52B10SEei95 2.13451 MOSS010

DIGHEM HI SURVEY

FOR

GRANDE PORTAGE

SHEBANDOWAN AREA

ONTARIO

N.T.S. 52B/7-10

RECEIVEDAUG081990

MINING LANDS SECTION

,13451

DIGHEM SURVEYS 8t PROCESSING INC. MISSISSAUGA, ONTARIO July 12, 1990

Douglas L. McconnellGeophysicist

Edited:John Gingerich

Division GeophysicistNoranda Exploration Company, Limited

CERTIFICATION

THE DIGHEM HELICOPTER EM/MAG/VLF-EM DATA OF THIS

REPORT WAS PURCHASED FROM

NORANDA EXPLORATION COMPANY, LIMITED

Grande Portage is hereby granted the exclusive rights to the

data as outlined on Map 1.

John Gingerich Division Geophysicist

Northwestern Ontario Division Noranda Exploration Company, Limited

o

SURVEY AREA

GRANDE PORTAGE HEM SURVEY AREAi SCALE 1^1/2 MILE ! ~" 7-^ -x--... ^" , i l ^ MAP 1 \

i;

SUMMARY

A DIGHEM111 survey was flown for Noranda Exploration

Company Limited, over the Shebandowan area in Ontario. The

survey comprised approximately 2620 line-km.

The purpose of the survey was to detect conductive

zones, and to map the magnetic properties of the rock units

within the survey area.

Numerous bedrock conductors were detected by the

electromagnetic survey. Some of these appear to correlate

with magnetic anomalies. The 7200 Hz coplanar EM data were

used to generate contour maps of the apparent resistivity.

These show the conductive properties of the survey area. The

total field and calculated vertical gradient magnetics yield

valuable information about the geology and bedrock structure.

The VLF data show numerous, moderately strong trends, some of

which may reflect bedrock structure or stratigraphy.

I " The survey area exhibits potential as host for both iconductive massive sulphide deposits and weakly conductive

[j zones of disseminated mineralization. A comparison of the

. T various geophysical parameters, compiled with geological and

** geochemical information, should be useful in selecting

l] targets for follow-up work.Li

Sea!e l: l .000.000

90 030' 52A/12, 52B/7-I2

FIGURE l

SHEBANDOWAN AREA

D

52810SEei95 2.13451 MOSS

CONTENTS

010C

(3O f]

Section

INTRODUCTION . . . . . . . . . . . . . . ......... . ................ l

SURVEY EQUIPMENT . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . 2

PRODUCTS AND PROCESSING TECHNIQUES . . . . . . . . .. . , . . . . . . 3

SURVEY RESULTS ...................................... 4Conductor Descriptions....,..................... 4-13

Sheet fl.................. .............. .. .. . . 4-14Sheet #2...................................... 4-18Sheet f3...................................... 4-22Sheet #4............. ......................... 4-24Sheet 15...................................... 4-25

BACKGROUND INFORMATION .............................. 5

CONCLUSIONS AND RECOMMENDATIONS ..................... 6

C[j

APPENDICES

A. List of Personnel

B. Statement of Cost

C. Statement of Qualifications

D. EM Anomaly List

- 1-1 -

INTRODUCTION

A DIGHEM 111 electromagnetic/resistivity/magnetic/VLF

survey was flown for Noranda Exploration Company Limited,

from January 20 to February 9, 1989, over the Shebandowan

area in Ontario (Figure 1). The survey area is located on

NTS map sheets 52 B/7-10 and 52 A/12.

The survey area was divided into three blocks. The

following table gives the details of these blocks.

Table 1-1 Survey Blocks

l

BOGU

Block

B

C

D

Lines

From

20010

30010

40010

To

21790

30700

41330

Flight Direction

150V330*o'/ieo'

014V194'

Line-km

1483

352

785

The survey lines were flown with a 200 m separation.

Tie lines were flown parallel to the survey boundaries.

The survey employed the DIGHEM111 electromagnetic

system. Ancillary equipment consisted of a magnetometer,

i;

o o

[J 13

- 1-2 -

radio altimeter, video camera, analog and digital recorders,

a VLF receiver and an electronic navigation system.

The survey results are shown on five separate map sheets

for each parameter. Table 1-2 lists the products which can

be obtained from the survey. Those which are part of the

contract are indicated on this table by showing the

presentation scale. These total 25 maps, 10 colour plots

and 7 shadow maps.

Recommendations for additional products are included in

Table 1-2. These recommendations are based on the

information content of products that would contribute to

reducing the cost of follow up, or increasing the likelihood

of exploration success.

- 1-3 -

Table 1-2 Plots Available from the Survey

NO. OF MAP [Parameter Number] SHEETS

Electromagnetic Anomalies [1] 5

Probable Bedrock Conductors

Resistivity ( 900 Hz)

Resistivity ( 7,200 Hz) [5,5] 5

EM Magnetite

Total Field Magnetics [2,2,6] 5

Enhanced Magnetics

Vertical Gradient Magnetics [3] 5

2nd Vertical Derivative Magnetics

Magnetic Susceptibility

Filtered Total Field VLF [4] 5

Electromagnetic Prof iles ( 900 Hz)

Electromagnetic Prof iles (7200 Hz)

Overburden TliieknesB

Digital Profiles

ANOMALY MAP

20,000

-

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

PROFILES ON MAP

N/A

N/A

-

-

-

-

-

MB

-

-

-

--

-

CONTOURSINK COLOR

N/A

N/A

-

20,000

-

20,000

-

20,000

-

-

20,000

N/A

N/A

-

N/A

N/A

-

20,000

-

20,000

-

***

-

-

-

N/A

N/A

-

Worksheet profiles

Interpreted profiles

SHADOW MAP

N/A

N/A

-

-

-

20,000*

-

-

-

-

-

N/A

N/A

-

15,000

-FM00li

N/A*** ***

20,( [ 3*

Not availableHighly recommended due to its overall information content RecommendedQualified recommendation, as it may be useful in local areas Not recommended

20,000 Scale of delivered map, i.e, 1:20,000The parameter number appears with the sheet number in the map title blockTwo additional sheets were needed to present the shadow maps due to differingsun angles for areas sharing a common sheet.

I

[j

D

- 2-1 -

SURVEY

i This section provides a brief description of the

geophysical instruments used to acquire the survey data s

Electromagnetic System

l ' Model: DIGHEM111

Type i Towed bird, symmetric dipole configuration, operated at a nominal survey altitude of 30 metres . Coil separation is 8 metres .

Coil orientations/frequencies i coaxial f 9 00 Hzcoplanar/ 900 Hz coplanar/ 7,200 Hz

Sensitivity: 0.2 ppm at 900 Hz0.4 ppm at 7,200 Hz

Sample rates 10 per second

The electromagnetic system utilizes a multi-coil

coaxial /coplanar technique to energize conductors in

different directions. The coaxial transmitter coil is J

vertical with its axis in the flight direction. The coplanar

y coils are horizontal. The secondary fields are sensed

.. simultaneously by means of receiver coils which are maximum

" coupled to their respective transmitter coils. The system

yields an inphase and a quadrature channel from each

transmitter-receiver coil-pair.

F

- 2-2 -

Magnetometer

Models Picodas Cesium

Sensitivity: 0.01 nT

Sample rate t 10 per second

The magnetometer sensor is towed in a bird 15 m below

the helicopter.

Magnetic Base Station

Model t Geometrics 6-826A

Sensitivity: 0.50 nT

Sample rate: once per 5 seconds

An Epson recorder is operated in conjunction with thert. base station magnetometer to record the diurnal variationsI

r of the earth's magnetic field. The clock of the base station i

is synchronized with that of the airborne system to permit

P subsequent removal of diurnal drift.

U VLF System

11 Manufacturer: Herz Industries Ltd.

Type: Totem- 2 A

l J Sensitivity: Q.1%

Df]

- 2-3 -

The VLF receiver measures the total field and vertical

quadrature components of the secondary VLF field. Signals

1 from two separate transmitters can be measured

simultaneously. The VLF sensor is towed in a bird 10 m

below the helicopter.

Radio Altimeter

Manufacturer! Honeywell/Sperry

j . Typel AA 220

Sensitivity: l m

The radio altimeter measures the vertical distance

between the helicopter and the ground. This information is

used in the processing algorithm which determines conductor

depth.

l]Analog Recorder

Manufacturer! RMS Instruments

fi Types 6R33 dot-matrix graphics recorder AResolutions 4x4 dots/mm

[j Speed! 1.5 mm/sec

The analog profiles were recorded on chart paper in the

aircraft during the survey. Table 2-1 lists the geophysical

N data channels and the vertical scale of each profile.

