1534 Lone Oak Rd -Electromagnetic reportATTACHMENT2
Report on the
Electromagnetic and
Magnetic Surreys
Conducted
at the
Dakota County's Lone Oak Site
Dakota County, Minnesota
for
Dakota County's
Environmental Management Department
App1e Valley, Minnesota
bp
Fromm Applied Technology
Mequon, Wisconsin
Julp 18, 1995
ABSTRACT
Electromagnetic and total field magnetic surveys were conducted at the Lone
Oak site in Dakota Coimty, Minnesota. The objective of both surveys was to
enhance the understanding of the spatial extent of tires lmown to have been
buried on site. The electromagneric conductivity results outline an area
characterizerl by low electrical conductivity. This area is interpreted to
spatially den"ne the majority of tire burial area. The magnetic results
indicated the presence of ferrous material in the southwestera portion of the
fill area Taken together, the results of the surveys provide an excellent basis
for establishing a program of direct observation_
afit081895a
TABLE OF CONTENTS
._;
Abstract
.
Table of Contents.........._...»....._..» .............».................. ._.... ii
».....».....»...........
List of Figures ..........................._........................................ .........
page 1
..._..._...__........
Introduction_ ....---------- .....---- ...------- _..»...»...............»....... page 1
gackground.....».........»._......».........»_.......»..... ..........».?.....?.... Page I
?--
Objetiives........._ ...................».......................-
page 2
......
?..
Theo .....» .............»....._..............
Instrumentadon and
..........
..... .__...».__........._...... . page 3
Methodoiogy. ...__ .........................»........._...............
............................. page 4
........._............» .......
Geophysical ?issumptions ......... _.....
.......
..... ...........»..._.»............... page 5
Resuits ...._ .........
.......................................
page 5
......_............»»
.»__.
Survey.»...._....»._.....»....» ...............».....
EM31 ? page 7
....._..."'".
Total Magnetic Field Survey ...................?.._......_. -.?.....-..."
» ...........
Conclusions .......................................................... .....»..........._......_...».. pase S
•
•-..._....._»..»......._.......» ...................... page 9
Quality Controi ...»...._._ .......................».
.».» ..................._..........»..............?....». page 9
Standard Disciaimer ............... »........... ...
.................».....page 10
......».
Geophysicai Figures ..... ......... .................. ........................ ...
Appendix A: Overview of Electromagneric Surveys
Appendix B: Overview of Magneric Surveys
Appendix C: Department of Energ,y docvment No. DOE/HWP-100
Appendix D: Daicota Co.'s Lone Oak Site-EiVt31 Data
Appendix E: Dakota Co.'s Lone Oak Site-iVlagneric Data
afrt081895a
LIST OF FIGURES
Figure I: Dakota Co.'s Lone Oak Site--Contoured ApQareat
Condudivities (mt71?mhos/meter) ...............?... ....-......_...»•.....-•••_........ _page 11
Figun 2: Dakota Co.'s Lone Oak Site-Contoured InQhase
Response ?PPt)............» ................................_.... ...........................».».......?age 12
Figure 3: Dakota Co.'s Lone Oak Site--Contoured Vlagrtetic
Total Field Residuals (gammas).....»..»......_... .......... .......... .........._..........page 13
Figure 4: Dakota Co.'s Lone Oak Site-Contoured EIevations (feet)...._..._.....page 14
afit081895a di
INTRODUCTION
Bacicground
During the Period of June 5 through 7uae 7, 1995, eleca'omagaetic aad magnetic total
fiefd surveys were conducted at the Dakota Couuty's Loae Oak site located ia Dakota
CountY, Nfimesora. The site was anpaready vsed for the storage of discarded rubber tires.
These tires may hzve been buried in place and are now possbly a concera to local tand
owners. In an aztempt to deicae areas posentiafly buried wirh vres, eiectr'Omagneuc
conducaviry (IIvG 1) and totai field magne4c surveYs were conduc:ed az the sise. The
basis for che conductiviry surveY is the assumnnon that btuied tu'es are very porous and
eiectricaUY inert gence, these praDerties og the buried tires wouid combine to geaerate
low near mrface eieczricai conducaviries. 'L1ris assumption was supported by a 1'united
eM31 survey conducted during he summer oi 1994. The magneric survey was iacluded
in the study because of the possibi3iry thai £=ous matesial was associazed with the tire
storage area.
