University of Nigeria Investigation on the Inyi Coalfield, Oji...University of Nigeria Research Publications Author ANEKE, Jack-Jacobs Uzoma PG/M.Sc/81/1288 Title Geophysical Investigation

University of Nigeria Research Publications

ANEKE, Jack-Jacobs Uzoma Aut

hor

PG/M.Sc/81/1288

Title

Geophysical Investigation on the Inyi Coalfield, Oji-River Local Government Area, Anambra

State

Facu

lty

Physical Sciences

Dep

artm

ent

Geology

Dat

e January, 1984

Sign

atur

e

GEOPIiYSICAL II?VESTIGATION OF THE INYI COAWIETD , On-IIIm IX)CBL GOIZRIQmTT P S U ,

ANANBRA STATE.

AXEKE JACK-JACOBS UZO1,U (PGD. sa/8?/1288 )

To Xris, for yow: unique

brotherly understanding.

TFi.BIZ 017 CONTENTS

INTRODUCTION .. . . a . . . . . . . . a

The Southeastern Nigeria sedimentary basin Previous investigations of the Nigerian o oa l f ields . . . . . . . . . . ..

Location . . . . . . . . 4 . . . Geonorphology .. . . . . . . . . . . Ge010w 0 0 . . 0

Tho ~sukka';ormation . . . . . . .. The bjaU Sandstone .. e r n . . L o .. Structures . . . . . .. . e . .

G2nPIflSICU SURVEY 6

Aims and S C O ~ of the ~ ~ r v e y .. a

Instrwnezitation and field prooedures e

Results of tho eurvey . . . . . a

Interpretation . . . . . . . . . . POUR

CONCLUSION .

LIST OF P I G ~ S

Ti t le - Ease map ahowina traverse lines of Gravity and ~ e s i s t i h t y l ~ & t u and newly esta3liskcd-coal bounCariea.

&p showing goology of p a r t s of Inyi

Geology map of Nigeria

Map showing location and acceaeibilftg of the stL~dy area.

Toctoixic m p of Southemstem Nigeria.

Coa l pzcas mp'ped in Migeria

Graph f o r &if% oonputation

Shows t h o layout of C-ravfty and Elec t r i ca l ronistivi-by trwerse l ime

Gravity p ro f i l e of GS-2

Gradty anomaly o f GS-2 reduoed t o an arbi trary d a m

Grade profile of GS-3

T i t l e - Page Gravity anom3y of GS-4 reduoed t o osl arb i t ra ry dalnm 45

Gravity prof i le of GS-5 4 7

Gravity anomaly of GS-5 reduoed t o tm aylbitzazy datwn 4 8

Graph o f v e r t i c a l e l e c t r i u a l soudine;, VES-I 50

Graph of Vertical E lec t r i ca l ~oundiilg, WS-2 5 2

Graph of Vert ical Elec*trical ' Sounding, 'GS-3 5 3

Graph af VarticdL Eleo-brical Sounding, VES-4 55

G r q h of EP-1 prof i le 56

Graph o f m-2 prof i le 5 8

Graph of EP-3 p r o f i l ~ 5 9

Graph of W-4 p o f i l e

G~aph of m-5 prof i l e

Graph o f GS-1 and El?-3 superimposed

Graph of BS-2 and EP-2 superimposed

Graph of GS-3 mr3 EP-3 superimposed

Graph o f GS-4 and EP-4 mperimposed

Graph o f GS-5 and EP-5 superimposed

Xap showing the comparison between the old a d new coal bounciazies,

Borehole aeotione at 1,-

Correlation of dopth p ro f i l e s at Inyi

Shows in te rpre ta t ion of vazious r e e u l t s of the v e r t i o a l E lec t r i ca l Soundings.

Table - 1 The lYigoria Strr~tigrhphy as representahd by -the

type 1-oc~l i t ios . , i n Southeaatem Nigeria. 2

D a t a of Gravity survey traverse GS-2 87

Data o f Gra-~ity survey traverse GS-3 88

Data o f Gravity survey traverse GS-5 90

Data of ver t ical Slectrioal eousrdfng VES-1 91

Data of Vertical Electrical Sounding VXS-2 9 2

Data of Vertical Eleotrical Sounding V3S-3 93

Data of Vertical Electrioal sounding VES-4 9 4

Data of horizontal profi- traverse E-I 95

Wta or" hoz%zon%al profiling trasrerso EP-2 96

Data of horizon'tal profiling traverse a - 3 9 7

Dnta o f horizontal pxofi3l;ng traverse m-4 98

Data of horizontal profil-in& traverse EP-5 99

M t h U a estimation of depth -to momrr3y f'2a gravity profiles

Calaubtion of regression IAne for drie aomputation,

iv.

My special md heaztfelt thanks i s accorded t o ny

supervisor, Dr. K.M. ONUOHA, f o r his unrivalled aocessibility

and helpless oontsibutions that aided ao eignally the cause

of the synthesis and timely produotion of t h i s work. Helpful

criticisms by 3r. L.I. Mamah 5s also aoknowledged wi!h thanks,

I am aluo p a t e f u l t o Messre C. Mbaai, A, Ohms,

T. Madu, G, Obi, A. U o , h e n e and Amobi Ozougwu a d Tony

(secretary) f o r the i r helpful oontributions in the various

stages of t h i s work.

Finally, other people who in one way o r the other

enhmoed the success of th ia paper - I say, thank you.

Ve

~ S S T ~ ~ C T

The I n y i area i n the O j i River Local Government Area

of Anambra State is under ldn by sediments o f tho N-a

Formation (~aleooene ) and the A jali Sandstone (Upper Maestri-

chtian), and bears one coal seam out of the s i x ae- tha t

have been disoovored i n Nigeria i n the past yyeasa, Gravity

and e lec t r i ca l r e s i s t i v i t y meaawrements wero conducted t o

provide a geophysiaal data - base fo r studying our aoal-

field#. The eleutriarrl moeteurements involved' both ver t i ca l

sowdings and horizontal profiling, About 80 gxavity atations

were established along 5 profi lea at lOOm intervals , while

r e s i s t i v i t y moaeurements were conduoted d o n g 9 profiles,

Negative Bouguer o~lomaliee and positive r e s i s t i v i t y ex ours ion^

were obtained over the ooa.!l-bewing layers. A now ooaL

boundary was established mound Obule-Inyi, and the

measurornents have helped t o define more 01earl.y the boun8m-

iee of the a e m in tho region, The resu l t s Indicate good - proapeots fo r using a combination of both the gravity md

elec t r loa l r e s i s t i v i t y methods i n mapping wer-burden

' thicknesses ma f'raoture zones i n our aoalfields, and oould

conoeivably be applied t o tad& the hydzogeological problems

in other o o d mining aroaa, ee. the Olrpixra and OnyemaMines

i n Enugu,

CWEEAPTER .OPE - INTRODUCTION

. .

1.1 The Southeasterp Nigeria Sediment- basin:

The six seam o f a o a l discovered w e r tho years i n

Nigeria ( Inyi , Enugu, Ezimo, Onikpa, Okaba and ~gboyoga.) are

located in the southeastern pa r t of the country, Pne south-

eastern Nigeria sedimentary basin, is believed t o be marked by

three major depositional cycles, which were controlled by

three main tectonio and epeirogenic phases (Short m d Stauble,

The e a r l i e s t tectonic a c t i v i t y s ta r ted i n Ewly Cretaceous

(see Table 7 ) as a. NESW trending fault-oontrolied depression

formed i n the West-African basement complex, This r i f t - l ike

basin ( the Benue-Ab&liki trough) is suggested t o be genetically

' re la ted t o the opening of the south Atlantic Ocean suld the

separation of Bauth Amerioa f'rom Africa ( E h g , 7950; Wright,

6968; Burke e t a a le 1.970)~ The oventual eepmation took p l m e

i n the Albian a d w a s followed by a widespread m i n e trans-

gression. This w a s the on-set of the first sedjslentaq cycle

which l a s t ed earn BEdbls A'Lbian t o Coniaoian (Short and

Stauble , 1967 1. Hoque ('1976) gave suggestive evidence t h a t

feldspathic szmdstones. amrsrtituted the products of t h i s cycle

since the neameaa of the provenmce (0ban Massif) would not

allow enough time f o r the ohemical w&hering of the l a b i l e

: Ef

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0 Y

62

t-'.