IJ

- 2-4 -

Table 2-1. Hie Analog Profiles

Channel Name

CX1ICX1QCP2ICP2QCP3ICP3QCP4ICP4QCXSPCPSPALTVF1TW1QVF2TVF2QCMGCCM3F

Parameter

coaxial inphase ( 900 Hz)coaxial quad ( 900 Hz)coplanar inphase ( 900 Hz)coplanar quad ( 900 Hz)coplanar inphase (7200 Hz)coplanar quad (7200 Hz)coplanar inphase ( 56 kHz)coplanar quad ( 56 kHz)coaxial s f erics monitorcoplanar s f erics monitoraltimeterVLF-totali primary stationVLF-quadi primary stationVLF-totali secondary stn.VLF-quad: secondary stn.magnetics, coarsemagnetics, fine

Sensitivity per mn

2.5 ppn2.5 ppm2.5 ppnt2.5 ppm5.0 ppm5.0 ppm

10.0 ppm10.0 ppm

3 m2^2.512^2.5125 nT2.5 nT

Designation on digital profile

CXI ( 900 Hz)CXQ ( 900 Hz)CPI ( 900 Hz)CPQ ( 900 Hz)CPI (7200 Hz)CPQ (7200 Hz)

CXS

ALT

MAGMAG

Table 2-2. The Digital Prof i leu

l;

J

ChannelName (Freq)

MAGAI/PCXI ( 900 Hz)CXQ ( 900 Hz)CPI ( 900 Hz)CPQ ( 900 Hz)CPI (7200 Hz)CPQ (7200 Hz)CXS

DIFI ( 900 Hz)DIPQ ( 900 Hz)COTRES ( 900 Hz)RES (7200 Hz)DP ( 900 Hz)DP (7200 Hz)

Observed paramBters

magneticsbird heightvertical coaxial coil-pair inphasevertical coaxial coil-pair quadraturehorizontal coplanar coil-pair inphasehorizontal coplanar coil-pair quadraturehorizontal coplanar coil-pair inphasehorizontal coplanar coil-pair quadraturevertical coaxial sferics monitor

Computed Parameters

difference function inphase from CXI and CPIdifference function quadrature from CXQ and CPQconductancelog resistivitylog resistivityapparent depthapparent depth

Scaleunits/mm

10 nT6 m2 ppm2 ppm2 ppm2 ppm4 ppm4 ppm

2 ppm2 ppm1 grade.06 decade.06 decade6 m6 m

- 2-5 -

Digital Data Acquisition System

j Manufacturer: RMS

Type: DAS8

l Tape Deck: RMS TCR-12, 6400 bpl, tape cartridge recorder

The digital data were used to generate several computed

parameters.

f Tracking Camera

Type: Panasonic Video

[ Model: AG 2400/WVCD132

Fiducial numbers are recorded continuously and are

displayed on the margin of each image. This procedure

l ensures accurate correlation of analog and digital data with

respect to visible features on the ground.

l;I

Navigation System i

Model: Del Norte 547niJ Type: UHF electronic positioning systemn Sensitivity! l m

Sample rate: 0.5 per second

l]l ' The navigation system uses ground based transponder

[ j stations which transmit distance information back to the

helicopter. The ground stations are set up well away fromV.J

[]

- 2-6 -

the survey area and are positioned such that the signals

cross the survey block at an angle between 30* and 150*.

After site selection, a baseline is flown at right angles to

a line drawn through the transmitter sites to establish an

arbitrary coordinate system for the survey area. The onboard

Central Processing Unit takes any two transponder distances

and determines the helicopter position relative to these two

ground stations in cartesian coordinates. These are

transformed into a known coordinate system (such as UTM)

during processing.

Aircraft

The instrumentation was installed in an Aerospatiale

AS350B turbine helicopter. The helicopter flew at an average

airspeed of 110 km/h with an EM bird height of approximately

30 m.

D

r-

- 3-1 -

PRODUCTS AND PROCESSING TECHNIQUES

The following products are available from the survey

data. Those which are not part of the survey contract may be

acquired later. Refer to Table 1-2 for a summary of the

maps which accompany this report and those which are

recommended as additional products. Most parameters can be

displayed as contours, profiles, or in colour.

pase Maps

Base maps of the survey area were prepared from l s 50,000

topographic maps. These were enlarged photographically to a

scale of 1:20,000.

Flight Path

The cartesian coordinates produced by the electronic

M navigation system were transformed into UTM grid locations

D during data processing. These were tied to the UTM grid onthe base map.

Prominent topographical features on the flight videos

Li

J

are correlated with the navigational data points, to check

that the data accurately relates to the base map.

- 3-2 -

rElectromagnetic Anomalies

Anomalous electromagnetic responses are selected and

analysed by computer to provide a preliminary electromagnetic

anomaly map. This preliminary EM map is used, by the

r geophysicist, in conjunction with the computer generated

digital profiles, to produce the final interpreted EM anomaly

map. This map includes bedrock, surficial and cultural

conductors. A map containing only bedrock conductors can be

j generated, if desired.

Resistivity

r

The apparent resistivity in ohm-m may be generated from

the inphase and quadrature EM components for any of the

frequencies, using a pseudo-layer halfspace model. A

T resistivity map portrays all the EM information for that

frequency over the entire survey area. This contrasts with

l the electromagnetic anomaly map which provides information

only over interpreted conductors. The large dynamic range

U makes the resistivity parameter an excellent mapping tool.

DG

EM Magnetite

The apparent percent magnetite by weight is computed

Li wherever magnetite produces a negative inphase EM response.

The results are usually displayed on a contour map.U

- 3-3 -

Total Field Magnetics

1 The aeromagnetic data are corrected for diurnal

f variation using the magnetic base station data. The regional

IGRF gradient is removed from the data, if required under the

j terms of the contract.

i Enhanced Magnetics

The total field magnetic data are subjected to a

processing algorithm. This algorithm enhances the response

of magnetic bodies in the upper 500 m and attenuates the

response of deeper bodies. The resulting enhanced magnetic

map provides better definition and resolution of near-

surface magnetic units. It also identifies weak magnetic

j features which may not be evident on the total field

magnetic map. However, regional magnetic variations, and

J magnetic lows caused by remanence, are better defined on the

total field magnetic map. The technique is described in more

[j detail in Section 5.

DO

Magnetic Derivatives

The total field magnetic data may be subjected to a i j variety of filtering techniques to yield maps of the

O following)

- 3-4 -

vertical gradient

second vertical derivative

magnetic susceptibility with reduction to the pole

upward/downward continuations

All of these filtering techniques improve the

recognition of near-surface magnetic bodies, with the

exception of upward continuation. Any of these parameters

can be produced on request. Oighem's proprietary enhanced

magnetic technique is designed to provide a general

"all-purpose" map, combining the more useful features of the

above parameters.

VLF

j The VLF data can be digitally filtered to remove long

wavelengths such as those caused by variations in the

transmitted field strength. The results are usually

presented as contours of the filtered total field.

O Digital Profiles

li Distance-based profiles of the digitally recorded

geophysical data are generated and plotted by computer.

1.1 These profiles also contain the calculated parameters which

are used in the interpretation process. These are producedf]

- 3-5 -

as worksheets prior to interpretation, and can also be

presented in the final corrected form after interpretation.

1 The profiles display electromagnetic anomalies with their

f respective interpretive symbols. The differences between the

worksheets and the final corrected form occur only with

respect to the EM anomaly identifier.

l Contour f Colour and Shadow Map Displays

The geophysical data are interpolated onto a regular

grid using a cubic spline technique. The resulting grid isi .

suitable for generating contour maps of excellent quality.

i

Colour maps are produced by interpolating the grid down

to the pixel size. The distribution of the colour ranges is

f: normalized for the magnetic parameter colour maps, and

matched to specific contour intervals for the resistivity and

l VLF colour maps.

PLi Monochromatic shadow maps are generated by employing an

n artificial sun to cast shadows on a surface defined by the

geophysical grid. There are many variations in the shadowing

techniques. The various shadow techniques may be applied toi."J

total field or enhanced magnetic data, magnetic derivatives,

Li VLF, resistivity, etc. Of the various magnetic products, the

O

- 3-6 -

shadow of the enhanced magnetic parameter is particularly

suited for defining geological structures with crisper images

and improved resolution.

li

o

DM

- 4-1 -

SURVEY RESULTS

i !

GENERAL DISCUSSION

fTables 4-1 to 4-3 summarize the EM responses on the

electromagnetic anomaly maps with respect to conductance

grade and interpretation.

J The electromagnetic anomaly maps show the anomaly

locations with the interpreted conductor type, dip,

conductance and depth being indicated by symbols. Direct

magnetic correlation is also shown if it exists. Bedrock

conductors are indicated by the interpretive symbols "D"

J (for thin dikes) or "B" (for other conductor geometries).

