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GSlO HECOHD NO. 7
SUH}'ACE Gt'OPHYSICAL STUDIES
IN THE MALABALA AlllcA (SEROVIE S'fUDY BLOCK)
NAY 1978 TO JULY 1978
R. J. PEAR'f ~i.Sc. D. LC.
1
CON'rENTS
Abstr<lct
Introd~tction
Geology and topogr<lphy
Previous geophysicG.l field vr·.:>rk
Held method
Description and discussion of l'Gs:.llts
COrlclU8iO~13 Qlld rGcommendations for further \·nrk
Hoferenccs
Fig. A Part of iI1.'lgnetometric tra'vers0 A, indicating
possible fuulting
FiG. B " " " "
}'ig ~ C " " " "
fig Q lJ " " " "
PiLi • E Pa:rt of app!lren t rasistivity profile ( traverse
indicatj.ng po,""i bIe faulting
Fi~~ & F Part of apparent resi3tlvity profile ( tr<lverse
indicat ing pOi3sible faulting
B)
D)
li'ig' • G l' art of m~lg'n8tic traverse E indicating faulting
Fig. II Part of ill:.tGnetic travel's i:) D indicating faJ.lting
Fig. I " " " D " " 1<'ig. J " " " D " "
F' 18' • K " " " E " "
l"irr • L " " " E " "
141ig 0 H " " " -, v " "
• Fie. N A comparison of surface and airborne 111 agne ti c profilos
0\[21' zones both vlith and Cliti10Clt b<lsa1 t cover.
5
5
'7
8
11
33
11
12
12
15
16
17
17
18
18
19
21
23
2
Fig. 0 Surface maglletic profiles paf..tt four core boroholes
that penetrated basal t
O'l'iIEll FIGJRES PRESENTED ilITH TIns REPORT
Fig. 1 Location diagram - Scale 1: 125 ooe
Fig. 2 Traverses A, B, C and C2 (~!agnetics and
resistivity) 1:20 ooe
Fig. 3 Tro.verses D and E (Magnetics and resistivity)
1: 20 000
Fig. 4 ERTS lineaments in relation to neromagnetic
CCEtoUl:'S 1:500 oeo
Pig" 5 AerolT.agnetic profile map shOI'ling areas >lith no
basalt cover 1:500 ooe
FiG. 6 Aa l'omngnc tic con tour map sho'wing arer13 with no
base.l t cover 1: 500 000
Pig. '7 CCIT.parison of airborne and surface magnetic
profiles 1:125 ooe
Fig. S VES field and model curves at site of Cl
Ji1ig. 9 " " " " " " " " C2
Fle. 10 " " " " " " " " C3
Fig. 11 " " " " " " .. " C5
Fig. 12 .. " " " " " " " C6
:F'ig. 13 " " " " " " " " C7
Ii1ig. 14 Comparison of VES in are2.S with and >iithou t
basalt cover
l!1ig. 15 Corr.parison of VES in areas HHh and Hithout
basal t fover
Fig. 16 Comparison of VES in al-eaS >Ii th and '.thout
P !lCE
25
3
bad eLl t cove l'
Pig o 17 Gco-eJccLrical section of traverso ]) 1: 12'5
}i'ig~ 10 G(;O-- 01 GC t ricu} f)(;'ction of "Lrnvsl':Jc J!: 1:12)
F.L 1::. • 1') Ceo-elcclricnl 8octiori of t ruve 1'30;3 n and
Fig 0 ;~O Indu.Gcd pola]~isation sounding curve Gt e6.
}l:ig. 2] Intc:rpl'et[Jt.ion diogram 1: 250 ece
000
000
C 1: 12'5 000
4
ABSTRAC1'
Zones of Stormbcrg Buual t cover in the l-lalabala area (SorO\18 3tlldy
block) have beon readily delimited by surface and airborno magneticD.
Several major faults, some correlateable with gRTS 1ineurnentn, have
been J.oc~lted and these may be of hydrogeoloe;ical significD.nce.
It hus proved impossible to determine the tl1icknesE1 of tbu basalt
C~)\TV)' uG_ing' COmm(ID goophysical techniques (refraction oeiGmicG and.
l'(;;,:;j,sLivity sound.ing) but it is proposed, on the ba13iS of limited fluId
evtdence, that induced polarisation sOW1ding is likely to be ~;;uccessful
.in th.if-; app1ic:ltion.
5
IWPIlODUG1' llli'!
Su.rfaco g'('ophYGical studies \'18re undnrt;<'ih:(~l) by the BG:'::D on bohnlf
of the G SIC pro,j C?ci; over an ar(J<'1 of 1100 '''I . l<mn. , centred uomo ~iO j,:rn:::; •
to the He~) t of ::)oJ'owe (li'i o ' b· 1) • 11ho obj(~ctive~;_; of ti1is vlork \'lC re hiD-fold!
(n) to locate tl'1O ,SE tl'ondjng EHTS linu;.:l.HlCnts, which m,lY be of
hydrogeolo[;ical s.ign:i.LLcunc8, and
(b) to delimit those areaS of thi.n or non-Gxistnnt Stonnberg
Basalt covor on Cave Sn.-nufJ tone. ~f1hn indura-Led con tact be hmon
basalt and Cave So.ndntone iH frcquon!;l.y found. to be D. high
yielding nocond_ary aquifer and ideally l,vater bo1'oh010:-:) should
be sited in those areas where thi~3 contact occurs bolo1'1 the
both thin :lDd \'Ioathered (soft). Furthcnnol'o, in thOt}8 areas
imum !JcD:_,'i.!lle LhicknOiJU of Cave ;];:lnd:-:;tone, Lhought 1.0 he an
:il!lp(lrtitnt pl':Lm~-tr.y aqu.ifer, iu prcDoI'V'od.
