Paper presented at the First International Congress of Geosciences: Innovation in geology, geophysics and mining. Peru, Arequipa, UNSA 2017. GPR technique applied to mineral exploration.

PhD in Geology Kadurin Sergii

GPR technique applied to mineral exploration

Kadurin Sergii

PhD in GeologyAssotiate Proffesor of

Odessa I.I. Mechnikov National University (Ukraine)

[email protected]+38-0687524133

Faculty of Geology and GeographyONU

Why GPR?

• GPR is easy to use;

• You can get information about the deep structure quickly;

• The prices for GPR equipment and services are lower that for the another geoelectrical methods.

What is the GPR?

• GPR – ground-penetrating radar.

• GPR works with electromagnetical wave reflections.

The depth of study:

Limited by next parameters:• Electrical permittivity and conductivity of the ground;

• Transmitted frequency;

• Radiated power.

Electrical properties

• Conductivity is the ability of a material to support an electrical current (material property that describes the movement of electrons or ions) due to an applied electrical field. (units – Siemens/metere(S/m))

• As conductivity increases, the penetration depth decreases

General divisions of materials by electrical conductivity Low conductivity – excellent radar conditions (<10-7 S/m)

Air; Dry granite, dry limestone; Concrete

Medium conductivity – medium radar conditions (10-7 – 10-2 S/m) Freshwater, freshwater ice, snow; Sand, silt, dry clay, basalt, seawater ice.

High conductivity – poor radar conditions (>10-2 S/m) Wet clay, wet shale, Seawater.

Transmitted frequency

• Usually GPR uses high-frequency electro-magnetic waves;

• Higher frequencies do not penetrate as far as lower frequencies, but give better resolution.

Antenna frequency Maximum penetration depth Examples of potential use

900 MHz 1 m Pipe and void detection or assessing of concrete thickness

400 MHz 4 m Utility survey, storage tank detection, assessing structural integrity

300 MHz 6 m Utility survey, geology and archeology.

25 MHz 190 – 205 m Exploration.

Power of transmitter• The GPR transmitter produces the short duration high-power pulses of

energy that are radiated into ground by the antenna.

• There are two types of pulses can be used in GPR: oscillating and non-oscillating.

oscillatingnon-oscillating

Received data

Data analysis

Depth can be calculated:

c – propagation velocity in free space (3*108 m/s)

Vm – propagation velocity through thematerial

εr – relative permittivity

Dielectric constant

• Dielectric permittivity is the property that describes the ability of material to store electric energy by separating opposite polarity charges in space (units – Farad/meter (F/m))

• Relative dielectric permittivity (dielectric constant) is the ratio of the permittivity of a material to that of free space – 8.854*10-12 F/m (No unit)

Relative dielectric permittivity

• The range of relative dielectric permittivity is 1 – 81;

• Dielectric permittivity differences at boundaries cause reflections in the radar data, the strength of reflections is controlled by contrast in the dielectric permittivity;

• The value of relative dielectric permittivity is primarily controlled by water content.

Approximate electrical properties of various materials

Reflections• When the waves hits the buried object or a boundary with different

dielectric constants, the receiving antenna records variations in the reflected return signal.

Simplified diagram of GPR construction and profiles (adopted from Butler an al (1991) and Daniel at al (1988)

Some questions:

• How it works if we need to find subvertical geological bodies like veins, dykes or cracks and faults?

• How to detect a body with smooth geological boundary?

• Is that possible to detect mineralization in the rocks?

GPR “LOZA”LOZA series is commercial GPR produced by a Russian company VNIISMI and is widely used in industrial geology, archeology and civil engineering.

GPR “LOZA”LOZA GPR series has been designed specially for high-conductivity soils (wet clay, loam). To increase the devise effective potential we raise the transmitter peak power by a factor of 10000and replace stroboscopic transformation with direct registration. Note that despite the peak power enhancement the average power decreases by a factor 10 due to reduced repetition rate.