Surficial conductors are identified by the interpretive

[ symbol "S". An "H" interpretive symbol is used to indicate a

r broad or flat-lying conductive unit that appears to be

t 1 situated at some depth below surface. This may be due to

n either bedrock or surficial sources. An anomaly due to theU

edge of a broad conductor is given an "E" designation. ThePj interpretive symbol "L" is used to indicate a line source

such as a power line, or other response due to culture.

i ] The anomalies shown on the electromagnetic anomaly maps

are based on a near-vertical, half plane model. This model

li best reflects "discrete" bedrock conductors. Wide bedrock

- 4-2 -

TABLE 4-1

EM ANOMALY STATISTICS

FOR THE SHEBANDOWAN AREA. BLOCK B. ONTARIO

CONDUCTOR CONDUCTANCE RANGE NUMBER OFGRADE SEIMENS (MHOS) RESPONSES

7 > 100.0 36 50.0 - 100.0 125 20.0 - 50.0 574 10.0 - 20.0 1103 5.0 - 10.0 1192 1.0 - 5.0 237l < 1.0 224* INDETERMINATE 373

TOTAL 1135

CONDUCTOR MOST LIKELY SOURCE NUMBER OFMODEL RESPONSES

D DISCRETE BEDROCK CONDUCTOR 358B DISCRETE BEDROCK CONDUCTOR 151

i S CONDUCTIVE COVER 593E EDGE OF WIDE CONDUCTOR lL CULTURE 32

TOTAL 1135

O13 l! l] O

(SEE EM MAP LEGEND FOR EXPLANATIONS)

- 4-3 -

TABLE 4-2

EM ANOMALY STATISTICS

FOR THE SHEBANDOWAN AREA. BLOCK C. ONTARIO

CONDUCTOR GRADE

7 6 5 4 3 2 l *

TOTAL

CONDUCTANCE RANGE SEIMENS (MHOS)

>50.0 -20.0 -10.0 -5.0 -1.0 -

<

100.0100.050.020.010.05.01.0

INDETERMINATE

NUMBER OF RESPONSES

OO

12 31 41 83 71

118

356

L

O

CONDUCTOR MODEL

D B S H E L

TOTAL

MOST LIKELY SOURCE

DISCRETE BEDROCK CONDUCTORDISCRETE BEDROCK CONDUCTORCONDUCTIVE COVERROCK UNIT OR THICK COVEREDGE OF WIDE CONDUCTORCULTURE

NUMBER OF RESPONSES

7129

2302l

23

356

O (SEE EM MAP LEGEND FOR EXPLANATIONS)

- 4-4 -

TABLE 4-3

EM ANOMALY STATISTICS

FOR THE SHEBANDOWAN AREA. BLOCK D. ONTARIO

CONDUCTOR GRADE

765432l*

TOTAL

CONDUCTANCE RANGE SEIMENS (MHOS)

>50.0 -20.0 -10.0 -5.0 -1.0 -

<

100.0100.050.020.010.05.01.0

INDETERMINATE

NUMBER OF RESPONSES

O l

155098

181166219

732

L

u

CONDUCTOR MODEL

D B S E L

TOTAL

MOST LIKELY SOURCE

DISCRETE BEDROCK CONDUCTORDISCRETE BEDROCK CONDUCTORCONDUCTIVE COVEREDGE OF WIDE CONDUCTORCULTURE

NUMBER OF RESPONSES

12166

3872

156

732

O(SEE EM MAP LEGEND FOR EXPLANATIONS)

l - 4-5 -

conductors or flat-lying conductive units, whether from

surficial or bedrock sources, may give rise to very broad

anomalous responses on the EM profiles. These may not

l appear on the electromagnetic anomaly maps if they have a

regional character rather than a locally anomalous character.

These broad conductors, which more closely approximate a half

space model, will be maximum coupled to the horizontal

(coplanar) coil-pair and should be more evident on the

j resistivity parameter. The resistivity maps, therefore, may

be more valuable than the electromagnetic anomaly maps, in

areas where broad or flat-lying conductors are considered to

be of importance. Contoured and colour resistivity maps,

prepared from the 7200 Hz coplanar data are included with

this report.

L Excellent resolution and discrimination of conductors

p was accomplished by using a fast sampling rate of 0.1 sec and

by employing a common frequency (900 Hz) on two orthogonal

H coil-pairs (coaxial and coplanar). The resulting "difference

channel" parameters often permit differentiation of bedrock

y and surficial conductors, even though they may exhibit

p similar conductance values. The inphase and quadrature

" difference channels are displayed on the digital profiles.

OZones of poor conductivity are indicated where the

r

- 4-6 -

r -'l

inphase responses are small relative to the quadrature

responses. Where these responses are coincident with strong

magnetic anomalies, it is possible that the inphase

j amplitudes have been suppressed by the effects of magnetite.

Most of these poorly-conductive magnetic features give rise

1 to resistivity anomalies which are only slightly below

I background. If it is expected that poorly-conductive

economic mineralization may be associated with magnetite-rich

j units, most of these weakly anomalous features will be of

interest. In areas where magnetite causes the inphase

components to become negative, the apparent conductance

values may be understated and the calculated depths of EN

anomalies may be erroneously shallow.

rResistivity

rt Apparent resistivity maps were prepared from the 7200 Hz

coplanar EM data. These maps show the conductive properties

H of the survey area.

[j Power lines in the survey area have severely affected the

p resistivity contours. A herringbone pattern is evident inJ iII the contours in blocks C and D. This is due to the different

[ l angles of ascent and descent, depending on survey line

direction, as the helicopter crosses the power line. This

y

- 4-7 -

effect is common near large, high-voltage power lines.

tThere are also gaps in the middle of the grid in which no

j resistivity data was calculated. This occurs where ground

effect has been lost due to the bird height required to cross

' a power line. These gaps occur in narrow fiducial ranges in

the following line ranges: 21400 to 21440, 21450 to 21490,

21570 to 21640, 21780 to 21790, 30410 to 30470 and 30570 to

[ 30620.

Some of the resistivity anomalies correlate with magnetic

trends. This suggests that they may reflect bedrock

features. For example, an arcuate low resistivity trend on

j sheet B-l, comprising anomalies 20500B to 20470F to 20530D,

correlates with a similarly shaped trend on the total field

l magnetic map.

l]" Many of the narrow low resistivity zones correlate with

j] interpreted bedrock conductors. Some of these, such as the

zone associated with conductors 30030D to 30260E are

[j coincident with lakes. Conductive lake-bottom sediment may

n be influencing the resistivities in these locations.

[j; Surficial features appear to yield resistivities as low J

as 50 ohm-m. For example, note the broad resistivity anomaly

Ol'

- 4-8 -

associated with Middle Shebandowan lake on sheet 3 at line

i 30410. It does not appear to be possible to differentiatei

between bedrock conductors and surficial conductors on the

[ basis of resistivity alone.

' Magnetics

The total field magnetic data have been presented as

contours on the base maps using a contour interval of 10 nT

where gradients permit. The maps show the magnetic

properties of the rock units underlying the survey area.

The total field information has been subjected to a

j processing procedure which calculates the vertical gradient.

This enhances near-surface magnetic units and removes therl j regional magnetic background. This procedure provides betterp definition and resolution of magnetic units, and also

* J displays weak magnetic features which may not be clearly

[l evident on the total field maps.

y There is ample evidence on the magnetic maps which

suggests that the survey area has been subjected to t u deformation and/or alteration. These structural complexities

J j are evident on the contour maps as variations in magneticl j

intensity, irregular patterns, and as offsets or changes in

O

- 4-9 -

strike direction.

The stratigraphic strike direction as inferred from the

magnetics gently curves from northeast/southwest on sheet B-

1-2 through east/west to northwest/southeast on sheet B-5-2.

Numerous possible structural breaks are apparent. The

predominant orientations of these breaks appear to be

northwest/southeast and northeast/southwest. In area D,

several approximately north/south trending, magnetic, dike-

like features are evident.

i

Throughout the survey area, many of the magnetic bodies i-'

are long, narrow, possibly stratiform units. Some folding of

these units is evident, particularly in the southern portion

of sheet B-2-2, where a large "S" shaped fold is apparent

[ between lines 20960 and 21220.

A large oval shaped feature in the northern half of the

[l survey area on sheet B-l-2, between lines 20460 and 20750,

yields relatively high magnetic responses. This correlates

[j with a syenite body mapped on the Shebandowan Geological

,, Compilation map, West Sheet, supplied by Noranda Exploration

'' Company Ltd. The eastern end of this unit is transected by a

l i north-northeast/south-southwest trending fault, which appearsLi

to extend from anomaly 20430F to 20910B.

O

- 4-10 -

A long, narrow, strongly magnetic unit is located

coincident with the northern boundary of this oval body. It

also appears to continue westward, and is continuous except

for a few locations where it may be offset by faulting. It

is conductive in several locations, correlating with

conductors 20150C-20220E, 20230F-20340C, 20420B-20470C,

20531B-20560B, 20600B-20630B, 20710A-20720B, 20600B-20630B

and 20710A-20720B.

East of the oval magnetic body, between lines 21020 and

21310, is a large circular feature. The magnetics

associated with this unit are less active than that of the

oval feature to the west. This feature correlates with a

granitic body on the geological map.

i;L

A strongly magnetic, lense shaped unit dominates the

western portion of sheet 4, between lines 40330 and 30570. A

well-defined, north-northwest/south-southeast trending

F structural break is evident near the western end of this unit

on sheet B-3-2. This apparent break appears to extend fromP[j the south end of line 30480 to the north end of line 30450.

" The magnetic data in the vicinity of the power line may

[j have been affected to a very minor degree. A subtle

herringbone is evident on the calculated vertical gradient,

Qn

- 4-11 -

which may have resulted from variations in bird height and

bird swing as the helicopter traversed the power line.

If a specific magnetic intensity can be assigned to the

, rock type which is believed to host the targetri

mineralization, it may be possible to select areas of higher

j priority on the basis of the total field magnetic maps. This

is based on the assumption that the magnetite content of the

[ host rocks will give rise to a limited range of contour

values which will permit differentiation of various

l lithological units.

The magnetic results, in conjunction with the other

p geophysical parameters, should provide valuable information

which can be used to effectively map the geology and

j structure in the survey areas.