(;j ) 17.9 iGlU.> of rC::3iBti vi Ly traversing
( b) 91. '7 kms of IJla{(IH:;toffie tric trave 1'.'3 :Lng
(c) 30 Vertical electrical ciOUYlCU.ngD (VES)
(cl ) "(-<:-') lOllS of line PGgging ( 2') III in to 1.'V nlD)
(e) 0' .c 1-:11lD of line pegging (100 III intorvals)
(f) ~) .::> l{]llD o.f line? cutting
GEOLOGY AND TOPOGllAPHI
'ropogrnphic,'111y thc'; L1Toa cOrloiots of a flat uLwd.vc:Jcl p1 atcau, soma
1200nl. ~"l.:n;3.1, camJ)J eLLJJ:'i dcvo:i.d of (ll'aina(~u Ji11()f:) and diDplr.i.'j.Lng only
:.mbd1!ed local rel.ief provided by- pnJlG and anci.ont dunc; fCflturE)L~. rrtlO
6
'<'1hole n.rea is mantlod by ",hite, yellow, red and groy sands of the
1(fl'! : __ :tJ(lrj. Bedu that support a fairly dense vog'otation cover.
rrho Kal[Ulari Beds (variously coloured sands, sondston88, crete:::;
{tT,d. occasional marly horizonn} are of variable thickYlo:-38 (in excess of
()Om pl'ovcd) and overlie the Stormberg Basalts "nd Cave Sandstone, both of
· .. ,h:ich are units of the Stormberg Series, Karoo Super-group.
IJ'ho Cave Sandstone is an aeolian deposit consisting of a C101)8-
beddod ulterllDtion of fine and COarse layorB; it iD slightly folspaLhic
l'llli.Jc' to\','ards the base OCCur marls and shalon (tho Transition Beds).
Hesults of the pl'8sont geophysical work suggest that only a much reduced
SOCLiOl! of Cave SGJlrlstone remains in the relatively uplifted central-
Stormborg Basalt, extruded through linear vents as discreet flows,
bJrl.l1izctcd (-1 Cnve ,sanclston0 f3urface displayinc Dtrong rol:i.ef (up to )0
mCLrL~G in the SerOI'J0 uI'eR), probably a dune Durface, and this pr;;;rtly
expln:Lns the rapid local vQl'iationn in basalt thiclmess. 11he bai;w'lt
i tsclf cli.splays enormous variation in colour, texture and composition,
raJl.gin[,; from purple to black and arnygdaloidal to massive. (rho amygd.:l.lc;::~,
Wh:L ch n!ay be up to 15 cms long axis t are of quartz, calcite and
frequently chlorite and serpentine. 'rho clay minerals (secondary after
v.'roxone?) are ospecL'3lly abundant in the Heathered zone of the basalt:;.
COlT bOl'cholcs cl rilled duriJlg the present project sho", thu.t the ba::;al t is
CO liB" ,,1:; \Watllerod from its upper surface (sorr,Q 40 m belo>l ground level)
dOlm to about 75m. At onc of the borehole sites (e?) the ems "I t has
appnl'ont1y HndergoDo intcl'flow VI0athcring (ioo. an alternation of fresh
alld HPat.]wl'od bnsalt occHrs).
Post-Karoo doleri tie dykes trend WNil-ESE and maY be traced for many
7
miles. They are thought to have been intruded into major fault zones.
Doleritic sills are less common but at least t\10 are thought to have been
detected during the present \1ork.
PREVIOUS GEOPHYSICAL vlORK
Andrevl (1965) undertook seismic refraction and reflection 'IOrk in
the vicinity of Makoba, some 80 kIDs north of the present area, \1ith the
aim of evaluating the use of these seismic techniques in determining the
depth to the Stormberg Basalt/Cave Sandstone interface. Andrel'l concluded
that seismics l'lCre unable to detect this interface due both to a 10\1
horizontal velocity contrast between the hlO major lithologies and to the
presence of high velocity stringers (fresh basalt?) Vlithin the weathered
basalt horizons. Andrew also noted high speed layers "i thin the Kalahari
de')os:i ts (cretes and sandstones) and these velocity inversions preclude
reliable interpretation of refraction seismogrOJDs.
Robins (1973) made Schlumberger soundings and spot magnetometer
readings at 1. 6 km. intervals along the track from Sero"e to Letlhakane.
llobins believed that the depth of renetration for the Schlumberger array
approximated one half of the electrode separation a~d he interpreted
empirically to produce a :l<lrgely erroneous geological section. Some of
Robins' field curves have been interpreted quantitatively for this
report. The deepest interface detected is nOlf seen to be only some 45 m.
sub-surface, probably basal Kalahari.
Soundings made at the start of Robins' traverse, on the basalt
outcrop of SeroVle, indicate that a marked resistivity contrast (c.7:1)
exists betvl8en the basalt and the Cave Sandstone. However, electrical
anisotropy of the basalt here apparently causes an overestimation (by
8
some 60/;) of the interpreted thiclmess of this unit.
Jennings (1974·) also reports the existcnce of a strong resistivity
contrast bet"een basalt and Cave Sandstone and the successful application
of \1 Ollner array electrical soundings, interpreted empiricallY. in
determining the depth to this interface in the SeroV/e areao The examples
of sounding curves included in Jennings' thesis, ~Ihile of rather poor
quality, do indicate a marked resistivity contrast. HoV/ever, the
description and results of his method of empirical interpretation are
not convincing. It is no" accepted that empirical interpretation of
electrical sounding data cannot be successful. (Van Zijl 1977 etc.)
A natiomvide aeromagnetic survey l;as completed in 1976 and the
results of this, in the form of profile ~~d contour sheets, have proved
of immense value in outlining areas of basalt cover and ~lill be discussed
later in this report.
FIELD METHOD
Geophysical techniques employed during this investigation V/ere
electrical resistivity (both sounding and traversing), induced
polarisation (sounding) and magnetometry.
