- University of Nigeria Research Publications AGWUNOBI, Ohams Ethelbert Author PG/M.Sc./81/1188 Title Resistivity and Magnetic Surveys for Groundwater in Fobour in the Jos Area of Plateau State Faculty Physical Sciences Department Geology Date January, 1984 Signature
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University of Nigeria Research Publications
AGWUNOBI, Ohams Ethelbert
Aut
hor
PG/M.Sc./81/1188
Title
Resistivity and Magnetic Surveys for Groundwater in Fobour in the Jos Area of
Plateau State
Facu
lty
Physical Sciences
Dep
artm
ent
Geology
Dat
e
January, 1984
Sign
atur
e
RESISTIVITY AND NY\lsNEcIC SURUEYC FOR GROWATER IN FOBOCJR IN THE
JCE AREA OF PLATEAU STATE.
OF G E O E Y IN THE F W Y OF PPrYSZCAt
SCIENCES IN PNITIAL FULFIUENT OF
THE RKlUIfEMM'S RYI T k DSfGE OF MASTER OF SCIENCE
UNIVEEITY OF NIGERIA, H S W .
JANVMYv 1%
Mr. Agwunobi Ohms EtheIbert , a postgraduate stuclont
i n the Depar.tmcnt of Geology, University of Nigeria, Nsukka
has satisfactorily completed the requirements for courme
and project works for the degree of Master of Science ( E c
Applied ~ e o ~ h ~ s i c s ) i n Geology.
The wrk embodied in his Project Report is original and
has not been submitted i n part or f u l l for any other diploma
or degree of t h i s or any other University.
W R Dr, K,M, Onuoha Supervisor Gaology Department University of Nigeria Nsukka ,
irg
Dr. A X , Onyeagocha Acting Head Department of Geology University of Nigeria Nsukka.
L,DI(X'C ION ------
T h i s vork is dedicated to my wife
C O N T E N T S
L i s t of F igu re s i L i s t of Tables i i ~lcknovrlcdgemcnt i v Abs t r ac t v
PAGE
CHAPTER 3.
Ih!RODUCTION Id!! PREVIOUS !'YOiXS
I n t r o ~ l u c t i o n . . 6
Previous works i n t h e Arca a
GEFIEIUIL GEOLCGY OF THE AREA . . Gcncra l Gcology . Younger G r a n i t e s . C r y s t a l l i n e Basement . . F o l i a t i o n . . . . J o i n t s and F a u l t s . . .
P l a n and Ob jcc t ivcs of t h c survey .. 14 Ins t rwncn ta t i on and F i e l d Prcccdurcs ., 15 R e s u l t s of t h e E l e c t r i c a l R e s i s t i v i t y P~leasuremcnts . . .. . 23 hlaqnctic ~ s t i g s t i o n s . . . . . 59 Discuss ion - < f t h e R e s u l t s . . . 80
Topographic
Outline map
i
LIST OF FIGURES
T i t l e - P a m
map of Fobour and environs 3
of Nigeria, showing tho study area 4
Geologic map of the area 10
Map of t h e area showing layout/locations of of soundings and pro f i l e s 22
Extcxnal features of the Resistivity equipment 16
Extcrnal features of the Magnctomcter 19
Geo2lcct~ic dcpth section for sounding curves ES1- 41 ES *6
Resistivity c ~ r v e of ES.1 25
Resistivity curvc ~f €3.2 28
Resistivity curvc of ES,3 31
Cosistivity curve of ES.4 34
Resistivity curve of ES,5 36
Resistivity curve of ES.6 39
Resistivity curve of EF.l 44
fiesistivity curve of EP.2 47
Resistivity curve of EP.3 48
flesistivity curve of EP.4 51
Rasistivity curve of EP.5 53
Resistivity curve of EP.6 56
R~sistivity curve of EP.7 55
Magnetic anomaly curve of bP. 1 62
FAaynotic anomaly curve of FP.2 65
LIST OF FIGURE3 Cont,
Magnctic ancaaly curvc of MP .3
Magnetic anomaly c u m of MP,4
Magnc.tic anomaly curvc of MP .5
Magnetic anomaly curve of MP.6
Magnctic anomaly c u m c of EQ.7
Magnotic anomaly cuwo of MP.8
CoxrclaCion of Rcsistivity curves of EP.2, 3 and 7
Correlation cf ncsistivity curves of EP.4, 5 and 6
Corrclation of Magnctic anomaly curvcs of LP.3, 4 and 5
Corrclation of Magnetic anomaly curves of MP.5, 6, 7 and 8
Resis t ivi ty map of t h e area
Magnctic map of t h a area
Locations of recomncled sites
Tablc - 1 .
&ST OF TABLES
Title - Data of Elcctrical Resistivity Sounding
Data of Eloctrical Resistivity Sounding
Data of Electrical ncsistivity Sounding
Data of Elcctrical Resistivity Sounding
Data of Eloctrical Resistivity Sounding
Data of Eloctrical Resistivity Sounding
Results of Elcctrical Resistivity Soundings
Data of Eloctrical Nosietivity Profiling
Data of Electrical Resistivity Profiling
Data of Eloctrical Resistivity Profiling
Data of Electrical Rcsistivity Profiling
Data of Electrical Resistivity Profiling
Data of Elcctrical Rosistivity Profiling
Data of Elcctrical Resistivity Profiling
Data of Magmtic Profile
Data of Magnctic Profilo
Data of Magnetic Profilo
Data of Magnetic Profilo
Datl of Magnetic Profilo
Data of Magnetic Profilc
Data of Maghotic Profilc
Data of Magnotic Pro f i l e
I am highly grateful t o Dr, K. M. Onuoha whose advice,
cfiticisms and sllggestions immensely contributed i n making
ti is work succcssf ul,
My gratitucle also go$= to Abam, Amke Adfndu, Madu, Mbonu,
Wuka, Bob, Foetsr, Paul and especially my wife Chinela who
helped in one w7.y or the other during the period of the project.
Encouragements fran Augoc Law and 3key are gratefully
acknowledged,
v
msTW\CT
Resistivity and magnetic measurements were carried out to investi-
gate tho groundwabr occurrence i n F o b o u r m a o f Plateau State of
Eligeria, The area is underlain by Pre-Cambrian basement complex
in t rudsd by the Younger Granites, the Older Basalts and Newer Basalts,
Eight pzofil4:s of magnetic measurements were carried out with
Ja landm flux-gate typo of magnetometer, Seven e lectz ica l profilings
and five ~Iec t l - i ca l s o u ~ d i n g s were done with an ABEM T'CSAS 300 terrameter.
A s i x t ! ~ electrical sounding was carried out near a bore-hole a t Jengere,
The r e s u l t s showed two probable locat ions of potential aquifers.
'The aquifgr depths and thicknesses show variations from place t o place.
Maximum thicl:nsss of overburden of 107 meters is registered a t electrica;
sounding s ta t ion numbor I (=,I) . A minimum thickness of 4'7 meters is
recorded a t electrical sounding s ta t ion nwnber 5 ( S . 5 ) . ~ h i k r q of the h
aquffer is rrspectod southwa~ l s . Subsurface resistivity and magnetic
veriations suspected t o be structurally controlled were also observed.
The area af intercst has no bed sock exposura on t h e surface,
L;~te.rpmtation (by partial curve matching) showed maximum depth to t h ~
probable unfractured awl um3athered basement to be about 110 meters,
.:he ceoph-qsical r e s u l t s showed a resistive in situ or transported
hard dw top soil.. The thickmsses of this tog soil vary from Dn to
about 7 meters i n places. The resistivity values vary from m f i m to
over M 3 0 d ? m m . Unclmlying the top s o i l is the weathsred and aquiferous
medium of low resistivity values of 42i?-.rn t o 257$cCm, Tho aquifer
thicknasses vary from 35 meters to over 90 meters; This aquiferous horizon
gradually grades t o unf ractumd and unweatk'bd resist f ve b a s m n t .