I-'.

I-'. P E m

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2 g

bq

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P

g

(D

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u

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ul V

Gmity profile . Elechicoi sounding. Resistivity pro f~ le .

Roads.

Foot paths.

NPW coal horizons est? blished.

Isolated comoounds L buildings.

School C b u ~ h Mxrket

minerals, The sediments inolude the Asu River Group, the

0aukpan.i Formation, the Eze-ELku Shale and. the Awgu Shale which

were dated by Reyment (1.965)a

!Phis sedimentmy cycle ended with the Smtoulian tectonio

au t iv i ty , when the Benue-Abakaliki trough w a s folded and up-

l i f t e d with the formation of the Ananbra basin t o the west ,and

the Afikpo Syncline t o tho southeast of the r e s u l t a t Benu-e-

Bbaknliki mtic l inor iwn ( f i g s % & b), This tcctonism was

followed by the Cmpano-nlaestrichtian -t;rmsgcession (short

and Stauble, 1967). 1% w a s d u i n g t h i s E C C O ~ ~ sedimentavy

cycle t h a t the Ehugu Shales were deposited i n 8. broad ahal.10~

inner basin where oxygen gupply w a s limited,

. The sea became gradually shallower towards the close

of th . Maestr icht imj this led t o the fornation of an extensive,

low-lying.ooasta1 area with lagoons and. swaps , on which a , th ick suocession of coal meaanes mu orm mat ion) and fresh--

water sandstones were deposited, The l a t e r par t of

Maestrichtian t o Paleocene time waa also marked by thc roturn

of the sea when t rue f l u v i a l , continental sequence o f the

, M a l i Sandstone and the N s u k k s Formation wera Ceposi-bed,

This wag the time of deposition of the sediments of the

mapped areao

k t r j b a s i n a l d e f o m t i o n during post-Ihcstr icht im is

believed t o account f o r s e r i e s of fau$ts, jo ints wzd wazps i n

most rock-units of the MaYlu Folm~tion and Aja l i Smdstoiieo,

Sulixe ~ - a l erosion, a e t e r the defomxbion, ensued with the

sediments feeding in to the nioaem Piger del ta ,

1 ,I ? redous Investigations -. of the Nigerian CoaWielSt

C o a l m e 'first discovered i n $909 at Udi md 'oet~:sen 191 3-

..5974 at Inyi .by the Ximral S~uvey o f Sou-them Niger5.a

(~uulstan, 191 3, T9W) ; Subsequei~t ex@orntory work l s d t o

the discovery of the r e s t o f the coalf ie lds a8 ~ h ~ i n l I n

f i g m e 4,

Bain ($926) investiga.isod the geology of t he e ~ ~ : ~ p . o ~ i t

west of Enugu, and Wilson ( 1 9 ~ ) ma-pped s t r i p s of .the country

side along the ra.ilww f r o m Port-Haroour-b t o Etlugu, with Lhc

subsequent examination of the coals =and l i g n i t e s of Onitsha

m d Owexxi Prorinces (Yilson and Basin -1 928 ) , Outcrope

of c o ~ l t o the cast and southwest of Dokina ( ~ e n u z ~ t a t e )

were investigated in e a r l y 7926 by Rvss and -khe survey m a

extended northwards by Bain (j930) as far as Ogboyoga,

A great contribution t o the understanding o f the r s 1 a t i o n ~ h i . p ~

of the coal-bearing sediments was made by T a t t a m (9343) who

showed t h a t the coals did not a l l occur i n one formation, aa

was previously thought, but belonged t o two d i s t i n c t f o m ~ b i o n s ,

90

sepmated by a gmat thickness of sandstone,

No accounts of goophysical investigations m e obtainable

from the literature on Nigerian coalfields, It is hoped that

this a h d y would be followed by su3sequent, nore detailed m d

properly co-ordinated studies of the other coalfields in

Nigeria, especially the &ugu mines, where detailed geo-

-physical investigations will go a long way in solving the

problems created by mine flooding,

shown in Tablc 1 , but "i;ic major 0031 o c c m e n c c n m e knovm

Trom two distinc-t; f o m t i o n s ~ c a c l y , tho K s u k k a md T h i u

Formations; t h i c doos n o t inc lude -Lhc thin bcwi.ds and lonsca

of c o a l which ~xc? horn t 3 OUCUT i , ~ 1 otilcr f o m a t i o i ~ s .

It i s gcrhaps perb ine2t t o makc c few comments about t h e

Ikmu F o r m t i o n , w o n tl1011@r it docr;r n o t occur i n tix? ~ o E 0%

i . ~ v e s t i g a t i o n ,

The T ~ ~ I A F o ~ 3 3 t i on, 0 msj. s t h g of mmina int orcczldAons

w i t h c o a l sequanoes, :ims described by Reyxcnt (3965) f r o n i ts

type m o a , L'%wlu River i n Znu.gu metropol is , T h i ~ f o r n a t i o n

o v c r l i e a t h e Eitug~t Shnles 'and i s ovor la in by Aja3-i S,uldstono

~zni'i t h e NE;U~&C Formation, i n that sequelice, without any

break in ~ ~ c U ~ i ~ t & i o n , and m k s the bearLnj.ng of t h c h i c t o r y

of c o a l i n most p~w-i;a of So-cthcastcrn Ni,aor.ir, a long w i t h

i t s l a t e r a l equ ivn l sn t s i~ ~ t h o r weme The bkmu Formation

con ta ins a t o t a l or" five c o a U i e l d s aa rcpresontcd ia figure 4

w i t h t h e exception of t h e Inyi c o L U i c l d , The rock- - es of

, t h i s f o m a t i o n corn-prim a l t c r n a t i n g sandatom a , sha los X~Y she.l,os <and nudeCono8 wi th c o a l somm o r c ~ x b o n ~ c e o u o shales a*

v m i o u s horizons. Siinpaon ( 1 9 5 4 ) ~ de M m d t FIR^ Cascr (1963)

FIG. 4 Cod arws nwppd i~ i Nigeria. ( 6 . S. N. Bull, 2 8 ) X Study arm

noted the prosenceof a maximum of f ive coal seam i n t h i s

formation whfah they numbered Z t o 5 in ascending order of

a-trratigraphic column. A generalieed cyclotherm is as follows :

5 shale or sandy shalo o.t.c

4 coal, som~times s h d y at top

3 carbonaceous shale, passing downwmds in to shale

2 sandstone, with a fovr s h d y layers, or a l te rna t ing

sandstones md s h a h s

7 shale o r sandy shale

The result^ o f the proximate md u l t i m ~ t e mslyscs by

do Swmdt and Casey (6963) show tha t the coals contain low

a& with a r m k of sub-bi.t;uminous, pad ing between groups

A and B o f the mb-bi-hxminous c lass of A,S,T,M c lass i f i ca t ion

(American Sooiety o f Testing of Materials), Beyment (7.965 )

described the coals as m o s t l y oomposed of c lmiCm groundmaso

with d u l l sub-dass o f brightness, Thcy contain abundant

micro-slivers o f xitrim with joints pa r a l l e l t o bedding,

These fac t s , coupled with l o w fixed' carbon obtained by de

Swardt and Casey (7.963) mkes theooals compare unfavourably

with many coals avereeas. (~eyment , 1965).