Equipment used for the resistivity and induced polarisation ~Iork
included a Huntec 2.5 k.W. Time Domain IP transmitter (modified to provide
optional DC Oll tput, in stages, up to 2500 volts), hlO Fluke digital
multimeters, a Huntec Mk. 3 IF receiver and auxiliary oquipment (porous
pots etc. J A high pO~l8red/high voltage transmitter is required for
electrical techniques in this area vlhere dry surface :Jands, frequently
displaying resiGtivities in excess l ' 10,000 ohm-me 'es, overlie thick
layers of 101'/ resistivity (c 5 ohm-m ;res J.
9
The magnetometer used was a McPhar GP-70 proton precession type
;Ihich measures the absolute value of the earth's total field. Repeat
readings were made l1ithin each three hour period to allow corrections
to be made for the diurnal drift of this field.
Geophysical traverse, mainly confined to Anglo American Corporation
cut-lines (Fig. 1), ;Iere pegged as fo11ow8:-
'l'raverse A extends 12 kms. on the central El, line, commencing 34 kms
;Iest of the track to Serowe.
Traverse B extends 8 lans on the central EW line, commencing 22 kms
west of the track to Sero;le (at the intersection ;Iith the central
NS cut-line)
'l'raverse C extends 18 lans on the central EW cut-line commencing 3.75 kms
\'lost of the track to Serowe.
Traverse C2 extends 1.5 kms (betvTeen stations 7.5~1 and 9V1 of origin on
traverse C), 200 metres to the south of C.
Traverse D extends 31 kms on the central NS cut-line, for 10 kms to the
north and 21 lans to the south of the central EM line.
1'raverse E extends 30 lans on the easternmost NS cut-line, for 7.5 kms
to the north and 22.5 kms to the south of the central EVI line.
A combination of resisitivity traversing (gradient and Schlumberger
arroys) and magnetometric traversing 1;aS used in the attempt to locate
thc'mo SF~ trending ER'l'S lineaments. Initially the gradient array
(Cl C2 3 lan) was employed on traverse ll. HO;lCver, exceptionally 10;1
potential gradients ;Iere observed (due to deep, thick, 101; resisitivity
layers) and the computed apparent resistivity values displayed an
10
unacceptable signal to noise ratio. The gradient array was therefore
abandoned in favour of the Schlumberger array. An electrode separation
of AB 500 m (vlith MN50m) waS chosen after inspection of a trial VES made
at the intersection of the EVI and central NS cut-lines, and a total of
11.4 loo of traversing (25 m station interval! \iaS completed at the sites
indicated in figure 1. (Parts of traverses A, B and DJ
90.2 loos of magnetometric traverses and 30 VES (including 8 ortho
g~nal VES, Schlumberger array - AB commonly expanded to 2.6 Ions) vlere
made at sites indicated on figure 1 in the search for Zones of thin or
non-existent basalt cover. A faulted basalt contact, some 8.5 kms west
of Malabala borehole on the E-vl cut line, vias further investigated by
1.5 loo of resistivity traversing (AB 200m MNIOm) and 1.5 km of magneto
metric traversing on a line 200 m to the south of and parallel to the
major cut line. (Parts of traverse C and traverse C2).
Experimental borehole resistivity logging, employing a crude, 10'1-
vol tage DC power source, was tried at core-boreholes C6 and C5.
Observations made during these measurements of the slow voltage build-up
and decay at certain levels in these boreholes led to the conclusion
that the basalt may be highly chargeable and an bxperimental induced
polarisation sounding WaS made' at the site of C6, some 9 kms 'lest of
Malabala borehole. Unfortunately the IP receiver became unserviceable
'li th a recurring intermittent fault before completing this sounding.
11
DESCRIP']'ION AND DISCUSSION OF HESULTS
(a) Location of ERTS lineaments
A >lell defined ERTS lineament (ERTS E Nov. 1972) trending S1500
8
is seen to bifurcate approximately 45 kIllS to the N'l1 of the intersection
of the central N-S and E-W cut lines (Fig. 4). The >lesternmost branch
o remains \;ell defined, trends S150 E and appears to crosS the E--vl cut
line some 18 laIls >lest of the central intersection (investigated on
traverse A). This branch parallels tuo major faults, inferred as
bounding a zone of distinctive magnetic contour pattern (Fig. 6). The
easternmost lineament is not so >lell defined and does not appear to
extend as far south as the E-W cut line, but if projected >lould cross
this in the vicinity of the central intersection (investigated on
traverse B and the northern part of traverse DJ.
Traverse A
The magnetometric profile of traverse A (Fig. 2) displays several
major anomalies that probably indicate faulting:
(a) A narrou (100 m broad) negative feature (-350 gammas) centred
at 5.9 laIlS vi and originating about 50 m s.s. (Fig. A) Faulted zones
Hithin magnetic units frequently display negative magnetic anomalies
due to hydrothermal alteration and leaching.
200'6' I jfQUllt ~._. ," 1\ /".,; ... . /' \. --',. I ' .
I' "---'" ./'" / \. E W . ......----. ..../ . \ .. ,.-' \ I \ . Worn .)
70 60 50 Fig. A Part of magnetometric traverse A, indicating possible
faul ting.
12
(b) A broad (1 km) level change (plus 250 gammas to the Vlest)
centred at 3.6 kms W (Fig. B). Analysis of this anomaly is
complicated by the superimposition of short Vlavelength 'noise'
typical of basalt terrains, but it may indicate faulting at a depth
in excess of 1 km. Alternatively the feature may be due to a
gradual increase in thickness of a magnetic unit to the Vlest (e.g.
a gradual thickening of basalt in this direction).
fa u It? Ei
1\/\/,-. I .I' , \/',\r'\", ",
lOQ~[
50
f\ \,,\ ,0'_,
t.... \"-' i\ I\A' /""\ ! ',.' , / / ' I /
,0 ",T'" \ /' , " ~ I ,~, i
2QOm
40 30 }'ig. B l'art of magnetometric traverse A, indicab.ng possible
;faulting
(c) A 700 m broad level change (plus 100 gammas to the east)
centred at 0.2 kms E. (Fig. C).
w
100'( [
'-----'
200m
10 o 10 }'ig, C Part of magnetometric traverse A, indicating possible
faulting.