The sixth sounding at Jengere was used as a control for
accxa.te f n t o r p r e t a t i o n of results f r a a relatively virgin area of
F o b u r , From the analysis of the electrical soundings, the electrical
profilings anc! magnetic measurements two sites for bore-hole wre
recornmended f o r drilling, O n e of the sites i s located about 350m
south aa s t of Iyam - Fobour Junction and t h e other abouf; 900m t o
the w u t h c
Explo~ation geophysics has not played as imyj';rtant a part in
groundwater investigations as it has in petroleum exploration in the
C O U ~ ~ T Y ~ This situation has resulted mainly from economic considerations
t h a t have rsstricted the f inancial support to develope and apply geophy-
sical methods t o groundwater problems,
U n t i l mcen63y noli; much at.tent,ion was given to the solution of
g r ~ ~ w a t e x problms of some areas located i n t h e hard-rock terrain
(klasorncnt soinplex) of the country, With the exception of a few undetailed
I~ydrogeophysical surveys carried out by some companies like Mecon Services.
Alotis, Geocoi-mlt and others for their clients, most bore-holes are
s i t d and dri l led on ' @ h i t or miss basis",
In most of these works, ~nuch attention was paid t o tho *athered
horizon only, Ths repoz-2s lack information on the groundwater potential
of t!?e f r a s t~ roc l zon*s, Information capable of ai.dir.g t h e identif ic tio on
of geophysical :;ignatures a id hydrogeological parameters which can lead
!;CJ a stl.ccessfal location of sui table aquifers i n comparable d i f f i c u l t
t e ~ r a i n s are a lso lacking,
The present project was geared towards locating water bearing
horizons i n such hard rock terrains. The survey involved location of
such structures as buried l iver channels, fau l t s , highly .fractured and
j o i n ted horizons as well as weathered part of the bed sock capable of
hosting and t ransmit int ; water exploitable i n economic quantdties~
l e l r l Location and Axesrih4 1 i t ~
The survey area covers Fobour and environs i n the Jos,Plateau
Stake of Nigoria ard is bounded by longitudes 9°00'~ and WE; and l a t l t y d ~ s 9 ' 5 ~ ~ ~ and 9 '55#~ . Fobour is located about 23 Km
soutI+eas-t of Jos, An untamed lateritic road from Jos through larninga,
Jf rg in Sama and Iyam connects Fobour, (see figs, 1 and la) ,
L ,I . 3, phvsj:pgra~h.[
Fobour 5.5 oc n ~elatively f l a t and low lying area of approximately
13X3 meters Sove insan sea ievctl. The perimeters of the town axe defined
by chains of I~ilLs, To the North of Fobour is Shere h i l l s , t o the
South is the Fusa h i l l s and the Eastern perimeter is the Jarawa h i l l s .
The area is within the light Guinea Savana type 09 vegetationc
Light bushes alonc stream channels and s o w major joints on the vani t .?
hills dot t h c vegetation., The average monthly temperature is about
78% ( ~ 5 ~ 6 % ) ,
The rsirri season is nonnaly short a:~d lasts fo r about f o w months,
The rainfall. is Ilght w i - 3 1 moan annual precipitation of 110 - 150cm
(,F~~kins and Str~rnis:~_.:iacja 2906)~ The r a in f a l l i s characteriscd by a
high surfam run--off that %ed the seasonal streams, These stream
co.mses follow t h z j 0 9 1 C s i n t h c area,
1,'L Previog5VLo~ks ii.1 t h e fma_
The first recoxhd survey of tho N i g e ~ i a n tin fields dates back to
1919 under the direction of J.D. Fa:coner (Macbod e t al, 1971) b The
maj 3r results of the investigations by these earlier workers (eg Falconer,
R x l s u r ! ~ and grain; h a w been summarized by !lr!'xlsod e t al. (1971) s
Tho reports indicate t h a t the earlier survey of reconnaissance status
done maii:iy reveal the essential geological features of the region.
The fasc4.1ating rmdts of these surveys revealed the importance of the
Yotlnger Gra~itc Cmplex as a source of abundant tin mineralization,
T k J s Xed to t h e subsequent need -to establish the boundaries of the
Ycungor Granite Complcx.
The importance of tin mineralization i n the area was reflectsd by
t h e fmquert slirv~ys carr ied out in later years, Between i945 and 1948
a new s u ~ ~ e y carzied out by Mackay, G r e e i m d and Rockingham resulted
i n t h z cs lab2 i skmnt of a more priclse and w l l defined boundaq of t h e
Younger Grmite Complex. It also led to the location of several ring
structures and t h e recognition and study of t h e distribution of alluvial
t s n and colmbitP.
In TYriew of t h e i n c r e a s i ~ g indus tr ia l importance cf colmbite and
N!gerfafs prominent position i n the world columbite production then,
another surv2y 6:' the area began i n 1351 till l(359 under W,N, Macleod,
Ey 1%0-lW t h e manping was extended t o the crystalline basement
hy a k a m of geologists under the supervision of E,P, Wright,
VJrighS (1976) ;:eporkei t h e presence of basalt ic ash deposits of
over 7 meqers underlying lava flows in Jos area,
Turner (1976) showcl tha-t the Younger Granite iniruded the Pre-
Cambrian - L o w r Pa!.~ozoic Oascment of Northern Nigeria i n a N-S
direction and continued northwards into the Niger Republic. He also
pointed out that this direction of intrusion ran parallel t o the main
Pan-African trends i n the basement intlicating control by earlier
s*,ruc=tures.
Horley (1975) and Bowden (1976) carried out studies on the
rrlneralogical conposition of the rocks of the Younger Granites.
A j n h i y e ( l ~ 6 a & bj reported average densities of 2062 g c ~ 3
f o r Younger Granite and 2.68 for t h e Bascrnent complex, The
dansity d~termina t ions shoted higher values f o r the rock types of
Pro-Canbriai? - Lower Paleozoic age w e r t h e i r Younger Granite
comtmpaz ts , This is d d e r t in the mgatlve Bouguer anomalies over
the Yowg6-r Granite stt-ies,
Ras.l'is of some ear l ier yeophysLca!. investlyatiorls (as reviewd
by A:akaiye (1976a) 1, revealed t h e presence of buried channels along
which thz placer deposits from t h e Younger Granites viere deposited.
In the receni; Federai Govermnt of Nigeria 'b?ratsr Dorohole
P r o g r m , soma companies were contracted t o carry out geophysical
work for water bor+.holes in Jos area2 Hmdly any tangible
scientific result has emerged frm t h i s contracts,
Because of r ich mineral resources of the Jos Plateau a x a ,
most geological and geophysical works have been sentred on mineral
exploration, Very I l t t le has been done in the m e of geophysical
methods in the s eaxh of gyound water resources,
GENERTLL G E O L E Y OF THE ARM
2.1 Gen~cal Gee! ow
The area is situated i n the central part of +he 30s Plateau of
Northern Nigeria, covered by h i j u j u sheet number 169 in 1: 100,000 scale
of Geological Survey of Nigeria (GSN),
The area is underlain by rocks of the Pre-Gambrian~Lmr PaLoozoic
Easement Complex mainly mignatitcs, gneisses, Older Granites and tho
Younger Granites of Jurassic age, The Younger Granite Complexes are
donninantly granite porphyries and rhyol ites, S e d i m n t s are restricted
t o valley alluvium, Drainage 5s radial , the head waters of the Jarawa
and Federe rivers r i s e from Jarawa and Shere hills.
Geologically tho area has a daminance of Younger Granites, a
series of not+orogenic intrusives and associated acid volcanics. The
30s Plateau forms the focal area of the Younger Granites province and i s
the principal ccnf.re of t h e associated tin and columbite mineralization.
The early menlber; of Younger Granites typified by volcanic rock Cype?,
largely rhyolites and acid +-.uffa are presorvc3cl i n deeply eroded v d a c
., lhese are succeeded by granitic ring-dykes and plutons composed of
hornbler?cte - b i o t i t e - riebeck2te - granite (~acleod and Turner, 1971).