F!G. 5 Map of southeastern Nigeria showirrg Enugtr Escarpment (after Simpson,?!?54 ).

study area

760

2 , l Looation:

The area under hvest igat ion 5s bounded by l a t i t u d ~ a

6' 6'N and 6' 11W; and longitudes 7' 15'E and 7' l g f E .

It is bordered i n the sadti by the PT-S trending &ugu @scarp-

mont (cf f i g 5) and l i e s southweet of Enugu metropolis,

east of O r j i River, about a t o t a l distance distance of

twenty-f ive kilometezs (25km) f rom O r j i r iver station,

2,2 Geomorpholo~:

The mapped area is generally undulating on the wastern-

half and of gentle slope on the eastern-half, The general

alti tude i s about 122 meters (400ft) above the sea-level;

becoming greater towards the north-east, Tho topography was

described as mature by de Sv3mdt and Casoy (1963). One strik-

ing featuro i s that the eastern-half of the area i s devoid o f

streams md rivera, clnd vice veraa, Tho shale horizons i n the

Nsukka Fornation (disoussed l a t e r ) impound water i n the

sandstones below md arbesiaui conditions exist where the land

drops below the lowos t points o f intake on the sandstones.

And the shale (or o o a l ) ooourrenoos a m known f r o m tho

western-half of the map, w i t h a regional dip of 3' southwest;

hence the oonsequont streams take advantage o f the dipping

A d i v i s i o n of o z s t e n i Xigoria i n t o f i v e cl-imatic regions

w a s dons by Inym (1975). The mc%ppcd arm. falls w i t h i n the

f o u r t h rc ,g~ou. This region has f o u r m-3 jor months of rein-

fall , wi th a gcncrcl. spm f r o a Yhrch -to October while t h o

d ry season l a s t a from N:mem?xr t o Febmmy . II!he p r e c i p i t n t i o n

is l e s s t h a n 6 0 m w i t h t h e driest nonth hnving lces t h a a

f r o n February t o l~pxi.l, CCilc nica? a m u n l ntxnosphor5c prcseuro

is b e t w e n 1010 - 1012.9 nil1l.ba.r~. Ogaitoyinbo st. a1

(1978) notod -tihut t h c ol i rnzt ic fl.ucix.~.tions ir- Niger ia9

indccd ir- the r e s t o f Woet L f r i c n , ,CEO n o t local. but f o m

inost Cretaceous 2nd Xowor Yaleoc o m , r e p r z s m t e d by t h e

L jali SLmd& ones m d the MstWz. Fo~~.x- ,? t ion r c s p e c t i v d y , v i z :

2.3.1 The ITkkka Fomat.ion

This f o rna t ion f irst descr ibed as "Upper Coal 141cnsuzos1'

( T n t t m , 1.944) d e r i v e s tar; nmo froin i ts type l o c n l i t y M u

river i n N~nkka. It r o s t a conf orn~.bly on t h e A jali Sandotoncs;

the l a t t e r i s sandwiched between the NsuMca Formation and

tho lrltrmu Formation, The Nsukka Formation forms out l iers

on the Udi md Igala Plateaus (de Swmdt and Casay, 1963)

well the mapped area, and i n and Igala

the f o m t l o n i s nemly horizontal or dips a t a l o w angle t o

the Vest and South. The general dip i s 3' t o the wost

and southweat. The main rock-types include shales with

sandstone bonds; medium-pained t o g r i t t y emdstones,

with a few shale b~nds ; ahales md fine-grained sandstones,

with coal seams. It i s noteworthy tha t out o f the t o t a l

number o f coalfields shown i n figure 4 Inyi a.rea bears

the only coal outcrops of t h i s formation.

'Phe coal seam assumes a nearly N-S disposition* A good

outcrop i s exposed along Iyaya stream ( f i g ?a), very cloae t o

Obme Inyi. The ooals show considerable vasiation in

t h i c h e s s md quality along the s t r ike (do Swwdt and Casey,

1963). The sections penetrated show th i ches ses of 1.33

meters (4 f t 4* i n s ) , 0,97 meters (3ft 2jtins) and 0,88m

(2ft 11 ins ) f rom the bore-hole information,

2.3.2 .The IBm --- Smdstone :

AJnli Sandstones were first described by Sim~son (l954)

m d later by Reyment (1965) from its typo m o a , Ajali River

and ~ o l o g u in ITsukko m e n , It is the oldest formation in

tho study moa, Tho rock-masses m e confined to the

southorn-half a d emtorn margins of the study area (cf fig I)?

A good outorop of this formation is exposed on the hill-slopes

of Nkwiyi stream. It is ohnraoteristiocllly friable and forms

gullies at the slightest run-off. ! R e A j a l i Sandstone

cornprisos medium - to comso-gr&ned sandstones, typically

"cross-bedded on a large scale. !lkey me usually iron-stained,

but oontain orurbonnceous streaks and bmda at vmious

horizons.

2.3.3 Struoturee :

The sediments of the Inyi area striko a fow degces to

the north-west and dip at least 3' to the south-west. De

Swmdt &d hscy (1963) mapped rr, oonceded fault whioh is

located outsido the study area. No such faults were mapped

within the study moo by pr6vious workers. However, an

attompt by the prosent study to establish tho prosenoe of

such conckaled faults within the a r e a of investigation

d%d not yield succ~ssf'ul results.

e n v i r o m m t s o f EOX cc~~aplcx g e o l o r g , such cs the =ugu

coalfield vhich is now undcr ccononic axplo i t?h ion;

3 Assess the dopondabi l i ty of the p r o p o ~ i t i o ~ u put forvr,zrd

by m r l i c r workcra i n tkre we2 iil r c l ~ ~ t i o n t o t h a nub-

surf,zco s t r u c t ~ o s ;

4 Mvance virgin nrco(s), i f srj., of c o a l occurrcnccfr);

5 Properly ident i fy the fracture patterns m d f au l t s

within the mea, the concealed a tmctures whiuh cmrnot

be eas i ly ident i f ied from bore-hole information, which

would enhance the suuceas of any f'uture coal exploration

o r hydrogeological and geotecMca1 projects in -thc

mea,

3.2 Instrumentation and. m e l d ~rocedure.. '

Altineter Instrument :

The instrument used f o r the d e t e d n ~ A i o n of the

various a l t i - hde s (or clevztions ) f o r the grcvity sta%ions

was the Amric an Paulin System Altinotcr f r o m -the D~p~xtmonta l

Laboratory,

The density of a i r i s the parmeter measured by the

al t imeter i n s t m e n t . Since the density o f m y material is

a function of tenperature and pressure, any vmia t ions i n

the air-colunn temperature or barometric pressure would be

ref lected in the readings calibrated inside the instruiiont,

An increase i n elevation at a place within the day timu

would, e l l oonditions being equal, r e su l t i n decrease in both

air-colmn tenperature and the baronetric pres,sure, This

would cause a decrease in the air-column density which would

reg i s t e r an increase i n the instruncntal reading (specially

designed) The corrections applied under t h i s section would

different s t~. t iona, Since t h c -might recordc~l i i s a product

of t h o nass (which is cons tc~i t ) cad tho accelmat ion 4uo t o

grmrity, cmy v ~ z i ~ t i o n s dct~:c'ted i n t h o m i g h t of t h e o5jcct

w i l l , o t h c r f2c tors bein& equal, r e f l e c t v ~ i ~ t i o n a i n %hc

grnvit-ctj. o h s l LWC 0101~atim. The p ~ w i t y L!o~:^,o?L , t2lcrcf orc

which apprt.cic;bl.y d i f fe rent f ron those c,f surrounding

fomntions, f o r inatLance cocl m d sandstones or s h f i l ~ ~ r

Coal, with a density of about 1.19 g 0ng3 would register a

amctller mavify roading than either sandstone or shale with

derrsitieo of about (2.32 md 2.41) g reepectively.