VLF

CVLF results were obtained from the transmitting stations

Vli at Cutler, Maine (NAA - 24.0), and Annapolis, Maryland (NSS-21.4). Data from the Annapolis station were presented as

hlj contours of the filtered total field for blocks B and C, and

D data from the Cutler station were presented for block D. Adequate signals were not available during the flying of

lili;

h - 4-12 -

lines 20380 through 20561 on block B.

iThe VLF method is quite sensitive to the angle of

l coupling between the conductor and the propogated EM field.

r Consequently, conductors which strike towards the VLFi.! station will usually yield a stronger response than

conductors which are nearly orthogonal to it.

l Some of the VLF trends parallel magnetic features. These

may reflect conductive material associated with lithological

contacts or faulted contacts. There are some trends which

transect the stratigraphic strike direction as Inferred from

the magnetics. These are indicative of conductive material

associated with structural breaks. Other structural breaks

may be inferred where the VLF contours are offset or

li truncated.

rSome of the possible and discrete bedrock conductors

H yield well-defined trends on the VLF. Therefore, the VLF may

be useful as a ground follow-up tool. The filtered VLF will

IT j also show trends due to the edges of flat-lying conductive

sources, such as lacustrine clays.[jThe VLF contours have been affected by cultural sources.

Power lines and roads in areas C, D and the eastern half of

- 4-13 -

area B yield strong, narrow VLF trends.

iThe VLF parameter does not normally provide the same

i degree of resolution available from the EM data. Closely-

spaced conductors, conductors of short strike length or

conductors which are poorly coupled to the VLF field, may

l escape detection with this method. Erratic signals from the

VLF transmitters can also give rise to strong, isolated

anomalies which should be viewed with caution. The filtered

total field VLF contours are presented on the base maps with

l a contour interval of one percent.

l CONDUCTOR DESCRIPTIONS

r It is beyond the scope of this report to provide a

detailed interpretation of all the conductors within the

l; survey area. The Conductor Descriptions section deals with

some of the most interesting geophysical targets that occur

li within the survey area. It also mentions some of the

n structural and formational conductors which nay be important

as an aid for geological mapping. The anomaly lists appended

to this report should be consulted to ensure that no

anomalies attributed to bedrock sources are overlooked. AllnU bedrock anomalies can be considered potential targets forOB

further investigation,

- 4-14 -

Sheet #1-.

Conductors 20010B-20030A, 20010C-20040A, 20090B-20100B,

l 20140B-20150B, 20140D-20150D, 20160D-20181F,

l 20170B-20260F, 20181D, 20181E-20190D, 202206-

202306, 20240E-20290D, 20280B-20290B, 20340B-

1 20410A, 20350C-20700A, 20680A-20690Al\

l. These conductors are associated with a zone of active

magnetics, which occupies the northern third of the

survey area on sheet B-l. The conductors reflect narrow

i bedrock sources, most of which appear to dip to the

north. Some appear to be magnetic while others are non

magnetic. Those that are magnetic may reflect

pyrrhotite-rich sources or conductive material associated

L with magnetite, while those that are not may reflect

r? graphite-rich or non-magnetic sulphide-rich sources.

Some of the shorter strike length conductors (one or two

P line responses) such as 20280B-20290B may be more

attractive as exploration targets than the longer

I 'i l structural or formational conductors, except where thesel i appear to be altered. This conductor also yields

Imagnetic correlation and appears to be strongly

I j conductive. Conductor 20090B-20100B is a short, weak,

conductor which loosely correlates with a small limb or

[j fold of magnetic material.

- 4-15 -

Conductor 20350C-20700A appears to correlate with the

Obadinaw fault which is mapped on the Shebandowan

Geological Compilation, West Sheet, supplied by Noranda

Exploration Company Limited.

Most of these conductors yield well-defined

resistivity anomalies and some correlate with VLF trends.

Conductors 20260C-20330B, 20360A-20380A

These reflect narrow, weakly conductive, north-

dipping bedrock sources, which occur near the north

survey boundary. They may reflect conductive material

associated with a contact or faulted contact.

D G

O

O

Conductors 20130C-20220E, 20230F-20340C, 20410D, 20420B-

20470C, 20531B-20560B, 20600B-20630B, 20710A-

20720B, 20710A-20720B (sheet 12)

These conductors are directly associated with a long,

semi-continuous, strongly magnetic unit. This unit

generally strikes northeast/southwest except in the

vicinity of conductor 20420B-20470C, where it strikes

almost north/south. In this location, the unit appears

to fold so that it parallels the boundaries of a large

- 4-16 -

oval shaped magnetic unit (this unit is discussed in the

j Magnetics section of the report). This long, narrow,

conductive, magnetic unit yields negative responses on

l the inphase EM, which indicate the presence of

magnetite. In the vicinity of magnetite responses, the

1 resistivities may be overstated.

F:Remanent magnetization is the likely source of the

l strong magnetic low that is coincident with anomaly

20310E.

j Conductor 20020F-20090E, 20020G-20040F, 200306-200406,

20070B-20130H, 20080E-20130F, 201306-201506,

i 20150F, 20160E-20200H, 20260I-20300E, 20360E-

20380E, 20430E-20520D, 20541A-20550D

lj These conductors occur along strike with each other.

" Host occur on the north flank of a narrow magnetic unit,

H and may be associated with a contact or faulted contact.

Conductors 202006-20040F and 20030G-200406 appear to

[j directly correlate with the narrow magnetic unit, in a

p location where this unit is possibly tightly folded.

' ' Magnetite responses are also evident in this location.

UO f!

- 4-17 -

Conductors 20060D, 20060E, 20060Ff-,

These conductors are indicative of magnetic bedrock

f sources. Although anomalies 20060E and 20060F have been

- interpreted as two thin conductors, it is possible thati1 the response here may reflect a thick source (greater

than 10 m thickness). The high calculated conductances

and magnetic correlations are indicative of a pyrrhotite-

j rich source. The magnetics suggest a structural break or

tight fold in this vicinity. Further investigation of

this source is likely warranted.

Conductors 20190I-20220L, 20210G-20240H, 20210H-20230O,

[5 20220K-20230M, 20220J-20360H, 20220I-20240K,

20220H-20280G, 20300G-20400D

[iIT These conductors comprise a "J" shaped low

resistivity zone. Most of the conductors appear to be

R non-magnetic. They generally closely flank narrow

magnetic highs. The conductive material may be

[j associated with contact zones.

*-' Conductor 20430F-20480H

irThis conductor is indicative of a magnetic bedrock

u

- 4-18 -

source. It appears to be most magnetic and most

j conductive in the vicinity of anomaly 20440. Pyrrhotite-i

rich mineralization is a likely source.

lConductor 20760F-20790E

f This conductor reflects a moderately conductive

bedrock source. It correlates with a magnetic unit,

j which is evident on the calculated vertical gradient map.

Anomaly 207806 is a typical thick, massive sulphide-style

response.

i' ';

Conductors 20070E-20100H, 20460I-20490H, 20590D-20610C,

p 20610D-20680F, 20620F-20640D, 207606-207706

[. These conductors reflect narrow, discrete bedrock

j, sources. Conductor 20070E-20100H possibly has direct

'' magnetic correlation in the vicinity of anomalies 20090K-

H 20100H, however, most of these conductors appear to flank

magnetic units. They may be associated with contact

[j zones.

I i •* Sheet *2LI

Most of the area of overlap of sheets #1 and 12 has been

- 4-19 -

discussed under the Sheet #1 heading.t

rConductors 20720A-20730A, 20750A-20840Br

These conductors reflect narrow, north-dipping,

moderately conductive bedrock sources. They flank a

linear, narrow magnetic unit and are likely associated

with a contact zone.

l-Conductor 20760H-208206

This conductor appears to change in composition along

strike as some parts appear to be magnetic while others

[ M

are non-magnetic. Anomaly 20810F, and possibly 20800D,

reflect thick, magnetic, bedrock sources. The calculated

I vertical gradient map reveals important details which are

r not evident on the total field map, about the

magnetic/conduc t ive relationships in the vicinity of

[l these anomalies.

P[j Conductor 20910A-20921A

\- J This narrow, north-dipping, bedrock source has a

j; strike length of less than 400 m. Anomaly 20910A is l J

associated with an isolated magnetic low which may result

- 4-20 -

from remanent magnetization. Anomaly 20921A correlates

with a magnetic high.

Conductor 20931B-21010G

This conductor correlates with a well-defined

I magnetic low. It reflects a weakly conductive, non

magnetic, north-dipping, dike-like source.

liConductors 20981A-21180A, 20991C-21210B, 21150C-21180C

rThese conductors yield a narrow, arcuate, low

resistivity trend. This in part correlates with the

northern contact of the large circular body, which is

mapped as granite on the Shebandowan Geological

L Compilation. Conductor 20991C-21210B may change in

r composition from magnetic to non-magnetic along strike.

Conductor 20981A-21180A is non-magnetic. It appears to

F] become thicker in the vicinity of anomaly 21060B.