Minor basic ai~d intermediate rocks are alsc present ( ~ u r n e r , i971),
Macleod et al . (19'71) are of the the upinion t ha t a period of
sroston foll~wed the cmplac mntof the Younger Granites. This erosional
phase =sulted i n the formation of the major morphological units of Jos
Piatcau and surrounding plains. Alluvim then deposited by the Plateau
rivors in some places becam, covered probably in Early Tertiary times
Sv exkrlsive lava flov~s of basalt. Turner (1971) pointed out that these
hxalts have been largely w~athered to clays capped by laterite and
d i s ~ c t e d Ly scbsequcnt erosions. These deposits of alluvia are the
source; of nost of Nigeria Cassiterite productiv and ground water i n t h ~
area.
On tho bas i s of previous works by Bertidge (1971) , Wright (1971)
Masleod and Turner (1971) and Turner (1971)~ the generalised succession
shows a g ~ o i q i c a l sequence of rocks belonging t o f i ve main age groups
of:
(1) Quaternary New Basalts of lava flows and Volcanic cones,
2 Tertiary - Q u $ ~ r n a r y alluvium materials.
2 ) Lover Tertiary 0lde.r Basalts of Lava flows that is now largely
decomposed and overlain by the Tertiary Quaternary alluvia.
(4:f The Jurassic Younger Granites (mainly granite porphyries and
rhyolitvs - all strongly alkaline).
5 The P~c-Cenbrian Lo Scwirler Paleozoic: c~ystallino Basemnt comprisin.,-
of mignatitcs, gmisses and t h e Older Granites (of Pan-African Age! .
2= 2 L - ~ c ~ Y ~ o ~ _ n ~ r + + n$tee_&mp&
-r r~ t h d Younger Granite Complex belongs the Shere Cmplex. Wacleod
and Turner (19?1> dzscribed tho R'iere complex as a small ring-struc ture
superimposed upon thc North-east part of the 30s-Bukuru Yocnger Granite,
Ths Shere cmplex constitutes over f i f t y percent of the rock exposure in
the project, area, It forms a prcminent hill mass, Tho early intrusions
of Shem Complex were gabbrois* But Granitic intrusions dominate the
iater phases of igneous activities. These granitic intrusions show
peralkaline character,
The sequence of magmatic emplacement i n the part of Shere Complex
:si thin t ! ~ project area as shown i n Geological map sheet 169 1:100,00C
of Ge~IogZcaS Survey of Niger5.a (GSN) are as shown below-
! il Raf in Jakin grarite - porphyry (5) (2: 25ot i -k granite and microgranite (%) ( 3 Riekkite - aegirine - granite (W3)
(4j BiotLt? - aegirirle - gramite (Mt3)
( 5 ) Hori-dA~ild~ .- Ppilxcne - Fayalite - granite and microgranite ( M ~ ~ )
( 6 ) Qu~rbz - Olivine - gabbro (M'~) The luca'sions and distribut.ions of these ro:k types am shown i n
figure 2. A detailed description and ?.nalyses of the Shere Cumplex
a ! d its s i u n i t s are contained in Berridge (1971).
Another big unft of the Younger Granite Complex in the project area
is the ;'araqwa Complex, This complex is represented in the area by
larawa .- E o t i t e -, granite ( v ? ~ ) figure 2, This roek unit is located on
the eastern bourdary of the area. The occurence of three pmdaninant
jiihoiogical type5 wi th in the Jarawa biotite - qranftod has been
yeported by Berridge < iT7l). These llthologf ca: t y p s are:-
f . -. ?'' 2 morleratelj: c ~ s r s e inquigranular facies
(2; a mdimn-graj.ned equigranular f acies and
(3) a porphy~; t i c f ine-grained f acies,
The coarse ineqciqxnular facies is located at the central part of the
ccinplsx. To the nor th of the complex is found the m e d i ~ g r a i m d
facies while the porphyritic f i r e g r a i n e d f a c i e ~ OCCJWS the southerrl
ma?:gj.na
Undifferentiated migmatites including viened banded porphyroblastic gneiss. Some schist, amphibolite etc.
Porphyritic biotite and Biotite - Honblendc - granite Quartz olivine - gabbro Neils Valley granite - porphyry Biotite - granite and micro granite of Biotite-Granite Group f I (mdium grained)
Biotite Granite - Group I (coarse grained) 30s Biotite-granite
Rafin Jaki granite-porphyry
Gabbro
Hornblende - Pyroxene - Fayalite - granite and microgranite of Hornblende - Fayalite - Granite Naraguta quartz-pyroxene-fayalite porphyry of Hornblende- Fayalite-Porphyries
Fine a d medimgraincd biotite and biotite - muscovite granite of the Pro-Cambrian basement Cmplex.
Rock types M, OGP and E belong to the Pre-Cambrian - Lower
Paleozoic Crystalline basemnt while tho rest belong t o tho Jurassic
Younger Graqitc Complcxos.
-
FIG2 Geologic. mop o f the oreo shwing rock Vp? C;;iribution ( :cab '1 : 6667
2$3 The Cvctal l i@~asemen.t ;_
The Pre-Cambrian t o Lower Paleozoic crystalline basement had been
greatly ilrirudad a t various times by the Younger-granite rocks, the ch!72
E n s a l t s and Never Basalts, The less affected part of the basement
complex in fie area runs in ME-SW direction, It covers an appreciable
size of t h e arm. Like t h e Younger Granites, t h e c rys ta l l ine bassmonk
t13s various iithological types; both t h e basic and acid rock type are
present,
The najor units of the crystalline ks~men-t include the undiffere*
t i a t e d rnigmatites including vaired, banded and porphyroblastic gneisses,
Schist is a lso present, These uni t s are represented in figure 2 as M.
Ct%r rock t j ~ s are the porphyrit ic biotite and b i o t i t e hornblende - g a n f t e represented by O!P, The porphyritic - b i o t i t e and b io t i t e
hornblende-granite Is located a t t he central portion cf t h e NE - end of the undiffermtiatcd migina-tites. A fine and medim grainsd b i o t i t e and
bio kite - musr;oviP~ granite of t h e Pre-Cambrian Basement Cmplex
dxipnsted as 01T is lozated on the southern fringe of t h e mapped area
~ n d teminatcs i n the east by the bounrlav w i t h the Jarawa - granite complex,
AC the centrs: portion of the migrnatites and porphyroblastic gneiss
is a gabbro rock represented as E i n f igure 2,
To t h e North-most par t of t h e Basement complex i s the Federe
Granbts , a member of t h e Older Granite, It is located between
Shore and Jarawa h i l l s , The Federe Gran i t e forms a nunbcr of masses of
fine t o medim-greined granite within % h ~ ~?:r,eti+.cs,
Foliation 2 e a ----- This 5s a laminated structure resul-ting from segregation of
different minerals into layers parallel to schistosity, Foiiatod rocks
break app~oximately along the fol iat ion planes,
Deformational episodes are represented in the study area by
fo?iations, Foliateon is praninently displayed by all *he lithologic
unl.ts in the area i n varying degree. The rocks of t h e Pre-Cambrian base-
mm*, arrci the Older Granite9 show high degree of foliation which is
expr?ss~d by the pnrallei alliarnnent of micas, amphiboles, xenclitl~s
and quark ve ins , G n ~ r a l l y vertical foliations donginate,
Foliations i n the Fodere Granite are not usually so wll marked as
:.n latter granites, Tna foliations are mostly vertical and trend N S
parallel t o t h e gn?atest elongation of the granite masSss. They are
confoIlFdble with thc folia'iions i n the surroundirg migmatitos. The
nor them part of t ! ~ e Federe Granite shows shallowdipping to horizonial
foliationso
Som off-seto of %heso foliations are observed in places. This
iid!-ratm !.&era1 movement :.-2sultir.g frcm i aa l t ing , Most of thsse
0 0 iollations trend 1~30 -!40 aztmuth, k second set of foliation of less
pronilnetxe t r ~ n d marly N-S direction,
ToSwta and Faults 5 , -- .Jojnts are c m o n and the most affected are the hills. Two sots of
.joints, ~ s a r l y orthogonal a r e recognised i n the area. The major .joir,ts
t~ecd N &i1° \A!. The other set runs North-South. Both sets of j o i rhs are
f-..m near ~ r e r - L i c d to verticalc The rives channels, streai IS and i i ~ h t
f o ~ x t a t i o n in the area follow the joints, Most j o i r t s in the lowlying
are1 a m f i i h d wi th recrys-tallized quariiz. Some recrysta l l ized quartz
veins m a s a m 11p t c 30sm i n width,
The existcnco of f a u l t s have been represented i n the area by t h e off-
s5;ting and displacemen-k of some quartz veins. Some displacements are
upto 4.3crn. The analysis of both t h e magnetic and resistivity CtlCmal fe5
lend +thevselves t o thc bnterpretation t h a t t h e area must have suffered
some faulting, These f a u l t s are not observed on the ground surface, but
t h e i r presence cannot b ruled out,
3,l CbJe_c,t:>~ of the survey
Tho incessant problems of domestic, industrial and agricultural
water sl?ppl:r In areas of hard rock i n the coun+::y req:liro !,mediate
soluklon, Most boreholes found so far i n these areas were drilled
without an;r pre-g~ologlca?. or geophysical investigation, The result is
a ~ e r y low success r a t i o with a great loss in "man-hour" and fund,
Ti12 project was aimed a t using geophysical m t h o d s to locate,
delineate and assess the groundwater resources of Fobour areao This
cbZective was t o h? bshicved using e l e c t r i c a l resistivity and magnetic
ui-ospecting methods,
The clects ical roslst ivity and magnetic signatures of structures
and envisonnents vhich are favourable to hourlng and transmition of
wz.ter exploitable in economic quantity are easily recognizable, The
di : : tinct resist!.v!-ti contrast. betwen mter saturated overburden and
k d r o c k makes .msi st i - i i t y m t h w a prt ncipal exploration method for
tla'v2re LOW resistivity values i n electrical profiling and low rnag:e-tic
r a d i n g s across an area are directly related t o the extent of w a t h m i n g
?nd w!ater satmat<-on of the place. The small size, the ease i r l
handling and the low labour requirements of electrical resistivity and
magnetic in s t rwnJ- , s are other added advcintages to methods used.
