Borehole Resistivity Logging and Tomography for Mineral Exploration

Borehole Resistivity Logging and Tomography for Mineral Exploration

W. Qian, B. Milkereit, G. McDowell, K. Stevens and S. Halladay

www.geo-lt.com

Geoserve Logging & Tomography

WHY ?

A

B

M

N

• Continuities of conductors between boreholes

• Identification of conductors offhole

• Mapping perfect conductors

• Mapping poor conductors

WHY NOW ?

A

B

M

N

• Forward modeling studies

• Multi-electrode array instrumentation

• Very easy to deploy

• Can acquire vast amount of data rapidly

• Battery power

• Easy data QC

• Rugged design

Advantages of the System

Zn, Pb and Ag (Modest Conductor)

Ni-Cu (Super Conductor)

Vertical Resistivity Profiling

A

B

M

N Apparent Resistivity

Borehole intersects sulfides in conductive environment

Borehole pass by sulfides in conductive environment

Log10

Borehole intersects sulfide in resistive environment

Log10

Borehole pass by sulfide in resistive environment

VRP survey in a single hole will provide:

• Bulk background resistivity

• Information about off-hole conductors

Borehole to Borehole Electrode Configuration

A

B

M

N

B3

B1

B2

Projection Plane

B3

B2

B1

DeeperDeeper

Ore zone in B2

QC Electric Current Injected between B2 and B3

Current Electrodes in B3

Cu

rre

nt E

lect

rod

es

in B

2

DeeperDeeper

Ore zone in B2

Ore zone shadow in B3

Current Electrodes in B3

Cu

rre

nt E

lect

rod

es

in B

2

QC Electric Current Injected between B2 and B3

DeeperDeeper

Ore zone shadow in B3

QC Electric Current Injected between B1 and B3

Current Electrodes in B3

Cu

rre

nt E

lect

rod

es

in B

1

B3

B2

B1

Massive Sulfide Zone

Top

Bottom

Electric Current between two adjacent electrodes in B2

Ore zone in B2

Ore zone shadow in B3

Electric Potential between two adjacent electrodes in B3 [mV]1

mA

of c

urre

nt is

inje

cted

bet

wee

n tw

o ad

jace

nt e

lect

rode

s in

B2

DeeperDeeper

Zone I

Zon

e II

QC Electric Current Injected between B1 and B3

No Electrode Coverage

No Electrode Coverage

Zon

e II

Zone I

A: alteration bleached, no significant Zn mineralization or Pyrite-content, resistivity larger than 40 ohm.m B: brecciation, matrix Pyrite rich ( 5 – 10 % Pyrite), less than 1% Zn content, resistivity between 15 and 40 ohm.m C: strong brecciation, often more than 5% Zn content, resistivity less than 15 ohm.m.

Inverse Modeling Strategy

• VRP pseudo section as starting model

• Sharp inversion of only VRP data (Initial model is the main constraint)

• Build a model from the two sharp inversion models

• Fix the near borehole properties and let the tomography inversion work on the resistivity in the central region. The resistivity values can be fixed, semi-fixed (fixed in a narrow range) or completely floating

• Fine tuning the inversion model with different geological / petrophysical constraints

• Detect conductive zones within 30 m range around the borehole

• Provide independent estimate of bulk (4 - 100 m) resistivity data for calibration / interpretation of other EM datasets

• Map conductive zones between the boreholes 180 m apart

• Works for all conductivity contrasts

• Very easy field operation procedures

Conclusions

• Field test 3D tomography methodologies

• Develop IP data interpretation

• Move towards simultaneous data acquisition in multiple boreholes

• Build cables to deploy in deeper boreholes

Outlook

• Nash Creek, Slam Exploration

• Sudbury, Camiro, NSERC, CVRD, Xstrata, First Nickel

Acknowledgement