D Conductor 21010E-21030E

^ This conductor reflects a narrow, weakly conductive,

H bedrock source. The source may be magnetic in the

vicinity of anomaly 21020D. It occurs in an area with

D

- 4-21 -

complex magnetic contour patterns near the edge of the

aforementioned circular feature.

j Conductors 20991A-21030A, 21080A-21120A, 21230A-21250A,

r 21290A-21330A, 21330B-21340B, 21420B-21480B,

' 21490A-21510A

l;These conductors occur in a zone of relatively

j inactive magnetics, which is located coincident with the

northern survey boundary across most of sheet B-2. the

conductors reflect narrow, non-magnetic bedrock zones.

Conductors 20810E-20850G, 20910E-20950C, 20981F-21080F,

l 21020F, 21040E, 21050F, 210501, 21150F-21170G,

21370F-21420Di:*i These conductors occur in areas of active magneticsr

near the southern survey boundary. Most of the

D conductors are non-magnetic but flank magnetic units. ,They may reflect graphite-rich or non-magnetic sulphide-

Py rich material associated with contacts.

Anomalies 21150E, 21161F and 21180E yield bedrock

] style anomaly shapes, and discrete low resistivity

anomalies. However, they correlate with an area labelled

O f!

- 4-22 -

uli U

"Mine Waste" on the map. These anomalies have been

labelled B? and S? as it is not possible to rule out

culture as a possible source.

Sheet 13

f Conductor 21510C-21570A

l This conductor is indicative of a narrow, weakly

conductive, non-magnetic bedrock unit. This unit is

evident as a distinct low on the total field magnetic

map. It yields well-defined resistivity and VLF

anomalies.

Conductor 21590F-21710D, 30010A-30060B

t;

O The calculated vertical gradient magnetic contours

indicate that these conductors may occur along the same

H stratigraphic zone. These weakly conductive, narrow,

bedrock sources flank strong magnetic responses. They ri [j likely reflect conductive material associated with a

i f contact zone.

- 4-23 -

Conductors 21700A-21750B, 21720A-21780A, 21770C

These conductors comprise a non-magnetic, low

L resistivity zone on the north flank of a narrow magnetic

high. Although the conductivity correlates with a lake,r

1 the profile shapes are indicative of thin bedrock

i sources.

J Conductors 30020C-30060D, 30030D-30260E, 30070D-30110C,

30220E-30240D

Although these conductors are located coincident with

a long, narrow lake, the profile shapes indicate narrow,

dike-like sources. The conductors parallel a continuous

magnetic unit. Some are located coincident with the lowPl on the north flank of this high. Others, such as

ri anomalies 30140C and 30150B correlate with a narrow

magnetic high, which is apparent on the calculated

H vertical gradient map.

ny Responses due to this conductor were not detected on

p some lines near the northwest end of the conductor. The j

I j conductor axis has been extrapolated through these lines.

II It is possible that the conductor continues further to

the west. Excessive EM bird height was needed in this

O

- 4-24 -

area to clear high-voltage power lines. Coupling with

' the conductor was not maintained as a result on sometlines.

Conductors 30050G-30090F, 30050F-30060Ft i

These conductors flank a narrow, linear magnetic

high, which extends from line 30050 to 30100. The

j conductors are indicative of narrow, non-magnetic sources

which may be associated with a contact zone.

Sheet *4

f Conductors 30600G-40040G, 30680E-40040F, 30600F-30700D,

30600E-40110E, 30670D-40250H, 30620C-40220E,

L 40050E-40070G, 40290I-40351G, 40640J-40710D,

040640I-40680F

H These long conductive zones correlate with non

magnetic rock units. Host are weakly conductive, withPL the strongest calculated conductances occurring in the

, vicinity of lines 30680 to 40100. These conductors may

tj reflect narrow zones of non-magnetic sulphide-rich or

1 graphite-rich material. On strike, and in between

conductors 40290I-40351G and 40640J-40710D, are several

O

i" ~ - 4-25 -

shorter, non-magnetic, interpreted bedrock conductors and

numerous broad, surficial type responses. The

resistivity and magnetic contours suggest that these

i responses may be associated with the same type of source.

The VLF contours flank some of the surficial conductors.

This is not unusual, as the filtered VLF tends to show

the edges of broad conductive units.

l Conductors 40620C-40650C, 40650D-40660E, 40650B-40660D

1 These conductors may reflect narrow, weakly

: conductive bedrock sources. They do not directlyr-

correlate with any features on the magnetic maps, and are

l. likely non-magnetic. There are numerous cultural objects

in this area, but no direct correlation between the

l 4 anomalies and culture could be established by checking

O the flight videos.

R Sheet IS

n[ i Conductors 40821A-40850B, 40830A-40520A, 40880B-40930A,- 40960A-41000Asi

' With the exception of conductor 40821A-40850B, all of

these conductors occur on the north flank of a strongly

O

- 4-26 -

f -i

magnetic unit, conductor 40821A-40850B is located on the

south flank of a narrow magnetic trend. A likely source

for these conductors is weakly conductive, non-magnetic

I ' material associated with contact zones.r

1 Conductors 41010A-41020A, 41020B-41320B, 41020C-41070B,

j 41030C, 41150B-41200C, 41240B-41280B

f These conductors likely reflect non-magnetic,

conductive material associated with a contact or faulted

I contact. This contact is evident on the magnetic

parameter maps. A fault is also indicated near this

location on the Shebandowan Geological Compilation, West

Sheet.I i

l: Conductors 40980B-41020E, 41020D

PII Conductor 40980B-41020E correlates with a narrow,t l linear magnetic high. There is no evidence of a J

magnetite response on the profiles. Pyrrhotite-richW(j mineralization is a possible source. The magnetics

p indicate a fold or possible northeast/southwest trendingi

IJ fault in the vicinity of anomaly 40980B.

liConductor 4102OD is indicative of an isolated, thin,

O

- 4-27 -

non-magnetic source, which may be associated with the

contact zone at the edge of the conductive, magnetic

unit.

Although they have not been discussed in this report,

some of the.B? and S? anomalies may be of interest. They may

result from bedrock sources that are partially masked by

surficial conductivity. Isolated bedrock conductors, which

occur off to one side of a flight line, or conductors without

approximate thin-dike geometry may also be interpreted as

questionable (B? or S?). These anomalies will likely warrant

further investigation if they have supporting geological,

geochemical or geophysical information.

c

- 5-1 -

BACKGROUND INFORMATION

This section provides background information on

parameters which are available from the survey data. Those

which have not been supplied as survey products may be

generated later from raw data on the digital archive tape.

IRfrRfiTROMAGNETICS

.1

f DIGHEM electromagnetic responses fall into two general

classes, discrete and broad. The discrete class consists of

sharp, well-defined anomalies from discrete conductors such

as sulfide lenses and steeply dipping sheets of graphite and

l sulfides. The broad class consists of wide anomalies from

i conductors having a large horizontal surface such as flatly

dipping graphite or sulfide sheets, saline water-saturated

p sedimentary formations, conductive overburden and rock, and

geothermal zones. A vertical conductive slab with a width of

200 m would straddle these two classes.

*-'* The vertical sheet (half plane) is the most common model

p used for the analysis of discrete conductors. All anomalies Li

plotted on the electromagnetic map are analyzed according to

[! this model. The following section entitled Discrete

, -, Conductor Analysis describes this model in detail, including

l!

- 5-2 -

j the effect of using it on anomalies caused by broad

i/ conductors such as conductive overburden.

f The conductive earth (half space) model is suitable for

broad conductors. Resistivity contour maps result from the

use of this model. A later section entitled Resistivity

Mapping describes the method further, including the effect of

i using it on anomalies caused by discrete conductors such as

r sulfide bodies.

Geometric interpretation

The geophysical interpreter attempts to determine the

geometric shape and dip of the conductor. Figure 5-1 shows

i typical DIGHEM anomaly shapes which are used to guide the

geometric interpretation.

i:f . Discrete conductor analysis

The EM anomalies appearing on the electromagnetic mapP[j are analyzed by computer to give the conductance (i.e.,

pi conductivity-thickness product) in Siemens (mhos) of a

vertical sheet model. This is done regardless of the

j] interpreted geometric shape of the conductor. This is not an

unreasonable procedure, because the computed conductance l j [j increases as the electrical quality of the conductor

increases, regardless of its true shape. DIGHEM anomaliesOf]

en! m

Conductor location

Channel CXI A A AChannel CPI S M \

Channel DIFI **J \s* ^J V" ^ \f

Conductor - \

line vertical dipping thin dike thin dike

Ratio ofamplitudesCXI /CPI 4/1 2/1 variable

v y - \j~-

D 0 ""vertical or sphere; wide S * dipping horizontal horizontal H *thick dike disk; ribbon;

metal roof; large fencedsmall fenced area E *yard

variable 1/4 variable

————————— — ^s ——— -^

1 r s

conductive overburden flight line thick conductive cover parallel toor wide conductive rock conductorunitedge effect from wideconductor

1/2 0/4

Fig. 5-1 Typical DIGHEM anomaly shapes

- 5-4 -

are divided into seven grades of conductance, as shown in

Table 5-1 below. The conductance in Siemens (mhos) is the

reciprocal of resistance in ohms.