20
E
w
(d) A level change of 200 gammas (plus to the >lest) across 1.4 kms,
cent red at 2 loos. E.
E
200m
'. 10 20 30
Fig. D Part of magnetometric traverse A, indicating possible fsulting
It >las intended to investigate further these four features
using resistivity traversing but attention vias refoccused on the
eastern pert of the survey area after only 1.8 kms. of such
traversing had been completed. This resistivity profile displays
a steady increase to the >lest (5 ohm-metres per km); this may
indicate a thickening of resistive Kalahari sediments or other
shallow resistive unit to the ~18st. (Fig. 2)
Traverse B
This magnetometric profile displays a regional gradient (-36 gammas
per lan. to the ~lest) upon which is superimposed short ~lave length
anomalies. There is no definite indication of faulting. The apparent
resistivity profile displays a regional decrease to the ~18st (12 ohm-
metres per lan.) that terminates in a broad (1 km.) trough of low values
(50 ohm-metres) centred at 2.9 k. vi. (Fig. E). It is considered that
this trough represents a faulted zone and this is endorsed by the
broad apparent resistivity peak (85 ohm-metres) immediately to the v18s t.
w
14
Such a resistivity peak may indicate a rapid thickening of crete develop-
ment that frequently occurs adjacent
j fa u It
200m
1,0 30
to fault
zone?
20
E
Fig. E Part of apparent resistivity profile (traverse B) indicating
possible faulting.
A visiting German geophysicist (Herr H. Nickel) tested the Enslin
EM technique along part of traverse B (0-3.5 k.vl~ and recorded a distinct
reduction in apparent resistivity values (probably confined to the upper
30 mJ to the west of 2.5 k. '11., at 11hich point occurs the 10>lest apparent
resistivity value (38 ohm-metres) recorded during the Schlumberger
traversing. A pan is situated immediately south of this point and it
is thought that the low resistivity values are partly due to a thiclmess
of pan sediments. It would appear that pans are frequently alinged wHlLen
fault zones.
Traverse D (N section)
E\,rther resistivity and magnetometric traversing was undertaken
along the northern section of the central N-S cut line (Fig. 3). A
broad (1.6 kIlls) resistivity low (40 ohm-metres vlith shoulders of 70 and
90 oh:l-llletres) WaS detected, centred [) 2.5 lmlS' N. (Fig. F)
15
s
10~'" [
200m
10 20 30 .l"i{~·~ F P:.1J.'t uf ~LP.p[lT'cnt I'OS:l;3tjV:Lty prr)i'ile (tl't.lver::,o ,I)) iwLi.caLing
pOClsible faulting,
~-\110 dj_mension;;1 und relat.lvo cunplitude of this anolllaly al'e remarkably
Dimilar to that detected on traverse B (Fig. fl) and it is suggested that
both anom.:1J.ies reflect the sDIIle fault zone "[hich mUDt therefore trend
111. j\~4(30Eo EHTS .ljnea.ments i'lith nimila:c trend have been roported in th:L::.;
:'trCn by both B.ritisll and German photogeologistD (personal communicatJ.ons).
rrlley have boen ascri hed to sand dunes in the Br:L tj.sh report and left
undefined in the Gnrma..n.
1]1;10 follol'ling' core-·boreholes "'lore drilled to test this supposed
(;2 (Traven:38 B at 3.2 kms \1 )
C4 vrravorse D at 3.2 lans N)
C5 (lllraVOrf38 D Ht 2.9 l(Jn[J ;n
C4 HaG ab:mdoncd duo to the tochn.Lcul difficul tie:., of drillinc; th I~ough
bJocky ct'utcs. 'rho.re i'lt18 :little di.rect cv:tdenco of faultj.ng DoeD in C?~
but to\'lards the bottom of C5 (190-201 III S ,8.) a slickcEc;:ided nnd highly
:JiJoEll'ed doloY'i to in truuiOL \~;.1.S encountered.
It :is cor:vGr'ient to def;CTibG her8 ot.her roc~sible fa.uJts of regional
16
extent detected during mug"netometric traversing undertaken to delimit
zone;:: of b2)salt cover. Some of these faults may bo correlated Hith
Elrps linenmcnts; they are also readily :i.dentj.fied on the aeromagnctic
profile lllap (F'ig. ')). 1'he fault zones have frequently been intruded by
dykes '-Ihich serve to emphasise the features magnetometrically.
(a) 1'raverse E at 4.5 Ic.N. (l~ig. G). 'rhis feature, reprosenting
the southern margin of a do;m-faul ted block (in 'Thich basalt has
been prc::,,;ol'vod?) may be traced on aeromagnetic profile and contour
m::'ps for .'.:;8veral hundredn of kilometres, trending 395G
E (Figs. IJ- :lnu
'l) .
s I
1·\ ,// J .,._._/
'-'--._.--'-_. -"'-. / -._-., \. /_.
\.
fault \ Ni
\ "
'-
100'\[
200m
30 40 50 Fig. G f'art of mae;net:Lc traverse g, indic<':1ting f{ml ting.
The aeromn,gnetic anomaly is sorno\"hat Dubduod in the rog.ion of
profile D, Hhich displays 8. typical basalt signatuTG over its whole
lCl'!.gth, and it is suggested that the relative dO'l'mthroH to the nor-Lh io
not :LJ 8TC'at here (i. e. there has been faul t:i.ng, on a N-S trending
plane, bcbloon profi1()s D /lnd E, to the south of the major contact).