4bjecdves
The objective oi the efectromagneric surveY is to mag subsurface- annarent electrical
conducuvicies and to differenuate zones oi high conductiviry from zones of low
conducrivity. It was anucipaLad titat the conducaviry of the nauve soils would dir'Fer
significantiY from the conductiviry oi the lanafiil materiai. Fience, the extent of landfiil
operanons wouid be de5ned by an eleczicai conductiviry map. In addition, the
aut081895a page l
elecuomagaetic sarvey couid be intetpreted to imply the location of geophysicdlly
detectable mesal objecLS within the aear surface of the fll mateciaL
Tfie objective of the magnetic survey was to define the location of buried feaous metal.
Feirous material is commoniy assoaated with fll material and often extends out w the
bouadaries. The ability of the magneric suivey to detect ferrous metai, at greater deaths,
supplemeats the eiectromagnetc (EN resuits. It s6ouid be noted, hawever, that tke depth
of iavestigariaa of the magneuc method is depeadeat on the size and geometry of the
ferrous object(s), as is the EM survey.
IlNSTRUMENTA?'ION AND THEflRY
The eiectromagaaic surveys empioyed Geonics' Mvi31 canducrivity system. The M-Z I
system was operaied ia the vestical dipole mode and all mPas,rer,ents were electronically
stored on the artached data logger. A brief iatroduction to eiectromagneric theory is
presented in Appeudia A. Overview of Eiectromagaeric Surveys. The ma}?etic surrey
was conduc:ed with aa EG&G G-856 proton precession magnetometer. The insaumenc
empIoyed a single seasor t6at recorded tfie mtai magneric fieid. A brief introduction to
magaetic theory is preseated in ApQmd'u B: Overview of Magneiic Surveps.
32it081395a PaSe 2
METHODOLOGY
The recrangular caordiaate system referenced at the site incorporated a 3rid system
esrablished by Dakora Councy peisonneL Fourteea north-sourh aad two zasc west treading
lines were established by Fromm Applied Teclmoiogy pezsonnel under du direction of
Dalcota County's personnel. These lines were marked wirh pin 9ags at 5o foot
inQ.emenu. 'I1ie exact locauon of eac'a pin 9ag, in tezms of the coordinate system, was
established by Dakota Coumy surveyors. Each of the snaeea Gnes was 3eoPb5'sicaRY
yluveyed az 10 foot increments. The location of eac's geoPhyacai suc`'ey Poiac was
iuterpolated from the estabiished Pin flag locations A11 cesuits are rePorted in tenns of tfie
coordinace system used by the Dakora County survey team-
??ch E'vi; lstation, inaicated by the +'s ia Figure 1: Dakota Co.'s Lone Oak Site-
Contoured AQQarent Conductivities and Figure Z- Dakota Co•'$ Loae Oak Site-
' Contoured InQhase Response both conductiviry and inPhase measurements were
obtained. T"ae missing poincs encauatered on some lines are the cesult of ailtsuai feazssres
that made the stanon either inaccessioie or geophysic3llY invalid
The =s ia Figure 3: Dakota Co.'s Luue Oak Site-Cnntoured ylagneac Totai Fieid
Residuais represeirt the locations oi magneric svrvey stations. Ai each magnetameser
scarion, measuremems of che total magnetic field were obtained To remove tempors[
variations from the magnenc dam a base starion was mauuaine:l az location 535410E
116814N. The magneric field az this focarion was messvre3 approxmazellY every h°ur'
afnil81895a page 3
These base starioa measvrements were thea interpolated to yidd the base srarian reading
at the dme of each survey site measuremeut. AlI total field residuat valucs reported here
are the residuaI di$'ereace betweea the iatecpolated 6ase stadoa measuremmts and the
survey site measurements. Both the ENL31 aad ma}teric suiveys were cansisteat wiih the
iateat of inethodologies defined in the Departmeot of Energy document DOFJHWP-IOQ,
Geophysical SOPS, atcached as Appead'u C.