Table 5-1. EM Anomaly Grades

Anomaly Grade7654321

Siemens>

50 -20 -10 -5 -1 -

<

10010050201051

B

The conductance value is a geological parameter because

it is a characteristic of the conductor alone. It generally

is independent of frequency, flying height or depth of

burial, apart from the averaging over a greater portion of

the conductor as height increases. Small anomalies from

deeply buried strong conductors are not confused with small

anomalies from shallow weak conductors because the former

will have larger conductance values.

l! [j

Conductive overburden generally produces broad EM

responses which may not be shown as anomalies on the EM maps.

However, patchy conductive overburden in otherwise resistive

areas can yield discrete anomalies with a conductance grade

(of. Table 5-1) of l, 2 or even 3 for conducting clays which

- 5-5 -

have resistivities as low as 50 ohm-m. In areas where ground

i resistivities are below 10 ohm-m, anomalies caused by

weathering variations and similar causes can have any

j conductance grade. The anomaly shapes from the multiple

coils often allow such conductors to be recognized, and these

are indicated by the letters S, H, and sometimes E on the

electromagnetic anomaly map (see EM map legend).

r For bedrock conductors, the higher anomaly grades

indicate increasingly higher conductances. Examples!

f DIGHEM's New Insco copper discovery (Noranda, Canada) yielded

a grade 5 anomaly, as did the neighbouring copper-zinc MagusiF

; River ore body; Mattabi (copper-zinc, Sturgeon Lake, Canada)

and Whistle (nickel, Sudbury, Canada) gave grade 6; and

I DIGHEM's Montcalm nickel-copper discovery (Tinunins, Canada)

r yielded a grade 7 anomaly. Graphite and sulfides can span

all grades but, in any particular survey area, field work may

J show that the different grades indicate different types of

conductors.

Op Strong conductors (i.e., grades 6 and 7) are charac~U teristic of massive sulfides or graphite. ModerateF 'T

l conductors (grades 4 and 5) typically reflect graphite or

sulfides of a less massive character, while weak bedrock

[J conductors (grades l to 3) can signify poorly connected

graphite or heavily disseminated sulfides. Grades l and 2O

- 5-6 -

conductors may not respond to ground EM equipment using

frequencies less than 2000 Hz.

f The presence of sphalerite or gangue can result in ore

deposits having weak to moderate conductances. As ant: example, the three million ton lead-zinc deposit of

Restigouche Mining Corporation near Bathurst, Canada, yielded

a well-defined grade 2 conductor. The 10 percent by volume

r of sphalerite occurs as a coating around the fine grained

massive pyrite, thereby inhibiting electrical conduction.

l.Faults, fractures and shear zones may produce anomalies

iwhich typically have low conductances (e.g., grades l to 3).

Conductive rock formations can yield anomalies of any

conductance grade. The conductive materials in such rock

j ' formations can be salt water, weathered products such as

clays, original depositional clays, and carbonaceous

material.

nU On the interpreted electromagnetic map, a letterM identifier and an interpretive symbol are plotted beside the

EM grade symbol. The horizontal rows of dots, under the

[i interpretive symbol, indicate the anomaly amplitude on thel i

flight record. The vertical column of dots, under thej j[j anomaly letter, gives the estimated depth. In areas where

i- anomalies are crowded, the letter identifiers, interpretive

- 5-7 -

symbols and dots may be obliterated. The EM grade symbols,

i however, will always be discernible, and the obliterated

information can be obtained from the anomaly listing appended

f r to this report.

j The purpose of indicating the anomaly amplitude by dots

is to provide an estimate of the reliability of the

conductance calculation. Thus, a conductance value obtained

t from a large ppm anomaly (3 or 4 dots) will tend to be

accurate whereas one obtained from a small ppm anomaly (no

j dots) could be quite inaccurate. The absence of amplitude

dots Indicates that the anomaly from the coaxial coil-pair is

5 ppm or less on both the inphase and quadrature channels.

Such small anomalies could reflect a weak conductor at the

surface or a stronger conductor at depth. The conductance

j grade and depth estimate illustrates which of these

possibilities fits the recorded data best.

CFlight line deviations occasionally yield cases where

y two anomalies, having similar conductance values but

D dramatically different depth estimates, occur close together on the same conductor. Such examples illustrate thej ^ reliability of the conductance measurement while showing that li

the depth estimate can be unreliable. There are a number of

[j factors which can produce an error in the depth estimate,

including the averaging of topographic variations by theO

- 5-8 -

altimeter, overlying conductive overburden, and the location

and attitude of the conductor relative to the flight line.

Conductor location and attitude can provide an erroneous

depth estimate because the stronger part of the conductor may

be deeper or to one side of the flight line, or because it

has a shallow dip. A heavy tree cover can also produce

errors in depth estimates. This is because the depth

estimate is computed as the distance of bird from conductor,

minus the altimeter reading. The altimeter can lock onto the

top of a dense forest canopy. This situation yields an

erroneously large depth estimate but does not affect the

conductance estimate.

Dip symbols are used to indicate the direction of dip of

conductors. These symbols are used only when the anomaly

shapes are unambiguous, which usually requires a fairly

resistive environment.

A further interpretation is presented on the EM map by

U means of the line-to-line correlation of anomalies, which is

n based on a comparison of anomaly shapes on adjacent lines. This provides conductor axes which may define the geological

structure over portions of the survey area. The absence of

conductor axes in an area implies that anomalies could not be

correlated from line to line with reasonable confidence.

O

- 5-9 -

4 -

DIGHEM electromagnetic maps are designed to provide a

correct impression of conductor quality by means of the

conductance grade symbols. The symbols can stand alone with

f geology when planning a follow-up program. The actual

conductance values are printed in the attached anomaly list

' " for those who wish quantitative data. The anomaly ppm and

depth are indicated by inconspicuous dots which should not

distract from the conductor patterns, while being helpful to

i those who wish this information. The map provides an

interpretation of conductors in terms of length, strike and

dip, geometric shape, conductance, depth, and thickness. The

accuracy is comparable to an interpretation from a high

quality ground EM survey having the same line spacing.

l The attached EM anomaly list provides a tabulation of

r anomalies in ppm, conductance, and depth for the vertical

sheet model. The EM anomaly list also shows the conductance

h and depth for a thin horizontal sheet (whole plane) model,

but only the vertical sheet parameters appear on the EM map.

[J The horizontal sheet model is suitable for a flatly dipping

r i thin bedrock conductor such as a sulfide sheet having a

thickness less than 10 m. The list also shows the

; resistivity and depth for a conductive earth (half space)..l

model, which is suitable for thicker slabs such as thick

o[i

conductive overburden. In the EM anomaly list, a depth value

of zero for the conductive earth model, in an area of thick

- 5-10 -

cover, warns that the anomaly may be caused by conductive

overburden .

Since discrete bodies normally are the targets of EM

surveys, local base (or zero) levels are used to compute

local anomaly amplitudes. This contrasts with the use of

true zero levels which are used to compute true EM

amplitudes. Local anomaly amplitudes are shown in the EM

anomaly list and these are used to compute the vertical sheet

parameters of conductance and depth. Not shown in the EM

anomaly list are the true amplitudes which are used to

compute the horizontal sheet and conductive earth parameters.

Questionable Anomalies

r DIGHEM maps may contain EM responses which are displayed

as asterisks { * ) . These responses denote weak anomalies of

y indeterminate conductance, which may reflect one of the following! a weak conductor near the surface, a strong

Li conductor at depth (e.g., 100 to 120 m below surface) or to

p one side of the flight line, or aerodynamic noise* Those

responses that have the appearance of valid bedrock anomalies

M on the flight profiles are indicated by appropriate

interpretive symbols (see EM map legend). The others

L probably do not warrant further investigation unless their

[:

O locations are of considerable geological interest.

- 5-11 -

r The thickness parameter

DIGHEM can provide an indication of the thickness of a

l steeply dipping conductor. The amplitude of the coplanar

i anomaly (e.g., CPI channel on the digital profile) increases

relative to the coaxial anomaly (e.g., CXI) as the apparent

j thickness increases, i.e., the thickness in the horizontal

plane. (The thickness is equal to the conductor width if the

j conductor dips at 90 degrees and strikes at right angles to

the flight line.) This report refers to a conductor as thin

l when the thickness is likely to be less than 3 m, and thick

j when in excess of 10 m. Thick conductors are indicated on

the EM map by parentheses "( )". For base metal exploration

j in steeply dipping geology, thick conductors can be high

priority targets because many massive sulfide ore bodies are

[ thick, whereas non-economic bedrock conductors are often

., thin. The system cannot sense the thickness when the strike

^ of the conductor is subparallel to the flight line, when the

n conductor has a shallow dip/ when the anomaly amplitudes are

small, or when the resistivity of the environment is below

[j 100 ohm-m.

o

u r

Resistivity mapping

Areas of widespread conductivity are commonly

- 5-12 -

encountered during surveys. In such areas, anomalies can be

r generated by decreases of only 5 m in survey altitude as well

as by increases in conductivity. The typical flight record

f. in conductive areas is characterized by inphase and

quadrature channels which are continuously active. Local EM

peaks reflect either increases in conductivity of the earth

or decreases in survey altitude. For such conductive areas,

l apparent resistivity profiles and contour maps are necessary

r for the correct interpretation of the airborne data. The

advantage of the resistivity parameter is that anomalies

j caused by altitude changes are virtually eliminated, so the

resistivity data reflect only those anomalies caused by

conductivity changes. The resistivity analysis also helps

P the interpreter to differentiate between conductive trends in

' the bedrock and those patterns typical of conductive

r overburden. For example, discrete conductors will generally

appear as narrow lows on the contour map and broad conductors

(e.g., overburden) will appear as wide lows.