Electrical Resistivity Equipment:

The e lect r ical neasurernenta were done with the AZEM

Terrmeter U S $0 having c ompleto trm&tter/Receiver

system ( ~ e s f i g 6) with microprocessor distal display,

rechargeable Ni-Cd power pack i n clip-on battery tray,

Other conponents ~f the i n s t m e n t include Abem U33C (Universal

Battery Charger) f o r 1.0-30 V DC o r 100-240 V L C 47-63 Hz., ccm..-

fplefe w i t h two input cables and chm@r output cablc f o r

72 V battery pack; four very portable stainless s tee l

electrodes, 750x11 (length) 0.75mn (diameter) current cable on

plastio reel ; 2 x 50m 1 m potential cable wound on plastic

ree l and four r o l l s o f connecting cables. The US ( ~ i m a l

Averaging System) is a method whereby consecutive reailhga

me .Laen automatically and the resul%s are averaged

continuously. The SAS 30 used for the survey (seefig 6)

can be used alone o r supplemented as desired with the SAS

2000 Booster ( f o r extension o f voltage and aurrent ranges)

and SAS log 200 (well-logging unit) .

,Special Features of SAS 300.:. .

(i) Tho overal l system accuraoy o f the SAS 300 (with

o r without the U S 2000) i s _+ 2% (+0.07) of tho

reading plus 2 aigital increments @V or 3 ~ ) .

(ii) It oomprises a battery-povm?ed, deep penetration

r e s i s t i v i t y meter with an output suf f ic ien t f o r a

current electrode separation 2,000 meters under

good survey-ing conditions,

( i i i ) I t s c i r cu i t ry m d progrmming s e t t i n g i e such

t h a t .it d i sc rbdnz tes =and sepmates DC voltages,

self-potentials , m - m d e md natura l noiae f ron

the in-coming signal ,

( i v ) . The overall r m g e o f dv/= (resistnnce) i s

calculated and regis tered mtornaticaJ2y, extending

fron 0,5 mi l l - iohns . t o 1999 kilohns, which c m be

extended down t o 0,02 c l i l lohms by means of the

SAS 2000 Booster,

(v) Besides, s t a t i s t i c a l l y dis t r ibuted noise voltages

are reduced by n f ac to r o f IT (N i s thenunber o f ..

readings), especially when the instrunent i s used

in the s ignal averaging node (up t o 64 re&.ngs i s

possible) ,

( v i ) When the traulsmitter i s sending s ignals such as

20 d, the Terrmeter SAS 300 has a s igrr i f icmt

resolution of 0.5 ni l lohns f o r a s ingle reading,

With the SAS 2000 Booster giving out a current o f

500 d, the resolut ion f o r a single reading i s

0002 nilUohrls,

.. ( v i i ) The mi.croprocessor controls m d m ~ n i t o r s a l l

neaswements t o ensure optinal accuracy and

sonai t ivi ty , Heme, it i s possible t o know,

f ron the instrunent, the bat tery condition and

the usab i l i ty of selected p m a e t e r s , These m e

indicated i n form of simple e r ro r codes which

. can be checked up from the in,strunent nmual,

The neaswoments i n the r e s i s t i v i t y nethod i s deaigned

t o y ie ld infomat ion on fornations o r bodies having anomlous

e l e c t r i c a l conductivity, at a depth tha t depends on the

l a t e r a l spread of the electrodes by which the c w r c n t s w e

, s e t up. A coal s e m , f o r inst,mce, which has high r e s i s t i v i t y

would cause such a d is tor t ion i n the e l e c t r i c f i e l d signals, 5

The apparent r e s i s t i v i t y value ( d a, i n ohn-m) at the

position o f the measurement i s obtained from the following

( i ) For Werner Configuration,

P8, w = Brig

(ii) For Sohlmberger,

where "a? is the center-spread distance (n); R i s the

ros is tmce (ohns); L and 1 m e the horizontal distances (a)

between the current .md potential electrodes and the

observation points respectively,

Method o f b v o s t i ~ a t i o n :

The f i e ld work w a s carried out between the months of

.July aria September. A reoonnaisaance m e y was used t o select

suitable strai&t profiles, pazal lel in most profi les (Figs

la & 70); a l o a g whioh the detailed survey was carried out.

Detailea t o p o ~ a p h i o - as well as geologic-maps (1 : 20,000

$om = 0.2 h) were used as guidetj and f o r the correct

det e m k a t i o n o f elevations and observation points,

For the purpose of the gravity survey, s base sta-Lion aas

established at a spot with known elevation, very olose Lo

Inyi W k e t , This base-station w a s re-occupied every hvo

I . 2 Layout of Gravity and klec.tricrll resistivity traverse !iw:. .A D i r c h n d mavemen? .

hours during the alt imeter and gravity measwements (see

figurea 7a and 7b). The outl ine of the f i e l d layout is shown

i n f i w e 7c.

During the gravity measurements, the following pre-

oautions were taken:

( i ) The Crtnogonal?-y-di sposed it bull-eyeff bubbles were

always at oentralised' positions.

(ii) ,S 'lesmt two s e t s of perpendioular readings f o r

each s ta t ion were taken md the average computed,

( i i i ) Jiktraneous Bouguer %urces, for instance a very

lmge market mthering, whose ef fec ts would not

be corrected fo r , were avoided although the error

involved would not be appreciable.

( iv ) Great care was employed during he transportation

of the instrument t o ca ter f o r the in te rna l

oomponents and t o min im ize the instrumental creep,

For the alt imeter instrument r

(i) The thermometer and the alt imeter were allowed t o

stand f o r at l e a s t f ive minutes f o r each reading

t o a s m e the mbient temperature, while avoiding

direot rays of the sun.

(ii) For both the thermometer and the al tbteter , error

of pazallax was always elimimated by keeping the

instruments and the eyes a t the i r mutually, orthogonalb

- oentralised ,positions.

(iii) The themometer was NEVER held a t the mercury-end

of the glass-tubing during measurements,

( iv) For each re-occupation of the base-station, the

altimeter pointer was kept a t i t s preset reaang.

The eleotr ical r s ~ l i s t i v i t y profflee (wenncr) were cmried

out along the sane traverse-lines with the gravity measurements

f o r aomelation. During the entire res i s t iv i tx measurements,

the following p~ecautions were taken:

( i ) The t ravereo~ were disposed i n straight l ines since

t h i s w a s one of the a smpt ions in the model salcul-

ations.

( i i ) Battery oheck was always conducted ensuring re-

gistration of NOT l ess than 11,5 V. Short-aircui ting

of the battery w a ~ oamfully avoided since t h i s

could remit i~-L hi& dischmge currents that oause

dangerous heating and damgo t o the instmment;

besides, leakage u m e n t s , partiuulazly comcn on

wet conditions of the survey, was avoided since

they give erra t ic readings.

( i i i ) The power indicator o f the i n s - t m e n t was always

in the "offtv posi t ion af'tor each r e a m g and the

cablea mere disconnected t o a m i d shock o f any

degree; reduce expenditure of ba t te ry power, and

t o avoid damage t o the cables when posit ions were

changed,

( i v ) The depth at which the electrodes were harmzed

i n t o the pound, f o r enhanced e l e c t r i c a l contact,

w a s qui te deep (depth 2; %5crn) ; and a sepmation

of at teast 5m ( 3 5m) w a s maintained betwcen the

potent ia l and current oables.

(v) The r e e l s of the current oomponents were d i g n e d

parall-el t o the t raverse l ines , since they contain

many tu rns of wire, while the r e e l s of the

potent ia l components were kept perpendiculaz t o it;

beside^ the cables were prevented from touching.

These were t o avoid capacitive and inductive

coupling o r tlCrosstaUrn,

(H) Measurements were avoided during thunderat orma

o r high voltage aourcea ~l ince they aze capable

of induaing hundreds o f v o l t s i n long cable lay-

outs (common with t h i s type of survey) and t h i s

e n t a i l s r i s k f o r both the working crew and the

equipment.