Geophysical Assumptions
In a geophysical site chacacterizarion h7ce the one performed az the Dakota County's Lone
Oak site, no one method wiIl cieaziy answer alI the c}uestions ahaut the site. All method's
require certain assumptions and if these assumptions are reasonabie each method of
imvesrigarion should yieid desirable results. The assumpaons requured for this study, low
conducrivity for buried tires and the possble presence of fasous matetiial, appeac
reasoaable; heuce, there was a high leve3 of confideace thas EMi land magnetomeser
? measuremenss wauld delineate the boundaries of the 6II azeas, assuming minimal cultura[
imerference (i.e. buildings, fences, yard debris, and buried utiIities).
It is worth aoting the methodology used in presenting the isopleth maps. Eaca of the maps
rna;*+ra;ned a constant sca(e. In each of the plots, one will note azeas have been "bianiced
out." Tlvs miniurizes the effect of computer exteaQolazed comour lines thaz extend imo
unsurveyed areas. Areas thaz have questionabie validiiy are not intenriona?ly addressed in
this report. Computer extrapolazed areas were blaniced out to the exteat passible aad aay
rm^.ain+ng non-blanked areas can be found by deiermining where contour lines may have
aut081895a page 4
exteaded beyoad surveyed are3s. To desertnine unsurveyed areas, identitY areas with few
sration locations in the isogieth map of concan- For example, areas betweea roadways
and the bacic of a house and areas wich bcnldings, witbia the surveY area, Yleided mnes
with little or ao data The spatial exteat of smailer anomalies are ¢ot necessacily accurate
because of the lack of data. Contouring may distort the &eId data, thus, the locatioa and
eztent of azry aaomaly shoald be substan6ated 6y refereucing the originai data set
(see aPPendices).
RESULTS
EM31 survey
The af31 data obtained during the survey is taoulated bY station location and attached as
Appeadiz D:Daicota Co.'s Lone Oak Site - M01 Data. A cnior coded coirtour map
oi the measured efecuical conducaviries is presented in Fi3ure 1- In tlus ffpm the greea
areas represenc conductiviries oi 0 millimnos per maer ar less, the lighrt biue areas
- reareseat conducaviries benveen 0 and ZS millimhos per mete:, and the me:fium 6lue to
red areas are for conductiviues in excess of 25 miUimhos pec meter. The presence of tires
snould reducc the elecuical conduccivity aad, heacc, the iacerior of the landfitl area is
c,yaracterized by the centtal zoae oi low conduciivities, the L'ght blue azeas in Fgure l.
Wizh the exceQtion of buildiags and fencing, the low conducavizy cenual zone is
swrounded by intermediate to reSariveiy Irigh conducsiviues, the darica blue azess in
Figure 1. Two inserpreted examnies of cvitruai interiereace are the norrh-souch fence lines
aiong 535635E and 535720E. The highher conducuvity values outside the expected fi11 area
are interpre=ed to represenc a uansmon between fill and non-611 conditions. wrhin the fiIl
afrt081895a Page 3.
mateiiai, the Irigh conducrivity mnes are possbly a hansitioa betweea fill materiat aad a
cuitrzal feature (i&, a feace or a house).
Negative treading conduaivities are a sarong eiectromagnedc iudicanians of buried metal.
These conductivities have been cantoured ia green, see Fgure 1. All of the anomalies
cantoured in greea are rhought to be assoaated with buried metal, though possbly not as
spatially lar3e as shown in the plot The abnormal and very lagh conducwviries siwwn in
red are probably associated with buried wire or fencing. Though one would expect a
ae3arive resQaase, deeply buried mesal or metal aot directiy beneath the insaument caa
rnnce lugh conducavities. As an exampie, the area near 535720E 236715N was covered
with yard debris aad miscedaneous cefuse, whicii may contain eaough meral to weate a
conduc:iviry frigt:.
The rasults of tiie SN131 iaphase measuremeats are preseared in FgUre ? The inphase
vaiues, measured in parts per ckousand direcciy frnm the insuumem, are derived by a
comnarison of the prunary and induced electromagneric fieids as outiined in Apgendiz A.
