton precision magnetometerDATA
PROCESSINGDrift correctionThis is done to remove the effect of
diurnal variation caused by magnetic storm.CORRECTED
=(UNCORRECTEDDATUM) (BASE REFERENCE)To make accurate magnetic
anomaly maps, temporal changes (variation) in the earths field
during the period of the survey must be considered. During severe
magnetic storms, which occur infrequently, magnetic surveys should
not be made. The correction for diurnal drift can be made by repeat
measurements of a base station at frequent intervals.The
measurements at field stations are then corrected for temporal
variations by assuming a linear change of the field between repeat
base station readings. Continuously recording magnetometers can
also be used at fixed base sites to monitor the temporal changes.
If time is accurately recorded at both base site and field
location, the field data can be corrected by subtraction of the
variations at the base site. After all corrections have been made,
magnetic survey data are usually displayed as individual profiles
below. Identification of anomalies caused by cultural features,
such as railroads, pipelines, and bridges is commonly made using
field observations and maps showing such features.
THEORY AND METHODOLOGY
The survey can be carried out on the land, sea and the air. The
data is acquired along the traverse of say 5-50m. Measurements are
taken at the base station at regular interval. The data is
processed accordingly, e.g, by reduction to the residual,
correction for drift, etc. 15ADVANTAGES AND
DISADVANTAGESAdvantagesData acquisition is very fastLesser number
of man power is needed DisadvantagesHave wide variation of magnetic
anomaliesData is affected by spurious and non-relevant indications
such as diurnal variation.
Traverses and stations
CASE STUDY 1Sultan et al characterise deep aquifer of the study
area which is located in the central part of Sinai, an extremely
arid region in Egypt using Magnetic, gravity and resistivity method
for groundwater exploration.Ground magnetic and gravity
measurements were made at one hundred and fifty stations. The
combined interpretation of magnetic and gravity data allowed the
determination of the depth of the surface of the granitic basement
and depth of the Conrad surface.CASE STUDY 1
Base map of the study area, WADI AL GHUBBA, SINAI, EGYPT CASE
STUDY 1 RESULTTotal Magnetic field generated after reduction to the
pole.
20
RESULTS ContdGGoGeoelectric cross-section of VES numbers 1,2,3,4
and 5 (Sultan et al)
Geoelectric cross-section of VES numbers 11,12,13,14 and 15
(Sultan et al)CASE STUDY 1 RESULTThe results obtained from the
integrated interpretation of the resistivity, gravity and magnetic
data agree with the boreholes data and the geology of the study
area. The results revealed the existence of four major resistivity
units with resistivity values ranging from 2 and 412 ohm-m and
thickness between 1.4 and 462 m. The first unit consists of clay
and has low resistivity values. The second unit shows high
resistivity values representing limestone. The third one consists
of limestone intercalated with shale and has low resistivity
values. The fourth unit is characterized by relative high
resistivity values and correlates with sand, and sandy clay and
sandstone, which represents the main aquifer in the area. The top
surface of the deep aquifer is probably at a depth ranging from 300
to 1000 m, being shallow in the south western part of the survey
area. The interpretation of the gravity data shows that the main
structural elements are well correlated with the Gulf of Aquba
(NWSE), the Gulf of Suez (NWSE) and with the Nile Valley (NS)
trendsCONCLUSION From the gravity and magnetic profiles, the
thickness of the granitic basement is estimated to be within a
range of 1500 m and 3150 m, and the depth to the Conrad surface
ranging from 7135 m to 8783 m. The results also indicate that the
evolution of the sedimentary cover might be controlled by the
structural tectonics of the basement.
CASE STUDY 2 Godio (2000), used magnetic and electromagnetic
survey with low induction equipment to perform a test in an
industrial waste landfill site with the aim of checking the
reliability of data processing techniques for detecting iron masses
inside the upper part of the landfill.The study area is a waste
landfill in Torino, Italy . The study was carried out by the
department of earth sciences of the Torino Polytechnic, Italy.The
geophysical test was performed on an industrial waste landfill
where residues from two iron foundries were landfilled. The
landfill was cultivated , starting from 1989 till April 1998, with
a total capacity of about 180,000 m square. The maximum depth of
the waste disposal is about 15 m.
CASE STUDY 2
Magnetic anomalies generated by the presence of deep
ferro-magnetic materials buried in the landfill at
This shows disseminated ore-bodies. 26CASE STUDY 2Conductivity
meter survey using low induction number equipment in the vertical
dipole mode
Result continued
CASE STUDY 2 RESULTThe result of the magnetic survey delineate
the existence of high concentration of ferro-metallic objects; the
magnet behaviour of the waste disposal is affected by the presence
both of fine iron materials disseminated in sandy and clayey
material, derived from the cast iron foundry industrial process,
and ferro-metallic objects such as drums or parts of heavy
machines. Therefore, magnetic surveys could be useful tools for
detecting zones of high concentrations of iron particles in waste
landfill; this material can be recovered from the landfill and
recycled.
CONCLUSIONThe above results shows that magnetic method can be
used to delineate subsurface structures while prospecting for
groundwater. The structure can then be further investigated with
other geophysical methods in details to locate a good groundwater
aquifer. In environmental application, the magnetic method can be
used to directly detect the presence of buried ferro-magnetic ores
which can be recycled. REFERENCESAboud, E., & El Bishlawy, A.
(2011). Contribution of gravity and magnetic data in delineating
the subsurface structure of Hammam Faroun area, Gulf of Suez,
Egypt.Arabian Journal of Geosciences,4(1-2), 249-257.Aubert, M.,
Camus, G. and Fournier, C., 1984, Resistivity and magnetic surveys
in groundwater prospecting in volcanic areas - Case-history Maar de
Beaunit Puy de Dome France: Geophys. Prosp., 32(04): 554-563.Babu,
H. V. R., Rao, N. K. and Kumar, V. V., 1991, Bedrock topography
from magnetic anomalies - An aid for groundwater exploration in
hard-rock terrains (short note): Geophysics, 56(07):1051-
1054.Dobrin M.B and Savit C.H, 1988. Introduction to Geophysical
Prospecting (fourth ed.) McGraw-Hill Book Company, New York
867p
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