OD

The resistivity profiles and the resistivity contour

maps present the apparent resistivity using the so-called

pseudo-layer (or buried) half space model defined by Fraser

(1978) 1 . This model consists of a resistive layer overlying

1 Resistivity mapping with an airborne multicoil electromagnetic systems Geophysics, v. 43, p.144-172

[j

- 5-13 -

a conductive half space. The depth channels give the

apparent depth below surface of the conductive material. The

apparent depth is simply the apparent thickness of the

f overlying resistive layer. The apparent depth (or thickness)

parameter will be positive when the upper layer is more

resistive than the underlying material, in which case the

apparent depth may be quite close to the true depth.

l The apparent depth will be negative when the upper layer

is more conductive than the underlying material, and will be

zero when a homogeneous half space exists. The apparent

depth parameter must be interpreted cautiously because itrl will contain any errors which may exist in the measured

.,, altitude of the EM bird (e.g., as caused by a dense tree^

1 cover). The inputs to the resistivity algorithm are the

f" inphase and qaudrature components of the coplanar coil-pair.

The outputs are the apparent resistivity of the conductive

f. half space (the source) and the sensor-source distance are

independent of the flying height. The apparent depth,

lj discussed above, is simply the sensor-source distance minus

H the measured altitude or flying height. Consequently, errors t 'iin the measured altitude will affect the apparent depth

j parameter but not the apparent resistivity parameter.

uThe apparent depth parameter is a useful indicator of

simple layering in areas lacking a heavy tree cover. The

- 5-14 -

riDIGHEM system has been flown for purposes of permafrost

mapping, where positive apparent depths were used as a

measure of permafrost thickness. However, little

j ' quantitative use has been made of negative apparent depths

because the absolute value of the negative depth is not a

[ measure of the thickness of the conductive upper layer and,

therefore, is not meaningful physically. Qualitatively, a

i- negative apparent depth estimate usually shows that the EM

f anomaly is caused by conductive overburden. Consequently,

the apparent depth channel can be of significant help in

distinguishing between overburden and bedrock conductors.

[' The resistivity map often yields more useful information

r;, on conductivity distributions than the EM map. In comparing

l' the EM and resistivity maps, keep in mind the following!

c(a) The resistivity map portrays the absolute value

of the earth's resistivity, where resistivity -

I/conductivity .

n (b) The EM map portrays anomalies in the earth's

resistivity. An anomaly by definition is 'a

l ; change front the norm and so the EM map displaysi. ianomalies, (i) over narrow, conductive bodies

l j and (ii) over the boundary zone between two wide

oo

formations of differing conductivity.

- 5-15 -r

The resistivity map might be likened to a total field

map and the EM map to a horizontal gradient in the direction

F of flight2 . Because gradient maps are usually more sensitive

than total field maps, the EM map therefore is to be

preferred in resistive areas. However, in conductive areas,

the absolute character of the resistivity map usually causes

it to be more useful than the EM map.

t;Interpretation in conductive environments

Environments having background resistivities below 30

ohm-m cause all airborne EM systems to yield very large

responses from the conductive ground. This usually prohibits

the recognition of discrete bedrock conductors. However,

l DIGHEM data processing techniques produce three parametersl !

which contribute significantly to the recognition of bedrock

conductors. These are the inphase and quadrature difference

channels (DIFI and DIFQ), and the resistivity and depth

M channels (RES and DP) for each coplanar frequency.

OThe EM difference channels (OIFI and DIFQ) eliminate

l j most of the responses from conductive ground, leavingl-*

0 2 The gradient analogy is only valid with regard to the identification of anomalous locations.

- 5-16 -

responses from bedrock conductors, cultural features (e.g.,

telephone lines, fences, etc.) and edge effects. Edge

effects often occur near the perimeter of broad conductive

[ zones. This can be a source of geologic noise. While edge

effects yield anomalies on the EM difference channels, they!-

| do not produce resistivity anomalies. Consequently, the

resistivity channel aids in eliminating anomalies due to edge

effects. On the other hand, resistivity anomalies will

l coincide with the most highly conductive sections of

conductive ground, and this is another source of geologic

noise. The recognition of a bedrock conductor in a

conductive environment therefore is based on the anomaloust

l"- responses of the two difference channels (DIFI and DIFQ) and

, the resistivity channels (RES). The most favourable

' situation is where anomalies coincide on all channels.

IIThe DP channels, which give the apparent depth to the

y conductive material, also help to determine whether a

conductive response arises from surficial material or from a

Li conductive zone in the bedrock. When these channels ride

H above the zero level on the digital profiles (i.e., depth is

negative), it implies that the EM and resistivity profiles

11 } are responding primarily to a conductive upper layer, i.e.,

*4conductive overburden. If the DP channels are below the

[iL j zero level, it indicates that a resistive upper layer exists,

n and this usually implies the existence of a bedrock

- 5-17 - ~~\

conductor. If the low frequency DP channel is below the zero

level and the high frequency DP is above, this suggests that

a bedrock conductor occurs beneath conductive cover.

i:The conductance channel CDT identifies discreter"

; conductors which have been selected by computer for appraisal

by the geophysicist. Some of these automatically selected

anomalies on channel CDT are discarded by the geophysicist.

r The automatic selection algorithm is intentionally

oversensitive to assure that no meaningful responses are

j missed. The interpreter then classifies the anomalies

according to their source and eliminates those that are not

substantiated by the data, such as those arising from

geologic or aerodynamic noise.

r 1.i Reduction of geologic noisel t l-JT-T—— — — —T — T.—.— .. JIT — -r. i— - TT ir— ~ . -.-T r nrjnLWJi:

[j Geologic noise refers to unwanted geophysical responses.

For purposes of airborne EM surveying, geologic noise refers

U to EM responses caused by conductive overburden and magnetic

M permeability. It was mentioned previously that the EM

difference channels (i.e., channel DIFI for inphase and OIFQ

f, for quadrature) tend to eliminate the response of conductive

overburden. This marked a unique development in airborne EMn[j technology, as DI6HEM is the only EM system which yieldsO channels having an exceptionally high degree of immunity to

- 5-18 -

i

conductive overburden.

Magnetite produces a form of geological noise on the

l inphase channels of all EM systems. Rocks containing less

than l * magnetite can yield negative inphase anomalies caused

by magnetic permeability. When magnetite is widely

distributed throughout a survey area, the inphase EM channels

l may continuously rise and fall, reflecting variations in the

l magnetite percentage, flying height, and overburden

thickness. This can lead to difficulties in recognizing

[ deeply buried bedrock conductors, particularly if conductive

overburden also exists. However, the response of broadly

j- distributed magnetite generally vanishes on the inphase

difference channel OIFI. This feature can be a significant

I aid in the recognition of conductors which occur in rocks

j' containing accessory magnetite.

[3EM mactnetite mapping

l J The information content of DI6HEN data consists of a

D combination of conductive eddy current responses and magneticpermeability responses. The secondary field resulting from. conductive eddy current flow is frequency-dependent and

consists of both inphase and quadrature components, which are

Li positive in sign. On the other hand, the secondary field

n resulting from magnetic permeability is independent of

1 - 5-19 -

frequency and consists of only an inphase component which is

negative in sign. When magnetic permeability manifests

itself by decreasing the measured amount of positive inphase,

j its presence may be difficult to recognize. However, when it

manifests itself by yielding a negative inphase anomaly

(e.g., in the absence of eddy current flow), its presence is

assured. In this latter case, the negative component can be

l used to estimate the percent magnetite content.

A magnetite mapping technique was developed for the

i coplanar coil-pair of DIGHEM. The technique yields a channel

(designated FEO) which displays apparent weight percentr '

r magnetite according to a homogeneous half space model.3 The

method can be complementary to magnetometer mapping in

1. certain cases. Compared to magnetometry, it is far less

j- sensitive but is more able to resolve closely spaced

magnetite zones, as well as providing an estimate of the

f amount of magnetite in the rock. The method is sensitive to

1/4* magnetite by weight when the EM sensor is at a height of p [j 30 m above a magnetitic half space. It can individually

,-t resolve steep dipping narrow magnetite-rich bands which are

'-' separated by 60 m. Unlike magnetometry, the EM magnetite

f j method is unaffected by remanent magnetism or magnetic

i] 3 Refer to Fraser, 1981, Magnetite mapping with a [! multi-coil airborne electromagnetic systems

Geophysics, v. 46, p. 1579-1594.

f; i;

- 5-20 -

latitude.

The EM magnetite mapping technique provides estimates of

magnetite content which are usually correct within a factor

of 2 when the magnetite is fairly uniformly distributed. EM

magnetite maps can be generated when magnetic permeability is

evident as negative inphase responses on the data profiles.

Like magnetometry, the EM magnetite method maps only

bedrock features, provided that the overburden is

characterized by a general lack of magnetite. This contrasts

with resistivity mapping which portrays the combined effect

of bedrock and overburden.