(b) Traverse D at 9.16 kms. S. 15.2 kms. Sand 19.') lans. S
(li'igs. H, I and J rCDpGctively.)
s
1000' [
........ , ...... ,~. .-.. _,-,-,-,-. /'1 200m
faul t
100 90 80 Fig. H .[Jart of magnetic traverse D, indicating faulting~
s fa u I t N
,A\ \ \
\ \,
"-'-'-""""'-', . , '...... /' "-. '--.,
'''. --........ A"'-"_ ...... -.......... '\,
\ 200m
160 150 140
}"lie;. I J' art of mL:.gnetic traver::;e D~ indicating faul ting.
18
s N fau I t
100~ [ \ '-'-'-. . ', ........ _/\
".
\,/'.-
200m
200 190 180
(c) 'l'raverse E at 15.6 k.S. and 19.5 k.S. (Figs K and L r8spectively)
fau I t s N
100\ [ _.-......... ~ "-._ ........ "'.
\ '--,/ ........ /._.-.... "./ . ' . ....... _ .
........
\ '",.
"-.-. 200m
170 160 150 14('
Pig. li.. .t'nrt of magneti.c traverD8 E, indicat:Lng faulting.
19
s
100'( [ \ '",.
200m
210 200
fOLl [t
"-. "-
1
190 • 1 FUL'L or l\ll){,~:nc~j.e l::C:l'I2TD':: i~, ).llclicaLing' faultin(",'.
N
Muc;netic [,'Iomahes mentioned in (b) and (c) above, and depicted in
f.L5'l) .. I'e~> 1I through L, indicate normal faulting ~'lith d':)hlDthroVl to the
sm\th~ rn1is faultinrr J?I'osumably occured post-basalt extrl.ltJion a~'1d
involved a sheet of bagalt of rel:).~.;ively uniform thickness that I'm::..;
dr)vmthro\'fll SUccGB~Jively deeper to the 81)uth Q r.ehis \'lould oxplain the
f<2:i.rJy regular decreases of ID8.s'netic fiold l.::::vel of approximately 100
{?,'':1]llmn:.;, between o.'1ch fault zono towards the south. If this hypothe~3i8 i:'3
COl'J'l::Ct if follows that the thickness of Kalahari bed~) \'Ttll increase
tOHnrds the s·)uth (for present topography is flat) and it ,,·juld also
explain l'J'l1y only the 1ot,'fer part of the Cave Sandstone 8UCc8Duion romai.ns
j.n the rolatively upliftod central portion of traverse E and the eastern
portion of the 8-11 cut l.ine (e.g. borel101c" Cl and C3).
Cotllparing shapes EU1d relative amplitudes of theso anomalieu the
folloi·ti.ng correla t ionD may be m,'J.de:-
Tr:.:V8t'SO D 9.16 k S with 'l'raverso E 15.6 k S (Figs. Hand K)
rfrnvcrse D 15.2 le S I'd th ~eraverso l~ 19~ 5 le S (]-'1' ('re:' b'~ 0
I and L)
20
Such correlations infer the existcnce of tvlO faults trending SllOoE,
which direction is similar to that of the El{lfS lineaments seen in li1ig. ~'G
It is sllggested that these are major regional faulh3 0
J-'artial surface magnetic coverage Has obtained along the E-W cut
line, betl<ecn the intersection l'lith the far east - and central N-S cut
Tines (traverse C, Fig. 2) to locato precisely Hhere basalt cover
commences along this lino. This maGnetic profile indicates a faul tcd
conLues at El.3 k.1'; ,rith basalt COVOl' occurinc; to the I'rest. (Fig. M)
21
C6 fo u [t C3 w E
100'6 [ 1 1 .............. ."" ,'" ..... -........... ~ ..... -.... -.--- .-_ .... _-.-. . - ....... . __ ..... -............... . -............. ... -., ~ - .' -. "-" .... '" " .. - ...... - ............... .
o.
200m
100 90 80
/\/1/\ " / . / '" .
/ b.
10.1(111 [ . f /\, I .
200m
90 80
:F'iG. M Fart of traverse C indicating fClul tine -
(0) 1~[~'Snetometric profile
(h) apparent resistivity profile.
22
Core boroholos C'j 211d 06 (at 7.5 v/ and 9 VI respoctivcly) prove th:U~
fault" C6 encountered 32 metres of \"feathered basa1t lihile in C3 thoro
",mS no b:-lsal t c::uld rrransi tion beds (basal Cave ,sandstone) Here penetrated
at only 80 metr(~::j bolavl surface"
rplw intensity of the m8p;ne·~tc field over the tasa.1 t cover is some
20 gammas lov.rer thc.w to the east and this suggest that some flovlS of
reversed remancnt mngnetisation occur i'llthin the basalt.
A resist:i.vity traverse (AB 200 m MN lOm) >!as conducted acrosS thi8
fa:l1.ted contact. (li'ig. 2 and Fig. ~I) A large resistivity contrast
OCCl.H'D across this feature Hi th val-J.Gs of 85 Ohll1-metres to Uu; east and.
only jU? ohm-met reD to the wost. This anomaly is clearly of shallm'J
oriGi.n and is thought to reflect the presence of thick crotes (4-4 m
proved in C3) to the oast, adjacent to the fault zone.
Traverse C2, made 200 m south of C, displays the Same magnetic
and indicates th!..1t the plane of faulting trends at approximatel;y
1 "0° . b JJJ this arOae
(b) A"nQs of thin or non-existent basalt cover
(1) l'iap;netomctrics
Areas of bQsalt cover moy be readily dolimi ted by observation
of surface - a~1d un filtered a2I'omae;netic profiles (Fig. N and Fig. r5)
The basal t f.:d.gnaturo is especially s troYlg on the su rface profile:-.:;;
where it is characterised by EUl abunuance of short l'IQvelength noi."Je
d:JO to variations of magnetic susceptibil i ty and flold geometry
\'lith:i.n the basalt. 11ho basalt signature remains fairly Hell defined
on aerolllagll..;t:i.c profiles, oxcept Hhore t!fenther-edging!1 of tho
lJc18rJ.lt cover occurD (i.e. v1hore the contact is not abrupt).