3.3 Reml t s of the Survey:

Gravity Survey:

In the gravity survey, measurements were taken along

GS-1.to GS-5 as shown i n f i v e 7c. The first four p ro f i l e s

05-1 t o GS-4 wcrc aligned pa ra l l e l , at about 120' azimuth

while the f i f t h prof i le GS-5 was disposed at about 70'

azimuth. The separations of the s t a t ions were at a constant

ir$ierval of 100 meters a d the movcment of each prof i le ms

pro-eaat . The raw gravi ty da ta were reduced i n the usual manner by

applying f r e e air and Bouguer corrections using a density

of 2.w 9.g omm3. No t e r r a i n c o r r e c t i o ~ s were applied since

the area conducted w a s r e l a t i v e l y flat.

Graphs were drawn with the reduced gravity values

plot ted against the l a t e r a l distances, as ahown i n f i v e s

8, 10, 12, 14 and 1.6. Separation of these graphs in to t w o

components using assumed (arb i t ra ry) datum is shown i n f igures

A general discussion o f each prof i le shown t n f igures

8, TO, li2, 14 and t 6 is given below:

Assumed regional -- + /-' Nk -kLv.I -F- \/--observed anomuly

NWLJ I L - - A SE 200 4CO m 8al XXX)

Distance (m)

FIG 8 (iravity profile ot GS-I .

FIG 9 Gruvity onomaty of GS-I reduced

to un arbitrary datum.

m o f i l o GS.-i Prof i le GS-1 represents the first gravity survey.

Measurement s t m t e d exactly from the v i l l age s q ~ ~ a r e ( ~ b u n e )

up t o a; s t r e t c h of 1.2 Im along I\NY-SZ di rec t ion (about an

azimuth of 1 2 0 ~ ) . me g M p h ~ , correspxding $0 %he r e a d k g s

presentoa i n l'abl-e 2, we shown in f i g s 8 and 9. The first

drawing ( f i g 8 ) roprosen-La .the f i e l d curve aepwated i n t o

the regional and res idual components.. Tho second &awing,

f igure 9, ropresentn -!Ale posit ion o f the res idual

coslponent r e l a t ive t o an assumed (arb i t ra ry) refercnco which

is equivalent -bu tiis regional hac'xgrovzzd, %TO anomlies

a r e qnito c lear , t h e l ~ x g e r one extending 100 m c ? t e r s

F r o m the v i l l age square (05une) t o about 509 meters zr.d the

ornaller one e ~ t e n d i ~ g 500 netcra t o 730 meters a11 from the

v i l l age square (Obune) . The mininw~ reading r e c o r b d f a l l a

wi;t;hin t h e Ismger momly, which. i s not grea ter t h m 17

millig&s.

Prof i lo GS-2:

Obuao-~ikwo junction (Pro-southeast ) narks the beg-

ginning of p ro f i l e E L 2 m d walks up t o 1.2 km along I - S E

-direction (about an azimuYh of 120'). The readin@ aro

presented i n Tablo 3. Figwea 30 and 1 4 a r e respectively

Topography a

Distance (m I

FIG 11 Gravity anorrmly of GS - 2 reduced

RAT UM

to an arbi t iary datum.

the two components of the f i e l d curve and tho reduction

t o ayl a rb i t r a ry d a b . A large negative excursion with a

very small anomaly at the t a i l end of the p ro f i l e m e

apparent, The l a rge r anomaly spans over 400 meters s t a r t i n g

from the 300 meter-mark while the smaller anomaly s t re tches

f o r about 200 meters from the 7 ,O k m - mark, The la rger

anomaly shows a very low grenrity reading ( 5 mi l l iga ls ) , t h e lowest reading in the gravi ty s m ~ y ,

Prof i le GS-3 r

Figmes 12 and 83 represent the drawings f o r prof i le

GS-3 with readings from Table 4, The prof i le covers a t o t a l

distance of 1.2 kn passing by the side of the premises

of G i r l s Secondary School Inyi,, The general b t i t u d e

. of movement i s pro-southeast (about an azimuth of 7 2 0 ~ ) .

The f i e l d curve i s a oomposite of two negative. exouxsions

o n . a regional background, It i s notewoAhy t h a t these h o

g r a e t y lows a s m e similar shapes, the la rger oovering a

distance of about 600 metoxsr from the s t a r t i n g point while

. t he other goes from 700 meters t o about 1 ,I km a l l from the sta.r+t-

ing point (a. span of about 400 meters), The lowes-ti reading i n .

t h i s p ro f i l e i s l e s s than 7 mil l igals ,

Assumed regional

l.hN+-'-4 I

200 400 m &XI m r ioo " Distance (ml

FIG 12 Gravity protiie of GS-3.

FIG 13 Gravity ~r io rnu ly of GS-3 reduced

to an arbitrary datum.

" f Pbser' ved ilnornuly

FIG 14 Gravity profile d GS-4 .

Distance (m )

FIG 15 Gravity unomuly of GS- 4 reduced to CL'I u~Lit1~~8r y da tu In .

~ r o f i i e M+:

Meammements f o r profile GS-4 starbed from about 900 meters

off Agbali ji--go jmution (towards ~kpugo) and terminated

exactly at the junction fxfore-mentfoned, aarjuming'a NVI-SE

direction (about 120' aaimufh). The readings are presented

in table 5 while the draninge of the f i e ld data and i t s

reduction t o an assumed (arbitrary) datum w e shown i n figures

14 and ?:5e Two gravity lllows" of uompmatively ma11 sizes

are qpetrentt ono goes from 300 meters t o 500 meters f rom the

starYt;Fng point while the other rangee 600 meters t o 800 metera

from the starting-point. The lowest reading recorded 'in t h i s

survey i a less than 23 m i l l i g ~ ~ l s ,

Profile GS-5,:

The t o t a l distance oovered i n t h i s profile is about 1,2 ,

km, It rzeeumes a different orientation, SW-NE (an m-Jmuth of

about 70') on acoount of the density of builikhgs and i t s

oonsequent spaae-problem, It suns a3most orthogonal t o the

other previous profiles. The readingg are presented i n table 6

while t h ~ drawings of tho f i e ld data and its reduction t o an

arbitrary da* are shown in figures 76 and 17, Ono gravity l o w

of an asymmetrical shape i s apparent and extends f'rom about

I .' Assurned regional

FIG 16 Sravity protile of 6 3 - 5 .

I

30'- - -- -. , " <-rC__ 4 - Observed onomaly

FIG 17 Gravity anomaly of GS- 5 reduced tom arbitrury datirm.

500 metera to 900 meters f'rom the starting-point. The

lowest reading in this profi 10 is leas than 26 milLL&ls,

It is noteworthy that profiles GS-4 and GS-5 have oompmatively

much @-eater gravity values and smaller sizes across the

anomalous zones than the other profilea. . . . .

Electrical Re sistivitg Sumex:

Nine (9) profiles were t e n for the electrioal re-

~Astivity survey, represented by VES-1 to VES-4 and EP-I

to EP-5. VES-2 and VES-3., representiag tho expanding

Schlumberger spreadg, were conducted at former drilling

&tea where the subsurface lithology was known, The

choice of the appropriate electrode spacing "an for the

horizontal profiling was made on the basis of tha results of the

sounding aata. EE-7 to EP-5 were conducted along the s m

gravity traverses GS-1'. to GS-5 although the stmting-points

differed in some oases,

The apparent roaistivities (in ohm-meters) of the weas

penetrated were later obtained *om the raw field data

(resistances in ohms) by multip3Jring it (Resistances, ohms)

with their reepective geometrio factore (8) .