• We use high power transmitter on basis of hydrogen spark-gap;

• Transmitter pulses are in asynchronous mode. Synchronization realize in the receiver as a waiting mode by air waves;

• We replace stroboscopic transformation by direct registration in the working frequency range;

• Transmitter and receiver have no electrical coupling;

• We use only resistive loaded dipoles as antenna. That antennas have the low level of “ringing”.

LOZA-N GPR low frequency, deep penetration

The signal energy dissipation in geological section

h

σε1

ε2

1

2

3

1. Signal attenuation from transmitter to geological boundary on depth h.

𝐴1 = 𝐴0𝑒−2𝑝ℎ

Where p is the attenuation coefficient p = 𝜔

с

𝜀

2( 1 + tan2 𝛿 − 1)

2. Reflection coefficient on the geological boundary

R = 𝜀1− 𝜀

2

𝜀1+ 𝜀

2

3. Signal attenuation from geological boundary on depth h to receiver.

𝐴3 = 𝐴2𝑒−2𝑝ℎ

A2 = RA1

tan 𝛿 =휀′

휀′′=

𝜎

2𝜋𝑓휀0휀𝜔 = 2𝜋𝑓

The signal energy dissipation of Loza-N GPR in limestone – granite geological section

h

σ=0.0005

ε1=9

ε2=4

1

2

3

limestone

granite

0

50

100

150

200

250

1E-08 0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10

De

pth

h

Returned energy A3A0 = 21kV A3 - ?

Receiver sensitivity

LOZA-N applications

Loza-N in Peru (Condor area)• Distrito de Pachaconas, provincia de Antabamba, Apurímac.

• Veins with mineralization are the main target.

Main objectives of the work:• Identification of fractures and cracks;

• Identification of mineralized veins;

• Identification of deep structure.

Section of Maiskoe Gold depositRussia.

Work process• The work was done 25 /07 /2016;

• 3 profiles have been done with total length 576 m;

Loza-N in Peru• There are 3 profiles have been made

1

2

3

PROFILE 1- 176 m

PROFILE 2- 192 m

PROFILE 3- 208 m

N

Profile 1 (50 m depth)

Profile 1 (200 m depth)

Profile 2 (50 m depth)

Profile 2 (200 m depth)

Profile 3 (50 m depth)

Profile 3 (200 m depth)

How to detect intrusive body by GPR?

Another project in Peru – Ica area.

Numerous andesite and quartz monzonite dykes and veins are crossed the area.

Granodiorite intrusion is the biggest magmatic body in that area

Zone of mineralization can be detected on the border between granodiorites and country rocks.

Geological map of studied area.

Profiles on the area.

Profile 2.2

• Andesite dyke can be detected.

• Deeping of dyke and underground structure can be detected.

Profile 2.0

• Dykes and veins can be detected.

• Intrusive body with weathered top part have been detected.

GPR profiles interpretation in 3D

How to detect ore body?

• The low frequency GPR allows to reveal the structural features (bedding,faults, intrusive bodies and other structural heterogeneity), as well as theidentification of polarizable and low conductivity areas.

• Parts of sections where the rocks can be polarized we can interpret as rocks with ferromagnetic properties, and the areas of low conductivity,we call “dielectric zones”.

Example of polarized ore body

Potential ore‐bodyFaults

Dielectric

Contact –Weathered / non‐ weathered zone

Project “Cobre”

Project “Cobre”

Project “Manuelito”

How we can compare GPR with IP?There are three veins can be detected on GPR profile (top).On IP profile only some roundanomalies can be identified.

Because of GPR profilingwe can identify the real shape of undergroundgeological body.

GPR and IP comparison.

Please note: steep topography has influence on our path through the area –so IP lines are not exactly the sameas GPR profiles because we had touse paths to walk on.

Hierro project with Hematite mineralization

Hierro project.

Ore bodies with Hematite – Magnetite mineralization

Conclusions about GPR LOZA-N using in mining projects:

• The low frequency GPR like Loza-N can give a good result for sub-vertical veins and fracture zone detection;

• Powerful transmitter (20 MV) allows to make geophysical profile to the 200 m depth and to see the deep structure and position of veins on the big depth.

• Ore zones can be detected on GPR profile and distinguished from surrounding rocks.

GRACIASTHANKS