These measuremeass can be of value in desecting ferrous or aonferrous metal and also
serves for ;enerai quality comrol. In the presence of inetal, the inphase response is
normally associazed with values abnormally above or below background values.
dutside of the IandEll, che bacicground inphase response for the site is imemreted to be
approxitnuefy 0 ppt Within the laadfill area outlined by the eiecuical conductivity res[ilts,
Fgure I, tfre usphase response does not typically deviate from 0 ppt througEiout the entire
us[erpreted landfiil area. This suggests the (acic of large deposits oi bwied metat
throughout the site. Due to the composicion of the fill material, the inphase re,gulis did not
c:ear.v deiineate :he land&ll. As one would ecpect, in a non-conductive media with little or
no me:ai present. However, building structures and cultural features created the highesc
azicQ81895a page 6
amplitude anomaiies in the iaphase dasa As meauoned ia the conducdviry resutts, the
anomalous Irigh seea near 535720E 23671N is passibIy caused bY surfacs metal or
fencing.
Total Magaetic Fidd Sarvey
The magnetic data obrained during the sutvey is tabuiazed by starion locazion and attached
as AgQend'u E: Dakora Co.'s Lone Oak Site-Mar,nenc Data. A color coded map of
the totai fieid magnetic resilts is presemed in F'igure 3: Dalwta Co.'s Lone Oaic Site-
Cnntoured LMagnetic Tota[ Fe1d Residuals. The magaesic results complimmt aad
support the E?'i 1 results. These resulu are reSFonding aimost enrirely to the presence of
fe.?rous matetial. The response is typicaily a Irigh pasiuve peak accompaaied by an
associazed negative trou3h.
The fiil material coatains enough ferrous mateciai to creaze che soarially e+aensive magnetic
high (greater thaa I00 gammas) ceutered az appro;dmazeiy 5-35550E 236850N, seea in
- Figure ;, While sigaiucant, tius 6igh is law amplitude ia comparison to magietic
anomalies geueralIy observed in laudfiils. Thus. some ferrous material is present However,
it does not appear to be a major componeat of the fill mazesiaL There are no suvng
electromagaetic indications of buried metal in this ares, wmcli implies the metal is 6elow
the dececrion of the 261 or :s is not elecaically coupled to the gound. The negarive
trough that should be assocued with tius high is obscured by the ailturally induced
negative troua,k to the north. The r?ng magnetic anomaties in Fgure 3 are
imerpresed to resWt from cuieu:al inte*iereace's. Kence, siguncant amaums °f fesT°us
metal within the fr31 material anuears to be limited to the soucheast secion af the IandfilI.
afrt081395a page
Wittt the exception of the higfi vaiues along the northera aaem the landSll appeazs to be
aPFroximated bY resduaLs greater than 100 gammas, seen in Figure 3. Tlmugh this region
is less thaa what is approxdmated bY the cnnductivrty results, it suggem tiris azea has the
highest conceau'ations of busied ferrous metaL lhe tri3fi magnedc resduafs coataoced ia
daric red north of 231000N aze iatetpreted as culunal kterfereace•
THE FOLLOWING CONCLIISIONS SIIhID7ARIZE
Tf1E INCERPRETATION OFTHE FiLL MATERIAL
A14p'PHE L,iNDFII.L BOIIIVDARIES
1. The EM baclcground measurements outside of the laadfill indicazed diat
appareat conductivities are typicaIIy greater than 25 mfftmhos per meter with an
inphase resQonse of approumateiy 0 ppc The magnaic measuremeats yieided
bacicground residual values that varied besoveen -I00 and 100 -omnas.
Z The latezal oaeat of the bulk of the laadSll is appareatlY reP?ed by
canductivities less than 25 miIlimhos per meser. Tfus region is deaiiy represeated
in Figure I by the light blue areas. Deiineation of the landfill may be compromised
by rnimerous culriurai feanu'es. iMore precisety, trigh conducsiviry feanura may
destroy the contimiity of the (ight blue regioa This can 6e seen ia the nocth-canu'al
portion of the ste from 535600E 237070N to 535750E 237000N.