Groundwster exploration and exploitation are mostly limited to shaihw
depth. Resistivity sounccing has a better resolution than other
geophysical methods for shallow depth investigation* These factors
have made elec t r ica l res5stivity method the most prominent geophyqical
r ~ t h c d for gzoundwater exploration.
Unlike t h e e l e c t r i c a l method, mag;~et.ic method is s-till a t t h e
t2infazt ' ' s t age a s a geophysical method f o r water explora t iono Outside
t h e use i n mineral explora-kion, i ts use i n o i l explora t ion is l imi ted
t o reconnc5ssance s t a t u s f o r de l inea t ing and es t imat ing t h e th ickness of
sedimontaqr p i l e over t h e basement. I n magnetic . mcthod of s V m e y
areas of t h i c k sedimentazy p i l e appear a s magnetic lows, This magnetic
p rcper ty is used t o d e l i n e a t e a reas of t h i c k overburden f n hard rock
t e r r a i n , Thick r e g o i i t h , buried channels, f a u l t s , highly jo in ted and
fractured zones are t h e poss ib le aqu i fe r s s o r t f o r i n hard ~ o c k t e r r a i n ,
Thcse poss ib le water Scar ing s t r u c t u r e s appear a s magnetic anomaly
low, The a b i l i t y of magnetic method t o o u t l i r e these s t r u c t u r e s makes
magretics a usuful yeophysica'. method in water explora,tion.
It i s hoped t h a t in teg ra ted r e s i s t i v i t y and magnetic methods as
used i n t h e study would produce b e t t e r r e s u l t .
3e2 Jxt.~_n_&aJ-.ion arid f i e l d procedures
Two bas ic instruments used were
( i) 1\11 MEM t x r m c t e i - W
Ii13 A J a l a n d e r Fiuxgate magnetonleter,
3 a ' L o l E~r-yxnetey
The t a r r a m t c r used fo? t h e survey was an IJEM Terrmter (SAS 3m
model, f igure 4a). The SI'S s tands f o r S igna l Averaging System a method
whereby consscutivo readings a r e taken automatical ly and the r e s u l t s are
a v ~ r a g e d continouslye The terrameter SIS 30C contains thme matn U\ ti. ts;
t h e t r a n s m i t t e r , t h e r e c i c v e r and microprocessor, These t h r e e main
u n i t s a r e housed i n a s i n g l e cas!ng.
I n t h e r e s i s t iv i - iy survey mode, i t comprises a b a t t e r y - powered
deep pene t ra t ion r e s i s t i v i t y meter with an out put s u f f i c i e n t f o r a
current electrode separation of 2003 meters, With discrimination
c i r cu i t ry and programming incorporated i n t h e system the reciever
separates IX voltage, se l f potent ia ls and noise from t h e incoming
signal and measrzos voltags correlated with t h e transmitted signal I
current*
The microprocessor monitors and controls operation, calculates
v e l e c t r i c a l resistance (T) autcnnatically, and displays the r e s u l t in
d i g i t a l form i n ohms or milliohms. The range extends fran 0 3 m a
t o 1999 k%nd can be extended down to 0.02 II&L with a booster,
Measurements were taken with the cycles selector switched f o r four I
automatically averaged readings. ~ I
For data r e l i a b f l i t y , safety of t h e equipmen+ cer ta in precautionary
measures wra taken. Some of tho precautions included:-
(1) Hamwring t h e electrodes deep i n t o t he ground t o ensure good
contact;
(2) Ensuring tha t t h e reels of the wires were always positioned
with t h e axi.5 of symnetry perpendicular t o the traverse
t o minimise coupling and induction;
(3) A l l o w i n g the wires not t o cross o r touch one another;
(4) Taking measurements i n four automatically averaged readings
(occassional ly sixteen automatically averaged readings were
taken) ;
( 5 ) Running the traverses In straigh-; lines;
( 6 ) Keeping the reco-ding im*rmnk and the potent ia l cable
reel a t l ea s t 5 meters away from the l i n e of traverse;
(7) Looping the current cables t o firm objects t o prevent pull ing
down t h e recorder while moving with the cable qr generating
high stress o r tension a t the point of cable contact with the
3n2.2 y=netometer
The magnetic instrument used was a portable Jalander Magnetmeter of
a f l w g a t e -- type with type nz 6661 and i n s t r m n t no 7447 (figure 4) ,,
Some of the important compommts of the instrument show i n f igure 4
include: - push bottom whish controls the on and off of the instr~xnent when
taking rneasurcrnents,
po lar i ty knob which 1s used in determining the magnetic pota i iky
of t ho z>.a t ion I %nsi-ttvity s e l e c t i o n knob which ranges from 1 to 5 and used to
within keep the pointer d e f l e c t i o n A ' the scale and the ranges used
i n converting the f i e l d readings t o gamnas. i The bubble or bullqs eye which has to be centred during each
reading. I Measurement t a k i n g involves s e l e c t i n g t h e right poiarity which is
indicsted by tk po:nt?rs def l ec t ion towards the right into the calibca-t im I
region rthen the push bufton is. pressed i n t o polarity knob. S e n s i t i v i t y
selwLion knob is sa adjustai that the pointer does not go off the scale.
- i h ~ n the mxtsuremrit is taken with "f3ull-eye" bubble centred,
Csr ta i n precautions as ( i) centring thz "Bull-.eye" bubble,
(2) t ak ing up t o three readings for one s t a t i o n and the meav taken,
(3) avoiding c b j e ~ t s t n a t could affect t h e i n s t r m e n t reading in t h e
vicin:i.ty are o b s e s ~ e d during measurementsh
Precision of the readirqs frm this instrument ranges from 0 3 g m a z
Lo 12,5 g m a s w i t h most types being close to 5 ganmas, Dobrin (1976)
p509,
Figurc 4a illustrate how t h e f i o l d is measured by elements of t h e
flux-gate magnetometer, Two p a r a l l e l cores, each with a magnetization
curve of t h e type shown a t t h e upper left , are a l l igned with t h e i r axes
i n the d i rec t i f in of the ea r th ' s f i e l d . Primary c o i l s i n series
mzgmtizc t h e two c o r e s wi th the sane f lux density but i n opposite
d i rec t ions , s ince they arc wound opposi te ly around t h e rcspect iva
corcs. The e a r t h ' s f i e l d r e in fo rces t h e f ield set up by one of tho
c o i l s and opposes t h e f i e l d of t h e other . Each c o i l has, i n addi t ion ,
a secondary winding, the two secondarias being connoctcd t o a voltmeter
t h a t reads t h e d i f f e rence of t h e two outputs.