23
a. 100r[ 200m
s -_.-.-._-_.-.-.-._. __ .-.-. __ ._-._.-._._._.-._._.-
'-'-N
1000 [
200m b.
w
1250m 1\1
s
r'> basalt Fig. N A comparison of surface and airborne magnetic profiles over
zones both "Hh and \'IHhou t basalt cover:-
(a) from traverse E (no basalt cover)
(b) from traverse B (basalt cover)
(c) aeromagnetic profile (flight line 168)
Fig. 7 shoVls a comparison betVleen magnetic profiles D and E 1;Hh
the nearest aeromagnetic profile (for D this is 1600 m Vlest and for E
900 m east). Clearly the aeromagnetic profiles, flo;m at approximately
300 m terrain clearance, are attenuated versions of the ground profiles
and lack both resolution of the thinner bodies and much of the short
wavelength noise due to the basalts. Anomalies due to deep bodies are
hardly altered. In both cases there is a discrepancy of some 800 m (in
a N-S sense) between air and surface profiles.
It is clear that to facilitate precise delimitation of zones of
24
basalt cover (including areas of very thin cover) surface magnetic
traversing is essential; ho"ever, observation of aeromagnetic profiles
alone "ill indicatE! clearly zones of thicker cover (say in excess of
50 metres). In general the aeromagnetic contour mapS, "hich Vlere
prepared from filtered profiles, (e.g. Fig. 6) do not clearly shoVl
zones of basalt cover. The pattern on both profile and contour maps in
the area south of Sero"e is confused by the presence of doleritic sills
and it is not possible to predict "Hh confidence the occurence of
basalt here.
Unfortunately it has proved impos si ble to determine the thiclmess
of basalt from the magnetic profiles. The inhomogenic V of the magnetic
susceptibility (due both to differential weathering and variability of
mineral assemblage) and the highly variablc floVl geometry "Tithin the
basalts (probably including flat lying flo\lS of reversed polnrity),
coupled Vii th a generally low susceptibility range (observed d'Jring simple
field tests) prohibit mea;oingful quantitative interpretation of the
basal t thiclmess. At best it may be possible to develop some empirical
relationship beh18en the relative amplitude of the short Havelength
anomalies and the proved thiclmess of basalt, but such an interpretation
"ould remain highly su bj ective and would be applicable only to
individual areaS of limited extent.
Fig. 0 ShOHS surface magnetic profiles, dl'aVln to the same scale,
across the four core boreholes that penetrated basalt; thickness, type
(weathered or fresh) and depth to basalt re indicated on the right hand
side of the figure. As ~lOuld be expecte' the greatest magnetic
response is observed on N-S traverses Vlh ,e the least is shovm on the
25
B-W truverse across only 32 metres of 'Ieathered basalt (traverse C, C6).
Beyond this, however, it is evident that it ,Iill be difficult to find
any correlation between the thickness and type of basalt and the magnetic
response.
a
. ."..., ..........
s
\ '". "'-.-.
C7
1
C5
1 b ~.~''''. /\ /'" •.•. ...... -._ .... ; '..... '....... ... ... ' \.
N
N
\ ~' .. '-... '-'-'--. .. • _ ..... ./ ......... "'.J , ..... .........
\._. ...., J ,.... ... .....
w E , , ........... ..
"._. --'- ... , _ ........ . 'I
, ........ . ...... '" ......
...... \. .. ', ~ .-........ ..'_ ..... _.r .... __ ..... _ ....... ' .... ', . .;' , ............ -.. ' C
C6 w 1 E
...... .... -.~ ...... -.-.... -. .... --.- .. - ... ,-.- ....
,.-'-' .... - .- ..... -.-._ ..... - .-, ...... " .... _ .............. _ .... _. __ ...... - .... -. d
200m
Kalahari beds IttJ
weath. basalt 13 fresh basalt j'J~:1
.... : •. t\ :', ::.!~. I\.t,;, ~ •
y
" "
vv,,'t/.., vv'loJ 'V \I " ., v'VVY v.,,t,,, V
v •• ~ Vv"v
'~f"''::\''f: ' •• • \ •• r.. .. . :;.a?-~~
, "
50 m]
Vi :. 0 :Ju.l'face l!lug'l1ctic profileD past four core boreholeu thal; penetrated basalt;
(a) traverse E (ICS) (b) traverse D (N-S)
(c) traverse B lE-VI) (d) traverse C lE-vi)
50m
38m
43m
If
26
2. Klectrical Sounding
Recalling Jennings' (op cit) observation that a strone electrical
resistivi ty contrast exists betl;een basalt and Cave Sandstone in the
Soroll8 and Orapa areas, it lIas decided to undertake VE;] at 3 km intervals
alone traverses 13, C, D a;~d E to provide estimates of the thiclmess of
the basalt.
The VES curves lIere interpreted using auxiliury curves and the
resultant geo-electrical model lIas successively modified until fits,
better than 5to, Hero obtained bet\;een field- and computer derived curves.
(Figs. 8, 9, 10, 11, 12 and 13) This process resulted in an equivalent
solution for each curve i.e. one of many possible solutions that 1I0llld
yield an identical field curve. The result," tre presented as geo
electrical sections in figures 17, 18 and 19.
VES curves obtained at core borehole sites lIere fitted to the
lithological loes with the aid of Dar 6arouk curves (Zohdy 1974). For
those boreholes that had been logged geophysically a "particular" model
(i.e. a multi-layer model llith resistivity values for individual thin
layers p:Lcked from the resistivity log) Was also derived, and the
computed curve drawn. In these cases the geo-electrical layering above
the water table and below the base of th· borchole (i.e. beyond the
range of the resis tivi ty pro be) is taken L 1 the auxiliary curve
nolution.
Table 1 lists resistivity values for the Vc. )US lithologies,
obtained from the matched parametric VES curves an the 16 inch normal
resistivity log, corrected for borehole effects (h 'iameter 80 mm and
borollole fluid resistivity 7.5 ohm-met: 3).