3. IIecttnmagnerically daectab(e buried metal is present ia regions colored in
green and possibiy ia the areas colored in red. Magnaically, the fill material
conraias eaough ferrous metal to create a spatially extensive magneric trigt?
ceMered at apprmomately 535550E 236850N. The remaiuing higii magnedc
anomaties either coaespond to cu(turat iaterfference or EM aaomalies indicsting
the preseace of buried meraL
4. In order to ftuther eahaace the correlaticn of the geophyscs with curreat sne
conditions, a topographic map of the site was oeserated from Dakota Ca.s survey
data This dara suggests the region bouad by the 800 and 835 feet contouts in
Figure 4: Dakota Co.' Lone Oak Site-Conwured Elevatious cotrelates weil
vrith the area intespreted as fill in Figure l. This correlauon needs to be con6cmed
through either a boiing progam or eccavauon.
afit081895a page 3
QUALITY CONTROL
The quatity of the elecaroma3netic data was enRUed through extreme care duing the fieid
measurements and by repeated measuemerits at a calibrarion station established az
535410E 236814N. The opecator oP the EN131 coaducaviry meter oirtrined all
measuremeats with the Ioag axis of the insaumeat aliped paraltei to the uansect lines.
The inciniment was reuuned periodidIy ta the caLbrarioa srazioa The II1I31
at the calibrarion srauoa varied by less thaa 1 millimho per meter
throughout the sarveys duration
A magaetic base srarioa was esrablishe:i near scauon 535410E 236814N. The magneric
variations at che base sration were less than ?? SOnunas durmg any one hour pe:iod and
less than 1.7 jammAC throughout the durauon of the survey.
STANDARD DISCLAIMER
The objec3ve of any geophysical survey is ta define the edscence and configurnrion of
features az deQch. Aowever, these feanues may bear a highty comQlex relazianship to tiie
BeoPhYsical measuremeaa recorded. Therefore, conc:usians drawn, ao matter how
logically deduced, shouid not be misconsaued as facx. We shall not and wiil not, excepc in
the case of gross or willful negiigence on our part, be liahie or responsibie for any Iosses,
costs, damages or exnenses incurred or svsrained by anyoae resutting from any
interpretarioa made by any of our offsc=s, empioyees and ageats or by anyone else not•
reiated to Framm ?,pplied Techaology who mi,ht base iaterpretauons and opurions on our
geoPhysical surveys.
artU781895a pa4e 9
Geophysical Figures
afn08l89ia page 10
?
W1`r',111'e I I)al.ot<< Cn.'s i.one nal; Sile---Conlourecl Apparenl. Concluelivit.ies (millirnlios/meter)
,-1:17 I (.
,..:fr'U:
;':l`l(1(
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Q)
i '1::IEiJ`.
.?
.[
?
?.
? 236J1
y
u
?
? ?36N
n
I'..368
; 367
;?:3(i7
350
95
85
7.5
65
55
45
35
25
15
5
-75
Dislance GrAsL (feel)
.
I,Mie
ti: i'7 I (
rci roo
?
?
Q) ;?1,3695
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.?
?
;z ?:,i(iJ01
?
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a
,Ti
v
Q
;:;trstio?
;e;ics7no
S
40
35
30
25
20
15
10 .
5
0 •
-5
-10
-15
-20
-25
-3Q
_.....,.,,, aoa t uu 5351300
Ih:;hiuc:c N,'s1 (fcul)
.:>
a?
a?
w,
?
.>
c.
0
z
a,
U
C
?
Q
2:i710(
;L:i'7U5i
122700,
2:3Ei95
23Ei90
236E1,5
??36F30
l(i75
'.'.;3G I0
900
500
100
-300
-700
-110
-150
-]90
-230
-900
DisLance LasL (feel.)
Vigure '3: IYrilcolia ('n.'a I,unr. Ozil; Sil.e--CoiiLoiii•e<I
Magnelic 'I'olal Wield Residuals (Caninias)
FtL,ure =1: Da1;oLU C?i.'? I.,ouC Oal: Sile--C'uiiLoLu'ecl E'levalioiis (feel,)
,s: V
N?I
?
a?
a?