For Wenner Configuration,

R = 2 F a

For Schlumberger,

with tho ummlnotatfon (as given ear l ier ) , The apparent

ro s i s t i v i t i e s were plotted against half the current-

electrode sepnratione ( ~ / 2 , m ) f o r profi les VES-2 t o VES-4

while the apparent r e s i s t i v i t i e s were plotted against the

cent~r-spread df stances (m) fox profi les EP-f t o EP-5,

me drawings are ahown in figures ti8 through 26,

A general d i~cuss ion of each profile is given below: . .

ver t i ca l Elec t r ica l Sounding, VES-~ :

The beming of t h i s profi le is NW-SE and the oenter of

measurement i s located about 1150 meters f r o m Obunednyi

vi l lage square, The maximum spread f o r t h i s profilo i s

,about 1 . .2 .h (BB = 1.2 ka). Table 7 shows the readings obtainaiC

i n tho f i e l d md the various oonversions while t h o drawing

i s shown in figure T8. The graph shows a tcrminal positive

gradient of more-or-less gentle a t t i tuae , The rmgo of

values i a between 49.89 - 910.42 ohm-meters,

. .

t i ~ u e c t r i c Sounding, VXS-2 t

Tho depth probed f o r VES-2 was previously drilled by

the Geologioal Survey of Nigeria and located a t about

3.6 km *om Inyi market tow,wds AQugo, The beacing i a

roughly east-west -the. mcccbnum current-electrode spacing

i s about 1.2 k m (A3 = 1.2 km), Table 8 oontains the

readings obtained f rom the f ie ld and figure 99 shows . . the

drawtng, The range o f values is between 33.14 - 774.38

ohm-me Ths graph shows a terminal positive slope which had

ear l ier mintained negative gradient, . . . . . .

Vertical Electric Sounding, V E S ~ :

Profile VES-3 is the 1mt of the two locations that was

dri l led by the Geological Survey of Nigeria. The depth

,probed i s elooated a t Wout one kiloneter (I h) from llkpugo-

Q b a l i j i junction (Pro-southwest), The maximum spread f o r t h i ~

profile i s 400 meters, This value is low since the depth

t o the anomaly (a. prior information) is equally low,

Table 9 contains the data md the drawing is shown in

figure 2Ce The range of values is between 402,20 - 351*8,58 ohm-m, The graph s t h o d w i t h an i n i t i a l positive

gradient and a terminal negative gradient, having a

xmdmm point at about AJ3/2 = $0 metera.

Center spreud distance (m)

FIG22 Graph of E P - I profile.

V-er t ica Electric Sowding. VES-4:

The oenter of measurement f o r V E S 4 is located at the

Ore-thnuagu market, a l i t t l e distance awap from the c o d sub-

outcrop boundary, The beazir*g i s sllmost north-south and the

~1aJrimm current-electrode spaoing i s about I ,I km (BB =

1.1, km). Table 10 oontrzinn tho readings and figure 21 shows

the droving, The rango of values is between 225,49 9

8829,97 ohm-meters. The graph shows a n ascending positive

gradient a l l through the profile,

Profi le EP-I :

Measurements f o r . t h i s profile s tar ted a t =bout 3 0

meters f r o m the Obune-Inyi v i l lage squme and temrdnated

about 500 meters onw, i n a NW-SE direction (about 720'

,azimuth), ' The s ta t ion intervals were t&en at 50 meters,

!J!he readinga are shorn in Table ?I while the d4.awing i s

shown in figure 22* The graph shows, generally, high valu-es of

apparent r e s i s t i v i t y vexy close t o the st&ing point and

decremes, generally, away from t h i s p o h t ,

. . - -

N W LL . i - - . L - - c - L - - - - 40 1 60 2 8'3 ACU. 520 640 760 880 SE

Center spreud distance ( m)

FIG 23 Graph tor EP-2 profile.

Cantw spread distance (m)

FIG 2 L GI-itpti for EP-3 profile.

Prof i.le EF'-2 :

The bearing of this pro f i l e is NV-SE (about 1.20'

azimuth) and meamement s t a r t ed exactly I00 meters from

Obune-Bmaulkwo junction extending down t o 960 meters across

the MNviyi atream, Table 12 shows the f i e l d readings and the

conversions while the drawing i s shown in f i g , 23, The

p a p h shows a, high peak at &out 400 meters from the

s-bzting-point covering a rmge o f f60 meters t o 560 meters

f r o m the starting-point,

Prof i le EP-3s

A. distance o f about 2 km from Inyi market marks the beg-

ginning of prof i le IF-3 and extends Cfom t o about 880 meters

d o n g NW4E di rec t ion under s t a t i o n intervals o f 40 meters,

The readings m e shown in tab le 13 while the drawing i s ,

shown in figur~ 24. The maph shows a s h a q peak at about

400 meters fxom the starting point having a s p ~ m o f roughly

720 netera, The maximum reaifing i a greater than 5,5 x '10 3

oh-meters.

. Prof i l e EP-4:

Measurements f o r p ro f i l e EP-4 s t e e d about 1 ,I lan

f r o m a;kpugo-Bgbali j i junction and terminated at about 720 . -

meters, ctsswning a NW-SE direction, The readings EPQ

Center' spread distance (m).

FIG 25 Graph for EP-4 profile.

Center spread distonce(m)

FIG. 26 Graph of EP-5 profile.

shown in Table $4 and the drawing is shown in figwlo 25,

The g r w h shows a posit ive excursion r w n g roughly *on

600 meters t o 880 ne ters from the starbing-point, The

3 naximum value reoorded ~ t m d s above 2,4 x fO ohm-meters,

A U s t m o e of about 1:86 hi from 010-UFuma junction

narks the begginning of . p ro f i l e EP-5 and extends down t o

about 880 meters d o n g SWINE a r e c t i o n under s t s t i o n in te rva l s

of 40 meters, The readings a re &own in Table 15 while the

d r a w is shown in figure 26, The graph shows tGvo peaks

sepewlated by a negative exoursion, The mascimun value

3 recorded is over 2.4; x 70 ohm-meters,

3.4 Interpretat ion:

In c-ng out t h i s survey over the Inyi coal f i e l d ,

two type8 of geophysioalnethodg were employed, the gravity

m d e l e c t r i c a l r e s i s t i v i t y nethods, The gravi ty method

is based on tho measurement at the 'surface of s m l l v m i ~

t ions in the graxi ta t ional f i e l d , Small differences or dis-

to r t ions i n the gravi t ional f i e l d *om point t o point over

. the sur faoo of the o a t h m e caused by any l a t e r a l

var ia t ion in t he d is t r ibut ion of maes in the ear th ' s c rus t

( ~ e t t l e t o n , $9410). On the other hand, e l eo t r ioa l re-

a i s t i v i t y method makes use of var ia t ions in the electrical.

properties of rocks and' ninerals. - According t o Stdhl ( ~ 9 7 4 ) ~

oweing t o the large r e s i s t i v i t y of the oommon d ~ t r i t a l

rainerala, qcumtz, feldepms and oalci te , most of the current

passes through the f lu ids in the pore spaoes of sedi-

mentary rocks, and when rocks are completely dry, they a ~ l o

&ost perfect nonoonductors o f e lec t r ic i ty , If two rocks

differing in porosity are both saturated with water,

t.he more porous rock w i l l be the be t te r ~1ectr ica . l con-.

ductor, provided tha t the water i n both i s equally

oonductive owing t o ionization of dissolved g a t s .

mom the txbove preview, therefore, the gravity method which

is useful wherever the formations of in te res t hzve den-

s i t i e s tha t m e appreciably different from thoscof s ~ -

rounding formations, would show low gravity vzlues (negative

Bouguer momzlies) over a coal s e m which would be expected

t o &ve a good correlation with high r e s i s t i v i t y values over

the erne ooal seam, This w a s the basis f o r the choice of t he

two nethods w e r the coal s e a , m e magnetic method would

not be useful since the target , coal, does not show a-preciable

difference i n nagnotic properties i n contrast with sandstones

m d ahalcs, The s e i d c methods, which would have egznlly

given very re1iabl.e r esu l t s , w a s not umd in 'i3hi.u gtudy

FIG. 27

.. GS-1 profile --- EP-1 profile

DATUM

Distance ( m)

GS-1 and €PI superimposed for corrdation.