27
TABLE 1
SPECIFIC RESISTIVITIES (OHM-METRES) OBTAINED FROM MATCHED P ARAME'l'RIC YES
Borehole Kalahari Basalt (Heath.) Basalt (fresh) Cave Sst. + Trans. beds
Cl
2600 170 45 68
41/41
r-------------------r--------i-------1000
125 21 30
14 38
-----1---------------,.-------,.-------2200
220 39
108 (crete) r----r--------------,---------------
C~
2500 125
24 89 (crete)
30 219 43
1-----1------------------------------C6
3t300 266
14 34
40 40
~----~----------------------I-------
C7
12100 986 263
6?
6? 51
-----------------------------------FOmlA'rION RESISTIVI'l'IES FROM 16 INCH RESISTIVI'fY LOG (CORlF<;CTED)
Cl 5.6/24 -----1--------------- --------------
C2 26 72 32/4t3 1-----1------------------------------
C3 U N LOG G E D 1-----1------------------------------
CS 34 24 -----1------------------------------
C6 40 4t3/24 ~----I------------------------------
C7 24/56/72 ----------------------------------
x YES curve distorted - deeper layers affected xx Value for unsaturated Cave Sst. (i.e. above the water table)
28
This table indicates that the mOan value for saturated Cave Sand-
" stone (taken from parametric VES) is 41 ohm-metres and from the 16 inch
normal log it is 42 ohm-metres. The Vleighted mean value for all basalts
(from VES) is 44 ohm-metres and from 16 inch normal logs this value is 47
ohm-metros. It is immediately clear that the electrical sounding
technique is unable to resolve the basalt/Cave Sandstone interface and
therefore can be used neither to determine the thiclmess of basal t nor
even to predict its occurence in this area. l'igures 14, 15 and 16
demonstrate the similarity of sounding curves from areas both vlHh and
Vlithout basalt cover. It should be remembered vlhen comparing these
curves that abscissa shifts are unimportant in the contolCt of resistivity
ratios, they reflect only thickness variations.
Possible reasons liby the Stormberg Basalt displays such exception
ally 10Vl resistivity values arc listed below:-
(1) The extensive weathering (sub-surface, inter-floVl and
intra-fracture) and hydrothermal alteration of the basalt
has resulted in a high content of chloritc and other clay
minerals and these minerals serve to reduce the bulk
resistivity.
(2) The basalt is frequently highly vesicular and these vesicles
may contain static mineralised ~Iater of high conductivity.
(3) Inter~floVl deposits of silt, sands etc. reduce the bulk
resistivity of the basalt.
'1'here remain further obstacles to successful illterpretation of
sounding curves from this area •. For "lnstance, it is generally not
possible to determine the thiclmess ( Kalahari deposi s because:-
29
(1) The upper three layers constitute a Q. type CUrve (reflecting
variation in moisture content of the sand). The middle layer
is usually suppressed,
(2) The upper surface of the basalt is frequently calcretised
and is occasionally seen to be a completely gradational
contact ,/i th overlying calcretes of Kalahari age (e.g. in
boreholes C5 and C2).
(3) There exist rapid lateral variations in the nature of the
Kalahari beds, especially in the vicinity of pans.
E'inally the problem of anisotropy of geoelectrical layering sr,ould
be mentioned. It is clear from the geophysical logs of the boreholcs
in this area that there exist within both basalt and Cave Sandstone (and
presumably in deeper units) many intercalations displaying strong
resistivHy contrasts. (e.g, the resistivity log from C7 shows three
resistivity peaks, each several metres thick, that probably correspond
to distinct flo,/S). These intercalations do not possess sufficient
1 relative thiclmess' to be resolved by YES (see, for instance, the
comparison of particular- and simple model curves in E'igs. 8 - 13) and
yet, if ignored, Cause wholly erroneous depth estimates to be made.
This problem can normally be overcome if the depth to geo-electrical
basement (Le. an infinitely thiclc unit of high resistivity) is known
at a sounding site, for then a generalised coefficient of anisotropy
may be calculated and this can be applied to all interpreted depths to
basement. However, the depth to geo-electric basement is not lmown in
this area.
nom the foregoing it is clear that only a limited amount of
:information ma,y be extracted from the YES curves:-
30
(1) Occasionally, ~lhere basal Kalahari consists of high
resistivity cretes resting on "Gathered basalt, the thickness
of Kalahari deposits.
(2) Occasionally, tlliclmess of the complete undifferentiated
Stormberg Series ('lhere the resistivity of the Transitional
Beds (i.e. basal Cave S~~dstone) is not too 10". The
Resistivi ty of the underlying Upper Ecca mudstones and shales
is in the order of 5-15 ohm-metres,
(3) The presence of major fault zones,
(4) The presence of doleritic sills.
(5) Zones of deep or shall 0" geo-electrical ,'asement (Vlaterberg?).
But it must be emphasised that these depths ,lill 0, 'I be correct
in a relative sense- absolute depths CDJl only be determineu if a
coefficient of DJlisotropy is available.
Addi tionally, it is not possible to detect the water table on VES
curves from this area because the "ater table is deep (usually in excess
of 80 m), is probably a diffuse zone DJld is s~~d"iched behleen layers
of similarly 10" resistivity,
The three geo-electrical sections prepared from t interpreted
VES curves will now be examined but firsl t"o general ob" "vat ions may
be made:-
(1) There exists a large range of sm lOe resistivity values
(12,100 - 460 ohm-metres). It VIa: noted in the field that
areas of dark (bro,mish-gray), fi :-grained topsoil Vlere
31
characterised by lower resistivities while areas of pale
coarse-grained, sandy topsoil yielded the higher values.
(2) Crete development is greatest adjacent to fault zones.