.,,
23
?
?
?.
0
z,
0
G
?u
?
?
ra ; ;
?.
i:.
ti.
f3'l 5
8,10
E335
830
£325
820
£3I5
810
805
, 800
' 7:J5
790
535300 535=1UU o.soo%,v „?.,•••,•,
I)iSl,jucc I!;tiSl (fe.:l)
Appendix A. Overview of Electromagnetic Surveys
Overview of Electromagnetic Surveys
Elecsromagiedo sarveys attempc co measure near sudhce elecuical condncrivicy, stm urvuse of etear;cal
:esistivitY, ihrough eiearamagneric indaction meshads. The basc inshumrntarion ieqaired w accomplish
this is picsorially deSned in Figurc 1-A. A siu{ace uansmitter coil, Tx, is drivca with an audio f-equency
alternating current The magtetic field resuiting fiom current flow iA the +ra++<m;tter coil causes induced
cvrrents to flow in the gound. The induced graund current csGttes a secondazy magnetic 5efd which can
be sensed by a surface rexiving coii, Rx. The surface recciving coil wiIl also sense the pnmai9 magneric
fieid resuIUng &om current flow in the coil Tx but tbis primary fieid lags the szcondary field by 90
degrees, 'I'he phase shiR henveen the pnmary and secondary fields provide a basis for their sepazation
in general, the magaimde of the secondary field xnsed by rec-..iver rni1 Rr is a nonlinear funcuon of the
cod spacing, operanng frequency, and surface canductivity. Hocvevez, for selected vatucs of operacing
freqneary and coil spacing the magmtude of the secondary field is a linear funcuon or tbe sartace
conductivity. Hena. if an apQropnaze frequency and coil spacing are urilized. the magnitnde of the
secondary fieid phase shifted bY 90 degees from the primary fieid pn be used to imply the near surface
conducsivity. Tfte magnilude of the magnetic Be1d sensed by Rs and mphase with the primazv fieid c:m
- also be measured. This is Imown as an inphase mcuurement and pn be expressed in cenns af Parts Per
thousand (PPt) af the PrixnaTY field.
The peneuation depth of efecuromagnenc measu:emcnts is hiiglilY depeadeat upon surface conductivity.
Under idpl conditions, the EM-31 equiQment ased in this siudy can achieve -penetrauon depths of
appro:oma[eiy 15 to 20 feet for norntal conductiviry measuremenu. However, the presence of surfaca
metal can limit peneuation to che surfaca Thus, it is only possible to srate t6at peneuatian depth can be as
deep as approximateiy 20 feet and nore tba[ measumd conducuvities do not define a buciai deptiL A moce
conservatrve estimate for detec.uig 6uried meral is less tlian 10 feet In comparison and also under ideal
conditians, the EM-34 equipmenc implemenung a 20 meter coil spacing can achieve penetxatian depths of
approximazely 45 ta 100 Sea depending an tLe arientation af the cails. hori7vntal or vecticaL The
presence of s4wradic sua'ace mefal caa limit peamauan, bat aot osually as sevetely as the IIvII 1. Thus
and as it is with the EM32, rt is oniy pomble wsate tLat peaetarion dep[h can be as dap as
approxrimately 45 to 100 fea wiih a 25 merer coil spadng. Again, measuced coaducrivities do noc de6ne a
burial depth- With :esQect to metal detecton, the hasuvmwt, EM34, is not usuallY used to lotate lnuied
mcpl, unless the bu:ials aze eMremeLY lar-
Nhile elecuomagnetic mrasuremeuts pravide an excelleut method for the deaecsion af neaz surf= metal
abjects, the meastuemenis cannot define the type or fomt of metal imolved. For crampie? canduciiviry
mcsnuements cannoi distinguish between aluminum banels, iran engine biocks, concrae with sceel
:ei'nfotcing rod, or sted ldtchen appliances
Mefal witLin an eiectromagnerically sarveped azp can be detaxed with eitLer the conduciivity
measurements or the inphase measuremeats. IIecaicallY condncting material- geae:allY steel, within a
few fees af the suiface wiU cause elecuomagndic fidd distoctions whicit will :cmlt in zero or eren
negative values of ineasnred conductan,.. Deepu mehallic objear will caue less fidd distocdon and ]ead
to measared wnducrivities wluch are abnnnnaliy high in comQanson to ste bac3cground values. Inphase
_ mpsurements are believed w be more sensitive to metal than conducrivity mrasuRmrnts. Thus, inphase
anomalies may iadicate the presence of inetal az too grcit a depth to effect the wnductiviry rorasa•PmP,,•<
Heace. menllic objau withia rhe confines of a survey aica aze cha:acte:ized try one ar more of the
following:
i. Zem oc negative conducrivitias.