D i s tance (m)

F IG. 28 GS-2 ond EP 2 superimposed for corrdatim

FIG 29

GS.3 profile ---*- - EP-3 profile

6s-3 and EP-3 superimposed tor

correlation.

\ e-4-+- (55 4 profile - EP-4 profile

Dis tbnce ( m)

FIG- 30 GS-4andEP-4 superimposed tor correlation.

.. ._ --- - . -------

65-5 profile 69 - EP5 profile

Distunce (m) F IG. 31 GS-Sand W- 5 superimposed tor correlation

70 0

because of opcr~tiona3- problems with the equipment,

In the gravity survey, tho raw data ( i n mil l igals)

wore rdduced i n order t o convsrt them t o values which would

have been obtained i f the reaaings wore taken on an egui-

potent ial surface o r a datum plane s o tha t any deviation From

an wsumed trend of a homogmous subsurface would be a pointer

t o m anomaly, ! b e anomaljr i s usually tho target sought fo r

i n geophysical investigations, Phe observed Bouguer cmomtily

graphs ,after the reduction exercise was sepcxated in to the

regional md residual components, the l a t t e r consti tuting

the interest-group, as shown in f igures 8, 10, 12, 14 md

16. Usually, a p r io r i information suoh as geology and

borehole data would serve as guides i n choosing the regional

background,

The problem of poor exposure of rock solnples i n tho

w e a o f investigation mili tated against the colleotion nnd

meaeuroments of rock densities. Consequently, Nottleton's

(1.940) mothod of estinzlCion of surface deneities w a s applied

t o determine the bulk densit ies of the rook-masses between

the lowest ,and maximum olevations o f the respective tr,nrersea,

A density o f 2.24 g emg3 was selected from tho method and

used i n tho Bougu~r corroctions, The theory is based on the

fac t tha t both the Bouguer m a t e r r a in corroctions dopend

on the density of tha surface material within the rango of

71 o

the olevation differenoos of the survey. The mothod has

been cxperimnteJly shown t o be a good approximation with

a wide oonfidenco l i m i t ,

Pigures 27, 28, 29, 30 and 31 show the gravity and

r e s i s t i v i t y graphs superimposed f o r the purpose of cormla-

t ion, !he hatched areas are ahown t o represent the zonee

of anomaly, here believed -to be due t o the coal layers, The

reeul ts , as aeon from figures 27, 28, 29, 30 a d 31, obviowlg-

&ow e v e r y good match over the coal areas,

The excursions over the anomaly-zones are diametrically

opposite, as is expected; hence any excursion by e i ther

r e s i s t i v i t y or gravity trend which wacl not complemented by

the other w a s not regarded a s rn anomaly due t o the c o d

tha t , generally, t h i s capable

loading t o wrong interprota.tions especially when mea I s

made up of complex geology, but it is not applicable i n t h i s

area with simple geology, Figure 31a shows the bowrdries of

coal horizons established i n the present sufvey md thoso

boundrics as given by ~ m U o r workers i n the t area, A

comparison of the t r o reports, obviously, shows a close .

weemcnt. A new boundmy delineated i n the present study

i s located mound Obune-In. as shown i n figure la,

A qumti ta t ive interpretation of the wavi ty profi les ,

applying Smith s f o rmulm~ (Appendix B) , showed variations

72 0

i n tho location of the coal horizons below the surface f o r

tho various traverses. The U t of the Smith's method

used throughout the caloulations w a s tho " less t h m o r

equal to1' sign. Pro f i l e GS-1 gavo a depth of 54.3 meters.

GS-2 gave depth seotiona of 59.13 metors. GS-3 gavo a

rmgo o f 42.9 - 45.3 moters. GS-4 and GS-5 gave depth

sectiona of 87.7 meters m d 28 meters respectively.

Those f igures m y be considered good approximations when

compmed with the depth sections of the geoelectrioal

soundinga. !Rto data from the geoelectr ioal measuroments

( t rea ted in greater d e t a i l l a t e r ) showed t h z t tho anomalous

zones, here considered t o bo coal, were met at about 80

meters i n VES-2. VES-2 lies t o the west at about 3.6 lan

from prof i le GS-4 along m 'slmost east-west Ifne. VES-3

gave a depth of about 22.3 metere t o tho 0003 horizon;

VES-3 l i e s t o the wost on a northwest - southeast l i n e , m

approximate d i s t m c e of 2.8 km from prof i lo GS-5. VES-4

was conductod outside tho ooal area. The general increase

i n elevation P r o m the wost t o the eas t of the study asoa

(soe GS-7 t o GS-5) rnakos the f igure8 from tho e l eo t r ioa l

.and graxdty depth sootions compare even more favourably

since one would expect the coal horizons t o l i e closer

t o the surlace i n the west where the elevation i s lower

than tho east , This indicates good prospects f o r

using a combination of both gravity and e lec t r i ca l

r e s i s t i v i t y methods in mapping depth sections of momalias

below the surface,

Tho confLguration of the aoal boundaries suggests a

fau l t , However, it wae not possible t o establish a

f au l t from the mcasurments.

In figure 29 (GS-3/EP-3) the complemsntmy excursions

of gravity r e s i s t i ~ t y trends at about 800 msters were

interpreted t o s tand f o r the d j n l i Sandstones which are

re la t ive ly dry, It is important t o note the sharp negative

excursion at about 800 meters i n f i v e 37 (GS-~/EZ-~) which

could resu l t f'rom l a t e r a l variation in thickness of tho coal,

It is interest ing t o note the difference it makes in

eleatrode spacings the r e s i s t i v i t y values

seen between lP--1 (figure 22) and EP-2 t o EP-5 (figuros 23

throw& 261,

V k i l o the rcndings of EP-1 range 1'0 - 69 ohrL-netcrsp the 2 other f o u r p ro f i l e s r m g c (1-80) x 10 o h - x t o r s . EP-1

was t-ne first prof i le oonducted ~ m d c r center-sprezd in te rva l s

of 50 zmtcrs. Thc rcadincs 3f t h i s prof i lc pror~pted the

ohoice 02 a lower s t a t i o n f o r tho

r c e t o f t h a prof i les ajnce thc r end iws of the first

prof i le m-1' (fig, 22) dici not show w k e d r e s i s t i v i t y

contrast a l l alxy-; the prof i l o . ii l i ke ly explmcitiox f o r t h i s

i s 8irootl.y. p ropor t iond t o the center sprcad s t a t i o n

in te rva ls , It ia of C O ~ L O X I II3Lowled~e t h a t the dacper

i n t o isotropic

@cater tha conductivity (or lower i t s xcciprocnl, rosintivi . ty)

of the rock stratc.

Whcrcas r e s i s t i v i t y n2pping n c n m r e s tho apparont

resistivity of an area at oons tmt depth, on thc othar

' hmd, e l e c t r i c a l uoundings show t h i s r o e i s t i v i t y i n r e l a t ion

t o depth; t h c t i s t o my, it uccswes tho v e r t i c a l var ia t ion

of the ros is%ivi ty i n contrn-distinction t o horizontzl

v e z i ~ t i o n i n r e a i u t i v i t y noasmod i n r e s i s t i v i t y profiling.

T h i s was the reason w*jr two p ro f i l c s VES-2 ,ma VXS-3 (cf

fi,gwxs 19 and 20) wcro conducted at locations whore thore

had beclr sorio inforzation frm bore-holes d r i l l e d by the

workers of thc Geologiual S u v e y of Nipr ia , Zi,yne 32 uhowa

thc depths at which the vmioua cord a e m s werc a e t ~ . t ' Iny i .