Recognition of culture

f . Cultural responses include all EM anomalies caused by

" man-made metallic objects. Such anomalies may be caused by

H inductive coupling or current gathering. The concern of the

interpreter is to recognize when an EM response is due top culture. Points of consideration used by the interpreter,l ..jj 4

when coaxial and coplanar coil-pairs are operated at a common

l- frequency, are as follows t

1. Channel CPS monitors 60 Hz radiation. An anomaly on

i]

- 5-21 -

this channel shows that the conductor is radiating

, power. Such an indication is normally a guarantee that

' the conductor is cultural. However, care must be taken

j' to ensure that the conductor is not a geologic body

which strikes across a power line, carrying leakage

1 - currents.

h 2. A flight which crosses a "line" (e.g., fence, telephone

r line, etc.) yields a center-peaked coaxial anomaly and

an m-shaped coplanar anomaly. 4 When the flight crosses

IV the cultural line at a high angle of intersection, the

amplitude ratio of coaxial/coplanar response is 4. Such

- an EH anomaly can only be caused by a line. The

geologic body which yields anomalies most closely

resembling a line is the vertically dipping thin dike.

j Such a body, however, yields an amplitude ratio of 2

rather than 4. Consequently, an m-shaped coplanar

I ! anomaly with a CXI/CPI amplitude ratio of 4 is virtually .ia guarantee that the source is a cultural line.

Or* 3. A flight which crosses a sphere or horizontal disk

'J yields center-peaked coaxial and coplanar anomalies with

[1 a CXI/CPI amplitude ratio (i.e., coaxial/coplanar) of

1/4. In the absence of geologic bodies of this

4 See Figure 5-1 presented earlier.

y

i;

oo

l] o

- 5-22 -

geometry, the most likely conductor is a metal roof or

small fenced yard. 5 Anomalies of this type are

virtually certain to be cultural if they occur in an

area of culture.

4. A flight which crosses a horizontal rectangular body or

wide ribbon yields an m-shaped coaxial anomaly and a

center-peaked coplanar anomaly. In the absence of

geologic bodies of this geometry, the most likely

conductor is a large fenced area. 5 Anomalies of this

type are virtually certain to be cultural if they occur

in an area of culture.

5. EM anomalies which coincide with culture, as seen on the

camera film or video display, are usually caused by

culture. However, care is taken with such coincidences

because a geologic conductor could occur beneath a

fence, for example. In this example, the fence would be

expected to yield an in-shaped coplanar anomaly as in

case 12 above. If, instead, a center-peaked coplanar

anomaly occurred, there would be concern that a thick

geologic conductor coincided with the cultural line.

5 it is a characteristic of EM that geometrically similar anomalies are obtained fromt (1) a planar conductor, and (2) a wire which forms a loop having dimensions identical to the perimeter of the equivalent planar conductor.

- 5-23 -

6. The above description of anomaly shapes is valid when

the culture is not conductively coupled to the

f environment. In this case, the anomalies arise from

inductive coupling to the EM transmitter. However, when

|. the environment is quite conductive (e.g., less than 100

ohm-m at 900 Hz), the cultural conductor may be

conductively coupled to the environment. In this latter

r case, the anomaly shapes tend to be governed by current

gathering. Current gathering can completely distort the

anomaly shapes, thereby complicating the identification

of cultural anomalies. In such circumstances, the

[ ; interpreter can only rely on the radiation channel OPS

and on the camera film or video records.

P MAGNETICS

l j The existence of a magnetic correlation with an EM

anomaly is indicated directly on the EM map. In someHIJ geological environments, an EM anomaly with magnetic

p correlation has a greater likelihood of being produced by

sulfides than one that is non-magnetic. However, sulfide ore

M bodies may be non-magnetic (e.g., the Kidd Creek deposit near l,.]

Timmins, Canada) as well as magnetic (e.g., the Mattabi

l j deposit near Sturgeon Lake, Canada).

O

- 5-24 -

The magnetometer data are digitally recorded in the

aircraft to an accuracy of one nT (i.e., one gamma) for

proton magnetometers, and 0.01 nT for cesium magnetometers.

The digital tape is processed by computer to yield a total

field magnetic contour map. When warranted, the magnetic

j: data may also be treated mathematically to enhance the

magnetic response of the near-surface geology, and an

enhanced magnetic contour map is then produced. The response

j of the enhancement operator in the frequency domain is

illustrated in Figure 5-2. This figure shows that the

l passband components of the airborne data are amplified 20

times by the enhancement operator. This means, for example,

that a 100 nT anomaly on the enhanced map reflects a 5 nT

anomaly for the passband components of the airborne data.

l The enhanced map, which bears a resemblance to a

downward continuation map, is produced by the digital

T bandpass filtering of the total field data. The enhancement

is equivalent to continuing the field downward to a level p y (above the source) which is 1720th of the actual sensor-

0 source distance. .[j Because the enhanced magnetic map bears a resemblance to l. j

a ground magnetic map, it simplifies the recognition of

[j trends in the rock strata and the interpretation of

11 geological structure. It defines the near-surface local

n

-5-25-

i-..

rliO

UJo ^ \-no. S

CYCLES/METRE

Fig. 5-2 Frequency response of magneticenhancement operator for a sample Interval of 50m.

OO

O l!

- 5-26 -

geology while de-emphasizing deep-seated regional features.

It primarily has application when the magnetic rock units are

steeply dipping and the earth's field dips in excess of 60

degrees.

Any of a number of filter operators may be applied to

the magnetic data, to yield vertical derivatives,

continuations, magnetic susceptibility, etc. These may be

displayed in contour, colour or shadow.

VLF

,- VLF transmitters produce high frequency uniform

' electromagnetic fields. However, VLF anomalies are not EM

I ! anomalies in the conventional sense. EM anomalies primarily ireflect eddy currents flowing in conductors which have been

h energized inductively by the primary field. In contrast, VLF

anomalies primarily reflect current gathering, which is a

Li non-inductive phenomenon. The primary field sets up currents

p which flow weakly in rock and overburden, and these tend to

collect in low resistivity zones. Such zones may be due to

h massive sulfides, shears, river valleys and evenl*J

unconformities.u

— 6-27 —

l!

UJo

Q.

02

CYCLES X METRE

Fig. 5-3 Frequency response of VLF operator.

n

- 5-28 -

c-'

^ The VLF field is horizontal. Because of this, the

method is quite sensitive to the angle of coupling between

the conductor and the transmitted VLF field. Conductors

which strike towards the VLF station will usually yield a

stronger response than conductors which are nearly orthogonal

j to it.

i The Herz Industries Ltd. Totem VLF-electromagnetometer

measures the total field and vertical quadrature components.

Both of these components are digitally recorded in the

aircraft with a sensitivity of 0.1 percent. The total fieldl

yields peaks over VLF current concentrations whereas the

quadrature component tends to yield crossovers. Both appear

r as traces on the profile records. The total field data are

l' filtered digitally and displayed as contours to facilitate

P the recognition of trends in the rock strata and the

interpretation of geologic structure.

yThe response of the VLF total field filter operator in

U the frequency domain (Figure 5-3) is basically similar to

n that used to produce the enhanced magnetic nap (Figure 5-2).

The two filters are identical along the abscissa but * i different along the ordinant. The VLF filter removes long

wavelengths such as those which reflect regional and wave

li transmission variations. The filter sharpens short

i - wavelength responses such as those which reflect local

geological variations.

D

IJ

- 6-1 -

AMD

This report provides a brief description of the survey results and describes the equipment, procedures and logistics of the survey.

The survey was successful in locating numerous zones of interest. The various maps included with this report display the magnetic and conductive properties of the survey area. It is recommended that the survey results be reviewed in conjunction with all available geological, geophysical and geochemical information by qualified personnel. Areas of interest defined by the survey should be subjected to further investigation, using appropriate surface exploration techniques .

It is also recommended that additional processing of existing geophysical data be considered, in order to extract the maximum amount of information from the survey results. The use of Dighem's Imaging Workstation may provide additional useful information from the survey. Current processing techniques can yield structural detail that may be important in further defining the geologic setting.

Respectfully submitted,

DIGHKM SURVEYS t PROCESSING INC.

l TP-T5 f*Douglas L. Mcconnell

J Geophysicist

1 DLM/sdp

- A1056APR.90R

Fi:L o

APPENDIX A

LIST OF PERSONNEL

The following personnel were involved in the acquisition, processing, interpretation and presentation of data, relating to a DIGHEM* 1 * airborne geophysical survey carried out for Noranda Exploration Company Limited, over the Shebandowan area, Ontario.

Peter S.L. Moore Senior Geophysical OperatorMaurie Bergstrom Geophysical Operator/Electronics

TechnicianDan Chinn Pilot (Peace Helicopters Ltd.)Paul Bottomley Computer ProcessorDouglas L. Mcconnell GeophysicistGary Mohs DraftspersonSusan Pothiah Word Processing Operator

The survey consisted of 2620 km of coverage, flown from January 20 to February 9, 1989. Geophysical data were compiled utilizing a MicroVAX II computer.

All personnel are employees of Dighem Surveys S Processing Inc., except for the pilot who is an employee of Peace Helicopters Ltd.

DIGHEM SURVEYS fi PROCESSING INC.

. — .

Douglas L. Mcconnell Geophysicist

[l DLM/sdpli

p Ref* Report 11056-B-C-D li

A1056APR.90R

APPENDIX