2. Conductivities abnormally high or low for
site soils.
. Anomaious inphase values.
3
Appendig B: Overview of Magnetic Surveys
Overview of Magnetic Surveys
The magnitude af the eatth's magrietic field varies over the socfaa af the eazth for a aumba af tvsoas-
Shoct waveiength spatial vaziations of the field which occvr wiUrin teas of fea aze always due to the
presence of Fermus metal. assumnS the absence of powes linct. Hence, the detxrion of short aavelength
ypatial variauon in the cuth's &e!d is a certaia indicatian of the praena of fermus mate:ials-
The magaitude of the earth's magnetic field maY be mpsured with vazious tYPes of iastmmwtarioa Tbis
study empioyed a pmton procession magnetometa wtdc3i mctsum the magaiwde in gauimas of the
earth's total maBneric fieid. Because only spaaal variations aze of concan. all mcuured values are
ad'}usted to the value of some pazticiilar measurement Point within the sorvep azra 'i7iis mfaeaa point is
Imown as the basa siarioa Reported values are the magniaide b9 which the mtal fidd at a particular
locarion is above or below the mtal field ac the base station.
In addidon tn the magaitude of the field. the vectical change in the field per uait vecdcai distance pa aLso
be mcisured with the available magnetomecer. This quanaty, knowa as du verdcal gradient is related in
' par[ to the distance of the ^,^as,•emeM poinc from the fercous mateiisl. As the disrance fmm the fumus
- mmnaL incrcases the vertiaal g'sdient will deccease. Thus. assumi°B tile equahtS' af atha facio=s,
vertical gcadients which are high in comparison to badcground otadients are indicarive of very near
surface famus mateiial.
The =acc shape of a total field or gad'unt anamalY Over fen0us ma1°ig is lriehty dependentup°° the
geogcaphic loca[ion the direc[ion of the survey line, and the geometxy of the fexraus mataiaL Tbus. it is
diffcult to spatial identify the source aiea of most maBnedc anomalies. ty(nd found ia the field.
However, observed anomalies should generaily be chacacteiized by a anval positive Pcik Pftia11Y
surraunded by flanldng negaRve valucs. T7u eYact locraon of tlte fermus material is gmaai[Y asociated
with the point of uro crossing ar the uro contaar line. Hecnuse it is difficuit to define a typical magneric
anomaty it is aLso diffiailt m define a mapetic depth of invesdgation For a general poinc of cdaeace` a
100016 steel sphere could be detected at a depth of appcmomateiy 60 fea if there was no svofiaQ metal
a6ove the sphete. The ladc of asry sudace maal is critical as it is genaally nat posible m detat the
presence ofburied meml bmea[h sorface metal.
In enmmanr, magnetiG measurnmentc are on(f Cff'CC7Cd by (hE pICSERCC OF fCiIC1U5 mIC781 3nd 8[C
generallY independea[ of taPoBmPhy or siu soil conditians, assvming rha[ mosc narive soiLs do not cowazn
a sigmficaat amrnmt of mataiat with Iriglt magnaic suscepabilitim The detectioa range of magpaic
measuiemmts exceeds that of ather aeaphysicai mahads so tLat Sid spamngs rrn be increased in
compazisan ta other methods. The drawbaclc of magiaic measniemmts is tLat they aze strmg(y
intlumad by satface metal. inciuding maal teas of fea ro the ade of the mcuuermmt point, and the
e;cact locanoa of detxted metal can be diffiwit to prediet