Borehole nmber 1122 cormspondo - to S;ho desth-probe o f V3S-2

borcholo n~uibox 1108 correspocds t o tkc doptl.1-probe of

VES-3 ns indicated. i:a f i g m e 32, ~Wrn t l ~ c curve of TZS-2

(fig. 19), tha 1 m 0 m o ~ t lnpr sh3wn 5 t c ra inn l posit ive

slops nnd s w g s s t s a [email protected] r c s i s t i v o f urmtion. Ylla d q t h

narkha t h i s bounfi~pj wc?s in terpreted as tine beg5r;;imi:~g of t l x

c 0.~1 horizon, E'or largo elcctrodt. sepzrr..tiow, it x.y ba

n~surlled tha t prac t ica l ly sll the currect a m flowing ir, Lhe

uppsr tlEm S n y c r ~ ~ , p r a l l e l t o tho bedding plmos. %fie

cument dens i t ies i n t h ~ t h e e layera &auld 5e Invers~ly

propodionzl t o thc res i s t iv i tLoa i n the three layers,

- 3ozoholo 1122 shawn t b z t coal Tma net at c~bout 78.63 ~ e t c r f ,

an11 ev i l f i g m e 33, TlEfj-2 y io lda infamtio_n, tht hi&

resistive coal w a s ue t ct t ~ b o ~ t 80 a e t c r s aftcr the u a n l

1 Wcltherrd undstonc

cmo-i~a-tch.in.g proooduro, I n f ig . 20, tho CDC f o r VES-3

shows a t e n l i n a l negative slope suggests IZ iimked change .

fro3 high-rosiative fomcsl;ion t o a lov+rcsia~ive f omation.

The depth nemfdng th in 'boundcsy w c s interpre-bed as the te rn iac t ion

of o m 1 1-r~~ In other words, a f t o r tho weathered zone

wi-bh r o a i s t i v i t y of 480 ohiz-mters (of f ig . 33 & 34) the

intervening rack-strata were lunpad together wit-h coal

i i l t o c. gsoelectr ic ssct ion ha-ring a r e s i s t i v i t y o f 4320

ohrSnctcrs. The depth obtnincd a f t e r curvo-m.tching

( m e f i g 33 & 34) w a s 22.3 noters fron boreholo iltllnber 1101!

(corresponding t o VIE-3) the coe l s e n t eminc ted at about

23.62 neters. Thoso r e a u l l ; ~ , obviously, coapare f avoumbly

with the r e s u l t s of o w l i o r i n the a c a ,

F igwe 33 ahowo an httcnpt t o present a fence diagrLu?.

o f , the depths penetrated at VES-1, VBS-2 m d VES-3. WOE

tho d i q p m it ahowo t h a t tho weathered zone i n tho a r e a

rims fron north t o south. This i s Likely beoausoof greater

nunbor of river^, t o ~ ~ ~ r d s the eouth Ynai the north which

pronoto chonical w o a t h e r i , ~ ~ of t he lsthile aatoricls, Infact ,

'beyond t h e location of VES-f (5.0 0bune-1nyi) there is

somcoly my r i v e r mound (so0 f i g ?'a) . !Phis infomzticirr

-night servo as n usoful in i t in l -guido f o r a;ny f u t w c hydro-

FIG 33 Correlation of depth profiles.

generally sou.l;h-wcwt ciirectlon. It i a elso L;nown t h n t the coal

8 O'ZW. bo-m&xg l i a s host half of the moc surveyed

the a rea underlain by coal at Inyi l i c a fron the o o a l

bo~u13,my t o the west of the rizp. Imd f ron t h o

inf omation of t h o d o p t i scctiona , o m would ~ccc\r:n;icizd water

a check %zLainst ~ont~aa inn t ion by poisonaus c o r L p ~ ~ c l s fron coal.

F i , p r o 34 shows rm z t tccpt t o o o r ~ c l ~ t e the geoelectric

aections f'ron the s w o y with goology, The dcpth of ourrent

, very hit&- r c s i v t i v i t y -ralues, Thia scctior, is likalS; ~x.do

up of' e l e c t r i c a l i~ ls&xtors , t h z t i.9 t o m y rock-rmscos with

very l i t t l o wount of vmter, f ~ s shown in f i C r c la, VZS--4

representad the untlcrlying ~ e d b o n t ~ m y sequence, the Lj,U

Lrp soil sund -

8.28 w l h cool.

- -

Attermling sands tone

Md shah.

studies hclped t o cst8,bUsh 2.

c.3 shown in Piguro I s (w3und

l i e s very cl.om t o Obmc-Inyi

new c m l

Obunc-Inyi ) . Thc village squms,

whish w a s tho at,&ing point f o r EP-T/Gs-I (fig, 27).

Thc correln.tion of the two geophysiccl nethode helped t o

dolineshe t h i s bcundm?y.

CHAPTER FOUR

4.0 CONCLUSION

The geophysical invest igat ion oonducted over the I ~ J

area with i n s t m e n t s of high preoision has provided a

geophysical data-base f o r the area and the first of i ts

a n d over Nigerian coalfields. The r e s u l t s o f the surve!

helped t o delineate the coa l - sea boundaries and depth

sections which showed a close agreonent with the coal

boundary-positions m d borehole sections established by

e a r l i e r workers i n the area; besides, a new coal sub-

outcrop boundary w a s established mound Obunc--Inyi, The

r e s u l t s of the survey showed a propossive increase i n t l

depth of the weathered top-soil a d good geoelectric sec-

which night servo as a usefu l initial-guide f o r any f'utu;

hydrogeologicttl m d geotechnical projeots i n the mea,

Although no accounts of geophysical investigatione 2

obtainable from the l i t e r a t u r e on Nigerian coalf ie lds ,

. it is hoped t h a t t h i a study would be followed by subseguc

more detai led and properly co-ordinated s tudies of the o-

coa l f ie lds in Nigeria which would enhance the understand:

of the problens encountered in coal f ie lds and serve usefi

purpose as reference m t e r i a l s f o r nore conplex.

enxironnents, especially the fiugu mines whcre detailed

geophysicnl investigations w i l l eo a long way I n solvine;

the problems created by n iae floodin& Such resemchea

could load t o new disooveriee which would help t o re-

order our econonio setting,

mmn A

ISST OF GRAVITY BNP ELECTRICAL RESISTNITY DATA.

P ST TRPLWBSE' ( GS-I ) TYPE: G W I T Y SURVEY rnCATION: OBrnEINYf B;RING: . 620' 1~zinu'Gh

~ s t ~ ~ l c e (n)

2nd TRAVERSE (GS-2) TYPE: G W I T Y SURVEY LOCATION: OBUNE--0 JUNCTION

5th ICRAVXRSE ( G S 5 ) W E : GRllVITY SURVEY mCATIOn: 0IX)-UFm ROAD BEP3U:NG: 70' A~imuth

~ i s t m c e (n)

TRhvmss (VEs-1) W E : EIZCTRIChl; RESISTIVITY (SOUNDING) JXWXCION : OBUNEIRYI BFJRING:

T A L E 9

TRILKERSE ( m s - 3 ) !WE: E ~ C T R I C A L ~ S I S T I V I T Y (SOUNDING) IOCATION: Sk off AGUIJXJI-AKPUGO pro-SW

BET;RIIJG: North-South

TRAVERSE (EP?) W E : ELECTRICAL RESISTIVI2Y fRO'EI~TG I;OCATION t OBUHE-INYI BEARING t 120' b M % h STLTION INTERVAL = 50 meters

.DI_S'PL.WCE (n)

50

100

ICmm 1.2 lIRBVERSE ( ~ ~ 1 2 ) TYPE: ELECTRICfi DISTIVITY PBOFIIZNG rnCA!CLON : O-{o JUNCTION

T~~ T4 !rRAVERSE (m4) W E : EIZGCTRICAL RESISTIVITY PROFIUNG I 0 a BKPUGO-Irn ROm BEKRING: f20° Jbzhuth

bent er-~prca8 Distance (n) Apparent res is t iv i ty (ohn-n)

Expression of fornulass for estimation

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