For t h e magnetization of a s i n g l e core i n the absence of an ambient f i e l d as when tho core ' s axis i s nonnal to t h e earth f i c l d ,
thc sinusofdal exc i t ing f i e l d H (curve a of fig 4b) d r i v e s t h c core
past saturation at thc t o p am1 bottom of each cyclo (as shown i n c u m
b) . The secondary voltage is proport ional t o t h e r a t e of magnetic
f l u x *
If an ambient f i o l d is introduced which aids t h e magnetization
frm thc e x c i t i n g current , t h e sa tu ra t ion point is reached e a r l i e r i n thc cyclo than it would be in the presence of an ambient f i e l d apposing
t h e e x c i t i n g f i e ld (curves d and e). I f the vol tage ou tpu t s of both
coil8 are connected i n op;iosition, t h c r e s u l t a n t output (curve f)
consists of p a i r s of pips which arc proport ional t o t h e ambient magnetic
f i e ld .
This pip is registered as field reading. The f i e l d reading is
convorkccl t o appropriate value i n gammcs using a conversion table based
on the range used during mcasurc:i7cntr
3.2.3 Fie ld procedures
The f i o l d work was covered i n two phases, The first phase was of a
reconnaissance s t a t u s . This w?s done i n March, 1983 and lasted for a
per id of one we&. A t this s4age of t h c work, c l e c t r i c a l sounding
s t a t i o n s and l i n e s of p r o f i l e e f o r t h e e l e c t r i c a l 2nd magnetic
measurements wrc selected and areas of interest delimited from
appzrently unrewarding area,
The second phase which constituted the actual survey, was done i n
September, 1*3+ In Fobour, five electrical soundings were carried out
using t h e Schlumberger symme'trical spread with maximum A 0 separation
of 1,4W mters. The spreads were oriented obliquely t o t h e general
t r e n d of fractures and j o i n t s observed in the area. From a pre?.iminary
f i e ld i n t c - r g ~ s t ~ b i o n of some of these souildinps, an eiectzode spacing
of 5 h 'or Ihe Wenner configuration was considered appropriate for
a f o l l o w u p eiectrical profiling. Magnetic mea~urements were also
taken along the lines of the electrical profiles at 50 m intervals ,
Kith this arrangement common station points were established f o r the
elcctr ical and magnetic readings,
A to ta l of t w n t y one traverses wre taken, s i x electrical soundings
of spread :angthc 300 meters to a maximum of 1,400 mtcrs, Seven
e?ec t r ica l p r o f i l j ngs of lengths 603 meters t o 1,400 m and eight
m,zgr.~t i r ; t~air~xscs of lengths 400 meters t o 2,550 meters were also
t;lksn, The t o t a l length of the traverses amounted t o a-er 23,033 meterst.
Majority of t h ~ traverses run in NE-SW direction and additional
traverses wre l a id to cross these, The f i e l d orierrtation and exact
posi t ions of these traverses are shown in figure 3 0 The electrode
separation of 50 m was kept cons*,ant while the electrodes wera moved
along a straight line i n one direction during the resistivity profil ing
mc3asuremen"ts. In addition t o these M%SW traverses, two magnetic
traverses cut these magnetic and electrical prof i l es , h,!easurerner.ts
along these ' e re taken at 100 mters intervals, The NE-Slil orierrtation
Urctticol sounding stations (EW
- Line of e l e c t r ; c d ~ c f i l ; ; . ~ EP I ,
- - - Cine nwe~ t r o w s ? ( M W ,
HC - Heo!th centre.
Sch -!%hod. ,- a Recommended s i t ~ .
waz chosen t o h a w a t ransverse r e l a t i o n s h i p wi th tha strike of j o i n t s
and fractures in tho area,
O m e l e c t r f z s l sounding was done very close to a water bore-hole zt
Jerigere near Jos. !4ith the known s t r a t i g r a p h i c sequeqce i n t h e area,
it was hoped that the interpretation of the sounding should o f f e r some
contro l arid guid? fox t h e correct in te rpre ta t ion of the measurements
taken around F o b ~ u r , ~
The &ect r lca l soundings curves wre intswreted by par t i a l cume
- matching. he ceoe?ectric sestion of the soundings arc! shown i n table
7 and f i g u r e 52.
3.361 E I _ e ~ t ~ ~ ~ ~ ~ i ~ ! i n g .(BE?)
The spread i? oriented 230' azimuth and located XXm from Iyanb
Fohour junctio?, F:;.g. 3.
-- lhe maximum cursenC electrode separation is XWh. The f i e l d
appzren-i; resis.i;lvii;;- ~ead2ngs a w shown i n t ab l e 1, and apparent
r a s i s t l v i ' y versus nai f current electroda apread lograthrnic p l o t is
cho-m i n figure 5, The geeelectric para~eter as determined from
g x p h of f igure 5 by partial curve matching shows three layars overlying
the basemerl-t, This is also shown in tab?: 7,
The in te rpre-k3on shows an overbus. ,:en of about 10% t h i c k ovc?lyi;lg
t h e Ixd rock , From the rllrface, it shows a top layer of about 4m 'chick
and resistivity 82Q;k m which i s t h e dry l a t e r i t i c t o p sof 1. Underlying
it nra the se:;ilnd and third layers 15m and 8th thick respect.ive:v and o f
the water sa tura ted zone. Water level in n near by l i n e d open well
measures about 301m from the ground surface. This water is believed to
h3ve come frm t h a va:dose zone with thickness 1% and r e s i s t i v i t y of
about 1 4 5 c ~ ~ t .
Between t he lower boundary of t h e third layer and the underlying
be&-rock is marked by a high resistivity contras t , The underlying
msbtive lzyer of the order of 558am from depth of 157m is
i n t e r p r ~ t a C e d as %ha unweathered and possibly unfractured b~sement.
3.3 *2 g,l;eek.cal S mcli nq (@A)
The sounding s t a t i o n is located about 800 mctezs south of iyaw
Fobour junct ion a t a cross-road, It is oriented in a N-S direction with
an azimuth of 20'.
A maximwn spread length of 1,400 meters via$ covered. The apparent
resistivity readings are shown i n table 2 w h i l e t h e sounding curve is
shown in f i g w e 6 , The geoelestric sec t ion shows three layers of total
t9ickness 4? m.',qrs unclsr la in by the basement. This is shown in table 7 ,
A tc'p layer (dry l?'ceritic soil) 7 meters thick with a resistivity :-f
24OiIr5t.m is under le in by another layer 7 meters thick and having a
rosis'iiviJ~:r of 8~'3-?;_ril, The second layer is t h i n n e r than its l a t e ra l
equivalent in EZ,l arld probably contains l i t t l e o r no water as revealed by
its relatively high resistivity* From 1 4 i ~ e t e r s t o 49 meters is a very
lcw resistive medium with a thickness of 35 meters and an av:?rage
resistivtky of 42.. ':. T 5 ~ s horizon is believed to be a coiitinuation
>f the t h i r d laye: of ES.1 but thins southward i n t h e m:,pped a~rea.
3t3 low resistlvzty a: i n t h e case of t h e t h i r d layer of ES.1 prob?blq1
0 7 - 7 - , I I I I I I I I -- T I I , , ,
I-
3 A 3 ,Electrical soundinq (€,S,3)
It is located 1,300 meters from Iym-Fobour junction towards
Fobour siesion. The spread is oriented 205' azimuth (see figure 3).
A maximwn current electrode separation of 4N meter? was attained,
The resistivity readings are shown i n table 3, while the sounding
cu-rve is shown i n figure 7.
The fnterprsta.l.,ion of the graph shows a geoelettric section of
t h r e ~ layers underlain by the basement, Their respective thicknesses
and corresponding resist ivit ies are shown in table 7 , The layefs
above the bed rock have total thf ckness of about 56.1 rwters.