1'urning nOli to individual sedions:-
Section of traverse D (Fil';. 17)
The thickest section of undifferentiated Stormberg Series occurs
near the centre of the traverse between 3N and 3S. This series is
underlain by low resistivity sediments (less than 10 ohm-metres) thought
to indicate Ecca mudstones etc. - these appear to be shallow (less than
200 m s.s.) tovmrds the southern end of the section (18S). A major
faul t terminates these sedimen ts in this area.
HiC;h resistivity values in the centre of the section, occuring
bebleen 50 and 250 metres sub surface, have been assigned to a dolcrite
sill, bounded to the north and south by faulting. A highly sheared
doleri te Has encountered at a depth of 190 m in C5. Another doleri te
sill is indicated at depth at the southern end of the section (21S).
Section of traverse E (Fig. 18).
The Stormberg Series (?) are characterised by 101'1 resistivity values
(20 - 25 Ohm-metres) over a large part of this section. This may
indicate tl gr,'rlter clay/mtlrl content of the Cave Sandstone (i.e.
Transitional beds dominate as suggested previously) or a morc saline
groundwater.
The basalt occuring north of 4.5 N displays higher resistivity
values (138 ohm-metres) than elscHhere, and the surface magnetic
profile suggests that its susceptibility may be greater than other
32
baGalts encountered. Possibly this northern basalt is of different
character (in terms of both mineralogy and flol'l geometry) than that
occuring to the south.
Again, Ecca sediments are thought to occur at shallow depths ( c .100
m s.s.) to\1ards the so~thern end of the section.
Section of traverses B and C (Fig. 19) I
'fhc central doleri te sill of figure 17 is seen here to be bounded to
the \fest and east by faulting.
The supposed shallo>l occurence of Ecca sediments at 15V1 appears
unrealistic and the 101'1 resistivity value (10 ohm-metres) may be due in
this instance to a deeply "eathered zone (adjacent to a fault).
1'he 1'ossi ble application of Induced Polarisation soundings
Fig. 20 sho"s the results of a combined rP/resistivity sounding at
the site of C6. The sounding is incomplete (maximum AB spacing achieved
wail 200 m) but it does appear that a large chargeability contrast
exists across the Kalahari beds/basalt interface. This is supported by
the occurencc of two negative values (at AB/2 values 35 and '30 m) that
arc typically found on anomaly flanlm. 'l'he high chargeability of the
basalt is probably due to inclusions of chlori t, and other secondary
minerals and it is likely that a similarly large chargeability contrast
1'Iould exist bet1'leen basalt and Cave Sandstone.
Clearly 11' sounding, Vlith subsequent interprecation using type
curves, provides a possible means of determining b, ,alt thiclmess in
this area. Indeed, it may be the onJv geophysical technique that is
capable of doing so. It has been si ,m that ref otion seismic,
33
magnetometric and resistivity techniques are incapable of yielding
estimates of basalt thiclmess and it is unlikely that a sufficiently
large density contrast exists between basalt and Cave Sandstone for the
gravity method to be of value in this application.
CONCLUSIONS AND RECmIMENDA'rIONS
I'lagnetometric traversing has successfully located several major
fault zones, some of which may be correlated Vlith ERTS lineaments, and
has del imi tad areas of basal t cover. These fault zones should bc
further investigated by resistivity traversing techniques etc. if they
are thought to be of hydrogeological significance.
It has proved impossible to detennine the thickness of basalt cover
using common geophysical techniques but it is likely that Induced
Polarisation sounding Vlill be successful in this application. Ideally
further field trials should be undertaken before contemplating the
purchase of an expensive IP unit and a volume of type curves.
It is also recommended that the Department purchases a portable
core-testing unit so that the major geophysical parametcrs (magnetic
susceptibility, resistivity and chargeability) of a core may be
determined at the borehole site "hile the core is fresh. This Vlill
facilitatc better correlation between surface and borehole geophysics and
geology.
H. J. Pcart GEOPHYSICIST
RJl-' /SSlI
15th December, 1978.
ANDREW, E.M. 1965
JENNINGS, C.M.H. 1974
ROBINS, N. S. 1973
VAN ZIJL 1977
ZOHDY 1974
34
REFERENCES
The Seismic Method and Basalt-covered sand
stone in Bechuanaland - Geophysical Report
No. 29. Overseas Geological Surveys
(Geophysical Division).
The Hydrogeology 'If Botswana - Unpublished
Ph. D. thesis University of Natal.
A provisional analysis 0.[.' the Serowe to
Letlhakane resistivity truv8rse - unpubli
shed report NSH/1917 3
A practical manual on the resistivity
method - SIR Report FIS 142
The use of : .. '0' ZAllOUK curves in the
interpretatio, of vertical electrical
sounding data - USGS Bull etin l3UD.
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FIG, 7
, " " H 11
. , , , '--
, " 11
" 1\
" 11 , '
11
" Ii
" , 1\
I' 1 1 \ ,\
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Traverse 0
.', " , ,
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r ~ "1 + ' ", .!*, '''.
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11 --- -
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and surface [:\.,'''',] magnetometric profiles.
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1 cm, 125 gammas [surface J
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FIGURE 17
'15~ 1195 i 9S' ,65\ '35 0 (5 6·5N, 95N . _.-'
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GEOELECTRICAL SECTION OF TRAVERSE 0
Horizontal scale 1 :125,000 Vertical scale 1cm. : SOm.
,3
/0
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- 300m
- 400 rr
-500 m
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32S C1 0 3N 65N (proj) .
lQOOO 12100 7200 3'00 "", 11000 27000 1900 \>3000 15000 1900
21S C7 13S 11-5S 10S 9S 6'3S
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FIGURE 18 GEOELECTRICAL SECTION OF TRAVERSE E
Horizontal scale 1:125,000 Vertical scale 1 cm. 50 m.
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C A . €.
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FIGURE 19
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GEOELECTRICAL SECTION OF TRAVERSES BAND C
Horizontal scale 1 : 125,000 Vertical scale 1 cm. 50 m.
0 Stormberg serie~ ~ Dolerite . - ~
Kalahari beds
0 Ecca series - -
seAL!.
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