The sequence from top shows the top soil 1, l meters thick with
a resistivity of 4501crn. The secortd layer has thickness of 5.0
meters a d i s charactaised by high resistivity of m. Then
c m s a third la;rcr 50 meters thick with a resistivity 257JLrn.
T h i s third laye: i s underlain by the bed-rock of high resistivity o f
over 50(nrtrn, i3ls h!gh resistive bed-rock corresponds t o the
unwatheced and possibly unfractured basement.
Thr, unusually high resist ivity of t : ~ e second lager is interpreted
as resulting possibly from faulting and or mineralizatlm of
resistive materials such as quartz along t h e faul t plane. The
hf i l l ings of quartz veins as observed .long fractures and joints
plams i n the area and arourx! minitic, sites tend t o support suct. a
possibility.
Resistance R
(+I
303,4 Eefir-cal soundina (E S ,4)
' The sounding s ta t ion i8 located 6% West of ES.3, The spread
i s oriented 200' azimuth with a maximum separation of 6Xh. The
apparent resistivity values are shown i n table 4 and the graph of
resistivity versus the separation i n figure 8. Thicknesses and
resistivities of t h e various layers are shown in table 7. The
geo-electric section shows alternating high and low r e s i s t i v e
layers stndlar t o that of ES.3,
Frm the ground surface are tho followingz
1 2,311 top soil of resistivity 830am
(2) a sacond layer 8+05m thick and r e s i s t i v i t y 1245fi.m
(3) a third layer of interest of thickmss 0Qn and resistivity
415.5;; m.
F m a depth of about 91 meters is the bed rock believed to be
unweathered resistive part of the basement. The resistive
second layor is suspected to b a continuation of the second layer
i n ES,3 ar.d p o x f b i l y marks the trace of the inferred fault.
The sounding station is located behind Fobour Primary School
and oricrrted 230' azimuth. It has a maximum length of 6COn, f he
exact iocation and orientation is shown in figure 3.
The apparent r e s i s t i v i t y measuremnts are shown i n table 5 and
the graph of apparon resistivity versus half the spread length i s
in figuxe 9,
The interpr- tation reveals a serries of thin layers of 1-41,
1&, d,%, and 40.% overlying the infinitely t h i z k basement with
Resistance I:
(a)
Apparent Rss+istlvit~ (i iJ- m)
970.75
770.32
7945i
853,73
1071 ,?8
1031.12
620,15
485.90
379-61
405,265
me845
c169,14
487-73
E E a R I C A L SOUNDING 5 (ES.5) DATA
LXATIt7hl: EEHIN3 FOBOUR PRlh4ARY SCHOOL
res;stFvitLs of ? 8 O ~ - ~ r n , 109,<js, 3220 d - n, 1093)-*m and 20719- n
respestivtliy.,
Fron table 7, it is apparent t h a t the area has a l a t e r i t i c top
soil of ? t c ~ %I or more underlain by 5 t o 1% +I;ick udersaturated zone,
These tw layers are separated from the bed rock by an exploitable water
Sdaring horizon with mai<imm thickness of lOCLn at sounding station ES.1,
This exp?o5:hb:e horizon thins down towards ES .2 and ES ,5+
3-3,6 j3;,i&.~Lval sounding (ES #6)
T h i s soundir~g w3s done off the project area but in c? similar
geologic setting, It wa9 located cf ose t o a bore-hole of known strati-
araphic soq1.lence to act as a control.
The z~sistivity maclings are shown here i n Table 6 ; the graph of
resistivity versus half current electlode separation is shown in.
F5gwrs 10,
The ii~i:erpr,:tation of the resistivity curve shows a thzee layer
strttct.are. F:mi t h e surface are layers of thickrtesses 1.2Ckn, 9,h,
1313 2nd t h e basemeat of i n f i n i t e thickness with resistivf ties l l 2 - ~ ~ r n 9
163.f5, rn, 137~2 m, and 2749n-rn xespectively. The gebelectric section
.:mp~as satisfactorily with observed stratigraphic sect ion during
drj.llingm
The sequence of layers as observed during drilling obtained from
bore-hole records F,D.W,R Jos is shown below
0 - an = topsoil
2 - 10,5111 = sand
10,s - l?-c5m = weathered granite
l l c5 - 14m = w a t k x e d granite and sand
!?EtTLiCAL SOUNDL-ffi 6 (ES.5) DATA
LOCATYON: 1% FRCM F,D.W,R. BORE-HOLE AT JENGERE (The soreact was alligned parallel to the road)
TABLE 7 -- PZSUIITS OF ELE(JTR1CAL SOUNDINGS ES1-6
SOUN!lIEi STA i Thickness of rious layers I Resistivit~ of various lay~rs I L- , an$\ I - .--
bedrack Resistivi of Tapable Horizon
107 558 nz 2 8 3 layers
49 800 nz 3 layer
56.1 5140 n; 3 layer -. . . -
1 - 90.9 375+.. not attractive
47 2071 I no 3 8 4 layers 1
.A . .
23.2 2749 1 no 2 8 3 layers. 1
14 - l7m = fractured gran i te and weathered
17 - 18Jm = micacsous sand
18,:5 - 2:m = fractured gran i te
21 - 46m = very hard fresh gran i te with very few fractures. . From t h ~ f i e i d curve interpreta t ion and the lighologic log, the f i r s t
Zayei- from the sounding corresponds t o t he top s o i l , the second layer
i s reprcsent.ed by 8,5x th ick sand. The wathered grani te from depth of
10'3 meters t o 21 m t e r s corresponcls t o the t h i rd layer of f ie ld c u m .
3,3,7 Fieci;qica.Palrof jj 1 9 na i EP .I)
The profile is oriented 115' azimuth and runs transverse t o other
e:lsctrical p rof i ies i n f igure 30 I t covers a length of 8%. The
r e s i s t i v i t y and the corresponding centre of spread distdnce values are
shown i n tab le 8 while the graph of the r e s i s t i v i t i e s versus centre of
spread is shown i n f igure 11.
A look a t t h e EP.1 graph from NW end shows a r e s i r t i v i t y low frm
the beginiag of the p ro f i l e t o 40C)m. It's minimum is regis tered around
l m The resis-L?.irj.ty increases gradually t o a maximum a t 75Ckn witti a
sharp drop a t 70(Xn, The r e s i s t i v i t y low i n the region of 0 t o 4Cam
is interpreted t o indicate a water bearing horizon i n an area of deep
wea.tharing o r buzied r i v e r channel f i l l e d with water saturated a l l w i a l
matezial, The broad f lanks of the curve rnhy indicate a wide river channel.
ELECTRICAL PROFILE 1 (EP,~) DATA
LZATIO!L': THE SAVE IS MT.2 3 ~ ~ 3 1 hr; : 115'
Distance (m) Apparent Resistivity
p+h) 0 iDI-end 254, 47
50 242.85
. ',@!I 4634
150 88.54
200 m, 12
250 166 650
300 261.37
350 294.37
400 301 m59
450 372659
500 454e90
530 552.29
600 1 551.66
650 I 647 17
700 433e54
750 731 a99
800 414.06
SE-end. 306.95
I I I - 1 I -- I N o!? a60 L 00 b o o i 800
cent re , o f s p r m distance !m)
FIG11 Res is t iv i ty curve of E P i
3,3,8 Eiectl-ical Prof i l im (EP.2)
This p m f i l e EP.2 shown in figure 3 is oriented 245' azimuth and is
6 0 h long. Table 9 has t h e s tat ion distance and resistivity values,
?he graph of the resistivity vezsus centre of spread distance is shown
i n figicra 12.
From the SW of th5 profile, tbre i s no abrupt change i n the
resistivity betwen 5Qn and !XKh except some minor fluetwtior'ls that
show gradual incrcase in apparent res is t iv i ty , B s t ' e n 500 avid 5%
an abrupt, s:i,ange in resistivity to a very low value appears. Th i s to
resfst ivi ty anomaly appears related t h e obsemd a ~ m a l y i n C~400m A
of P,l of figure 11.
3-3.9 Electrical Profilinq (EpC3)
The profile is oriented paralled t o EP,2, located a t 20rh south of
9.7 . The apparent resistivity and centxe of spread distances a r e
showi I n table 10 arid figure 13 shows the graph of apparent resistivity
A SVdcNE clescri;?tfon of EP.3 shows a rt31atively M g h res is t iv i ty value
from 0 to 13Q-n. Betwen 15U and 20an shows an abrupt change I n
a.p~arent resistivity w!-~ich continues with gradual decrease -from 'LOOm
t o across 800ms 1:n~nl
The consistency in thentrend of the resistivity ananally low as
Ind?.sated i n EP.1, 2, and 3 lends itself to a structurally corrtrollod
anom ly source,
TABLE 9 ---.--
ELECRICAL PROFILE 2 ( E P . ~ ) DATA
LOCATION - rJJl BEARING THE SAME AS IN W.3
>q-- Distance (n) Apparent Resistivity
sL,
EXCTRICAT, PROFILE 3 (EP, 3) DATA
LKATIOIJ P,ND BEARING - THE SAME AS IN W . 4
Distance (m) Apparent Resistivity
--- OSW-End
' 50
100
150
200
250
300
350
400
450
500
550
600
55c
700
750
m0 IP End,
I n s t . i : w n t Reading (Resistance <-R>j
3A10 ,Flectrical Prof i l im (~p.4)
The NF end of EP,4 is located 1% east of tho NE terminus of
8?,3. The trend of the traverse ( E P , ~ ) is d~v ia ted to a bearing of
210' to avoid scme obstacles, but a general SW-M trmd is still
maintained, The traverse is 10Xh long.
Fig, 3 shows tne location of the traverse on the surface. The
centre of spread distances and station readings are shown in table 11,
Figure 14 shows the graphs of the apparent resistivity versus the
centre of spread distance*
The SW--hE graph of tho apparent resistivity shows an anomaly
b2twoen 13Qn ami POO~IL betteen 350 meters ancl 650 mters there is
no appreciable variation in resistivity. A resistivity h'lgh is
observed bo-kwecn 790 r,,eters and 900 meters with its peak at €03
meters, ?ran 95C meters t o the NE end of the profile another
resistivity Losv shows up, Field observations show that these
resintiv~.l.f M ~ h s correspond t o areas of th in regolith and areas of
exposad basecent,
3+,3.,11 i;'J1ectricalL.-?~f~7.i:w (EP.5)
kF,5 is separakd from D.4 by 280 meters ancl runs paraliel t o
EP.4, It is 1200 mtess long, Figures 3, 15 and table 12 show the
Zo.:ation, qraph of tl~e apparent resistivity versus the centre of
spread, and centre of spread distance and corresponding resistivil;{
Z G S ~ ~ C ~ ~ V O ~ Y ~
The SW-NE graph of the measurements shows minor distortions
batwen 50 meters ar.J 200 meters and an abrupt resistivity low
DLstance (m) 1 Apparent Resistivity I n s t r m n i . Reading I ( a m ) (Rcsistanc:e)<n>
.C.?&LE - 1 -- ----A:':
S?2ECTRIC.4.L PROFILE - 5 (EP.5) DATA
'~.OC:J-~~:!:!Y A I D FEARTFG - THE SMIE AS IN kP,..6
LOOATION;: 10Cm FROM W, 4 -ie 300m FROM f YAM-FOBOUR J U K T I O N
Acading I
- Polarity
-- Svs
-1-vs
station distance ( rn)
F G 22 Magnetic
A s u b s ~ r f a c e s t ruc tu ra l connection bstvreen theas lows ct tile
YZ ends o f tile magnetic p ro f i l e s can not be ruled out,
I 3.46 --- Mxpst ic Profile (MP,~)
0 T h i s is located 15011 south of MP.5 on a bearing 210 . The magnetic
prcfi3.e is 1850m long, The measured magnetic in tens i ty (az, and ths -
Instrument Re acli ng
--- ?20
243
360
240
200
190
:a3
1.6d
210
200
220
?40
243
380
'330
200
280
220
rn 260
185
19C
220
--
- Range
- 3
FIG 2 3 Mags-tic anomaly curve of MP 6
Instrmcnt Reading
180
' 180
1 60
140
140
150
160
i 60
180
220
160
160
160
230
349
130
-
- Range
-
Time Polarity
+ve
Reading in Gammas - 2 ,814
1814
1613
141 1
141 1
1522
1613
1613
1814
2218
1613
1613
16 13
2318
3427
C G Z L Magnetic anomaly c g r v e of MP 7 , i
T.'...RLG 22 --- -<->-
hUiGN.CTiC PROFIE 8 (MP,8) DATA
5 30Qn AFTER FOBOM? PRIPJARY SCHOOL, ON KUDEDU RCAD
Range Tine Polar i ty 1- 1 !
station distance (rn l I
I
anmaly grapho
SbV-.NF description of the anomaly from 0 t o 300 maters shows a
nsgative uniform anomaly which has a l inear re la t ionship with t h e
negative anomalies observed i.7 MP.6 and W.7, There is a sharp rise i n
the magnetic anomaly betwen 300 meter$ and 350 meters t o a value about
25W ganmas, The NE pa r t of the prof i le , marked by high posi t ive
magnetic anmaly coincides with the col separating two in t rusions of
Bumblenda -. Fayali te - Granitee. These intrusions designated a s M2 i n
the southern par t of the mapped area i n f igure 2 is believed t o have
co:ltributed -to the high aagnetic reading a? the vicinity.
3.5 Discussion
A combination of t h e e l ec t r i ca l sounding3, e l ec t r i ca l p rof i l ing and
magnetic measusements 1s i s the case i n t h i s study gives a good idea of
t he subsurface feature i n the area i n three dimensions, Magnetic
signatures of thicker weathered and water saturated overburden over
hard rock t e r r a i n ' h i ch have been reported by earlier workers, i f
i o s ~ r z e d and cornbi,.:d a i t t t h e h e c t r i c a l sounding and profiling results,
shou;J ix able t o confirm t h e presence o r ~ 5 s e n c e of economic exploitable
w a t ~ r i n at-. area,
The tnterprcta t ions of e l ec t r i ca l somdings (ES, 1-5) presenttd i n
tab le 7 shows t h a t the thickness of the overburden varies from place t o
p l a ~ e . A maximan ovarburden thickness of about 107 meters and aquifer
thtclmess of about 103 meters are recorded i n ES,1. The aquifer
r 5 s i s t i v i t y values range botwecn 6s-m and 1- m, Stations ES.2
and E P , 5 located south of Eel have smaller thicknesses of 49 meters
and 47 meters zespectively f o r the overburdenr The aquifer thickrms
$5 a l s o small a t t h e s e s t a t i o n s south of ES,l. S t a t i o n s ES,2 and ES.5
show squil 'er th icknesses of 35 and 50 meters respec t ive ly indicating
th in ing of t h e aquifor .southwards (see Figure 3, 5-9, and t a b l e 7).
The e l e c t r i c a l and magnetic anomalies are a t t r i b u t e d t o the presen~e
o f m r i o u s rock typos and probably s t r u c t u r e s ~ i k e f a u l t s , fractures,
and buried r i v e r channels.
- & h e c lose resemblance of apparent r e s i s t i v i t y measurements of
p r o f i l e s 2, 3 , 4 and 5 as wll as t h e semblance a l s o noticed among are
t h e magnetic p r o f i l e s of 6 , 7, and 8 ' , i n t e rp re ted t o ind ica te a near A
su r face homogeneity i n t h e azea.
The anomaly s o u c e responsib le f o r t h e approximately l i n e a r
r e s i s t i v i t y lows trnnding North-South on the western end of t h e
e l e c t r i c a l measurenlcnts of p r o f i l e s 2, 3, 4 and 5 with broad anomaly
f l anks , appzared as magnetic low in p r o f l l e s 6 , 7 and 8 of t h e magnetic
rreasuremcnts,, Yhe l i n e a r i t y and broad f l anks of t h e anomaly could have
i x . x i t e d from pwbably f a u l t of much l a t e r a l displacement cr a l i n e a r
s t ruc5ura l iy control-icd buried rivex- channel. The presence of a 'ir)
p-obable second fau1.t t rending North-South and located on the easLern
p a r t of the p r o f i l e s has a l s o been in fe r red from t h e ana lys i s of
EP,2-4 (see figs, 25a, 26b, 27a and 27b), Unlike t h e f i r s t irrferrecl
f a u l t , t h e second f a u l t is not very apparent on the magnetic p r o f i l e ,
Stahl (1973, 1974 and 1975) showed t h a t i n s t e e p l y dipping Faults:
t h e r e s i s t i v i t y p r o p e r t i e s may be divided i n t o t h r e e d i s t i n c t parks
namely the r e s i s t i v i t y o: blocks on each s i d e s of the f a u i t and t h e
z s s i s l i v i t y cf t h e f a u l t zone . i t se l f , H e a l s o pointed out t ha t var ious
cornkinations are poss ib le , f o r example
( 1) the r a s i s t i v i t y of t h e t h r e e blocks may be unequd
CIG XJ R n c e i iog~am cl E P 2,3 ?. 7
FIG 27a Fmce diagram OF Y P 3 84
(2) the two fault blocks may be of equal r e s i s t i v i t y , while t he f a u l t
zma ma-? appear as e i t h e r more r e s i s t i v e or more conductive than
the bloclcs,
But whether t h e f a u l t i s more conductive o r more resistfve, t h e electrica
r~cis-Livi ty curves of t h e p r o f i l e s a r e angular a t the point!; where an
elec t rode crosses the f a u l t . Conductive fau l t s appear as mlnimum while
a r e s i s t i v e one manifests i t s e l f a s a maximum, S t a h l (1974:l futher t
sho:rrr?d t h a t a r e s i s t i v i t y anomaly f l a t e n s out with increasi.rq e l ec t rode A
sepe~ztiorl , T h i s i n any case may have contr ibuted t o s m extent t o t he
a ~ p a r e n t broad f l a n k s of the resistivity curves that iridicated t h e
first inferred f a u l t . Satpathy and Kanungo (1976) noted that. hard rock. terrains show
low t o t a l magnetic i n t e n s i t y wer t h i c k weathered overburden. The lower
t h e t o t a l magnetic i n t e n s i t y i n such ai> area (hard rock ten'aj-n) t h e
th icker t h e overburden,
Henkel atic! Gvzman (1977) attribtittld l i n e a r magnetic rniriimum t o
l i n e a r fractum zows o r l inear structurally controlled buried river
c h a n x l . They alzo pointed out that t he magnetic minimum rr!sults
from t h e oxidat ion md hydration along the l i n e a r f r a c t u e >:ones,
The readings and the r e s u l t s of electr ical sounding ES,1 ( t a b l e s 1
and 7!, t h e very low magnetic i n t o n s i t i o s as obsarved i n Fm, 3 ancl F.P.4
of figures 20 and 21, the low resistivity values of EP.3, W.7 of
figures 13, 17 and 28 ind ica te that the overburden is thicker In the
Nortbeastesn part of the surveyed area of interest. That tihe overburder
and t h e probable aquifer t h i n southwards and poss ib i iy less saturated
with water are indica ted by t h e increas ing r e s i s i t i v i t y and rnagnc-tlc
i n t ens i t - f due & x t h , ?!I@ assoc ia t ion of low resistivity and: low
87
magnetic i n t e n s i t y values i r t h o surveyed area agrees well with the
observations of Apparao and Roy (1973), Apparao (1977), Satpathy and
Kanugo ( 1976) . Another t h i c k ovcrburclen apparent on the w s t c r n perimeter
t rending approximately NorthSouth i s also suspected. This can be
a t t r i b u t e d t o l incar s t r u c t u r a l cont ro l ,
The r e s i s t i v i t y may (figure 28a) shows a r e s i s t i v i t y c o n t r a s t
t h a t d iv ides t h e area into two major port ions. The western por t ion
charac ter isad by high r e s i s t i v i t y corresponds t o the Younger Grani te
Complex (Ms of f i g u r e 2). Tho high r e s i s t i v i t y probably i n d i c a t e s
nearness t o the surface of the umathc rcd Younger Granlte . The
1XX) f lm contour l i n e along t h e vestern baundary of t h e survey area
de l inea tes the Younger Granitc with r e g o l i t h from tho unwathered
outcroping portion.
A g r e a t par t of t h e area expccia l ly tk e a s t e r n ard southern
parts g e m r a l l y show low m s i s t i v i t y va lues defined by resistivity
contour lines of 100s m t o 4OOn m, Few isolated spo t s i n t h e
ccntral p a r t of t h e area with r e s i s t i v i t y values of 60C t o 7 W m
are also present. This area belongs t o t he Pre-Cambrian t o Lower
Paleozoic D~scment com~lcx, The low r o s i s t i v i t y probably indicates a
t h i c k water bearing and weathered horizon.
Unlike t h e r e s i s t i v i t y rnTep, the Younger Granite boundary with
t h e Pre-Cambrian Baserncnt is well defined by tho magnetic anmally
c o n t r a s t , The Younger Grani te i s defined by t h e very low magnetic
67
anmalies of -400 t o -700 gammas on t h e western part of t h e area.
The - 200 gamma contour line defines t h e boundary of the Younger
Granite with t h e Basement. The Pre-Cambrian Basemcnt i:s chncrscteris
by high magnetic i n t e n s i t y v s lues except a t the North-wstcrn part,
Two areas within the Pre-Cambrian Basement (along P.P,6 of f igum 28b)
of exceptionally very high msgnetic .values correspond to the out crop
of k i l s Valley granite-por hyry (Y). P
Two bore hole sites have been recmended on the basis of the
magnetic and e lec tr ica l signatures which correspond to ~mcjnetic and
e l e c t r i c a l lows (figures 28 and 29),
FtG 28a Resistivity m a of the survey .. .
CHAPTER FOUR
CONCLUSION
The r e s u l t s of the survey showed two probable loca t ions of
p o t e n t i a l aquifers . The aquifer depths and thicknesses show v a r i a t i o n s
from place t o place. Maximum th ickncss of overburden of 107 meters
is reg i s t e red a t e l e c t r i c a l sounding s t a t i o n number 1 (S .I). A
m i n i m u m thickness of 47 metors is recorded a t e l e c t r i c a l sounding
s t a t i o n number 5 (ES,5). Thininc; of t h e aqu i fe r i s suspected south-
wards. Subsurface r e s i s t i v i t y and magnetic v a r i a t i o n s suspected t o
be s t r u c t u r a l l y cont ro l led were also observed,
The area of i n t e r e s t has no bed rock exposure on t h e surface.
The geophysical r e s u l t s sharacd a r e s i s t i v e i n s i t u 01. t r anspor ted
hard dry t o p l a t e r i t i c s o i l , The th icknesses of t h i s t o p s o i l vary
from lrn t o about 7 meters i n places. The i r r e s i s t i v i t y va lues vary
from 450 m t o over 2000 m. Underlying the t o p !;oil is t h e
weathered and aquiferous medium of low r e s i s t i v i t y va lues of 42 m
t o 257 m and th ickness of 35 mctcrs t o over 90 me.ters. Th i s
aquiferous horizon gradual ly grades t o unfractured 2nd unweathered
r e s i s t i v e basemcnt .
On t h e b a s i s of near surface homogeneity and f i e l d masursmnts
the two sites f o r t h e rccommcnded boreholes ( f i g u r e s 3 and 29) are:-
( 1 ) t h e area enclosed by t h e NortbSouth road t o Iyam, EF',7, EF,4
and t h e road .to Fobour mission and mining camp,
(2) t h e area north of Lenge-Kudedu road, west of EP,5 and vest of
E P m L .
P-A present magnetic me+hod is r m e l y used f o r ground water prespec-
.Ling, Thz r e s u l t ob tn imd i n t h e survey with magnetic method is
encouraging, The use of magnetic method t o outline and delineate
structures capable of housing t h e aquifer required in hard rock
terrain would makes it a potent ia l geophysical prospecting method
fo r water, From these attributes a quick rscogni t lon and wide spread
use of magnetic meti-:eel in water explora t ion i n had rock t e r r a i n i s
anticipatedo
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