Modelling and Survey Results of In-mine Electromagnetics ...

Created by Peter Downing – Educational Media Access and Production © 2011

ACKNOWLEDGEMENTS

The limitations of performing in-mine inductive geophysics have been partially

overcome during the course of this survey. The limitation in instrumentation from

working in a physically confined space, leading to small depth penetration, have been

allayed by the smooth and repeatable response in the data. The physical variations in

geology were consistently detected in both components and from a variety of TX-RX

separations, proving that certain geological features on the other side of the salt are

resolvable from in-mine induction.

This project has had a lot of support from both my supervisor and from

PotashCorp. As such, there is still much work to do in processing and analyzing the

in-mine TEM survey data, including comprehensive modelling of the data and

interpreting resistivity horizons from the profiles. Additionally, a focus will be paid to

current environmental EM systems on the market – matching functionality to need –

with the aim of improving the quality of geophysical work in this area.

Mine Layer

Salt

Salt

Shale

SaltShale

Carbonate

Mine Layer

Salt

Salt

Shale

SaltShale

Carbonate

Mine Layer

Salt

Salt

Shale

SaltShale

Carbonate

Mine Layer

Salt

Salt

Shale

SaltShale

Carbonate

Mine Layer

Salt

Salt

Shale

SaltShale

Carbonate

Mine Layer

Salt

Salt

Shale

SaltShale

Carbonate

SOURCE

SOURCE

Electromagnetics is a

geophysical induction technique

that responds to spatial variations in

the conductive properties of rocks.

These variations are stimulated via a

time-varying magnetic field; created

and observed through the placement

of two loops of electrical wire (TX

and RX). The magnetic field decays

slower in material that is conductive

and faster in material that is

resistive.

First and foremost, I wish to thank my Mitacs supervisor, Randy Brehm, for his patience and technical

expertise in organizing and implementing my Mitacs partnership this fall. Without his herculean effort

much of my work would simply not have been possible. Additionally, I wish to thank Arnfinn Prugger and

Craig Funk for being so supportive over the years; Tanner Soroka and James Isbister for their geological

expertise (and the latter’s labour in assisting with the data collection). Matthew van den Berghe for his

mentorship and training in all things geophysical. GDD Instrumentation’s Circé Malo Lalande and

Geonics’ Rob Harris for their instruction and correspondence. A special thanks to Joel Grunerud of

Patterson Geophysics, whose knowledge of electromagnetic instrumentation is unmatched. Last but not

least, I want to thank my research supervisor, Dr. Sam Butler, without whom this project would not exist.

This project was funded in part by the Mitacs Accelerate program and PotashCorp.

The Dawson Bay carbonate

members are the target of this project.

They are members of the Dawson Bay

formation that includes a basal shale

layer, the Second Red Bed, and a

locally present cap layer of evaporite,

known as the Hubbard. Above this

formation lies the First Red Bed shale

body and the Souris River formation.

Below it the Prairie Evaporite

formation, which includes the mined

potash ore zones.

The upper carbonate member of

the Dawson Bay (the Neely) is

typically porous and fairly conductive.

The lower Dawson Bay carbonate (the

Burr) is almost exclusively tight and

resistive. The case where it is not is the

purpose of this research project.

Safe mine expansion has been an essential focus for potash mines in

Saskatchewan over the years. One of the primary areas of care and attention has been

mitigating the potential for sub-saturated brine inflow. The source of sub-saturated

brine in-flows are anomalously porous geological layers above the mining horizon. In

this project we are proposing, through computer modelling and in-mine surveying,

the possibility of detecting the presence of these anomalous zones using geophysical

electromagnetic methods.

Areas of increased brine-filled porosity in the salt are known to produce a

conductive response to various geophysical techniques. However, what is not well

known is if the same response is measurable in the carbonates. The time-domain

electromagnetic (TEM) method has been selected for this investigation as it has a

variety of benefits to it’s application, including higher resolution depth sounding

capabilities (when compared with frequency-domain electromagnetics) and low-

power requirements to excite geology on the other side of the salt layer (unlike direct

current resistivity).

These anomalously porous geological features are outside of the norm, and their

genesis is not perfectly understood. However, there has been found a geospatial link

between areas of increased brine and the absence or partial destruction of overlying,

younger evaporite members. One such area was the target of a time-domain

electromagnetic survey that the author participated in as part of a Mitacs Accelerate

program partnership with Potash Corporation of Saskatchewan Inc. (PotashCorp) in

the fall of 2017.

INTRODUCTION

PRINCIPLES OF ELECTROMAGNETICS

GEOLOGICAL MODELING PRELIMINARY DATA ANALYSIS MODELLING

Modelling and Survey Results of In-mine Electromagnetics for Brine Layer Detection T. J. LeBlanc, S. L. Butler

Fig.1: Illustration showing the principle operations behind EM surveys (Unsworth, 2009).

([email protected]) ([email protected])

IN-MINE TIME-DOMAIN ELECTROMAGNETICS SURVEY RESULTS

REFERENCESAlhstrom, J. H. (1992). Geology And Diagenesis Of The Dawson Bay Formation In The Saskatoon Potash Mining District, Saskatchewan (Unpublished

master's thesis, 1992). University of Saskatchewan. Retrieved May 01, 2016, from http://hdl.handle.net/10388/7025Butler, S., & Sinha, G. (2012). Forward modeling of applied geophysics methods using Comsol and comparison with analytical and laboratory analog

models. Computers & Geosciences,42, 168-176. doi:10.1016/j.cageo.2011.08.022Butler, S., & Zhang, Z. (2016). Forward modeling of geophysical electromagnetic methods using Comsol. Computers & Geosciences,87, 1-10.

doi:10.1016/j.cageo.2015.11.004Chouteau, M., Phillips, G., & Prugger, A. (1997). Mapping and Monitoring Softrock Mining. Proceedings of Exploration 97: Fourth Decennial

International Conference on Mineral Exploration,927-940.Das, U. C. (1995). A reformalism for computing frequency‐ and time‐domain EM responses of a buried, finite‐loop source in a layered earth. SEG

Technical Program Expanded Abstracts 1995. doi:10.1190/1.1887562Duckworth, K. (1992). Detection Of Brine Layers Overlaying Potash Mine Operations. Canadian Journal of Exploration Ggeophysics,28(2), 109-116.Dunn, C. E. (1982). Geology of the Middle Devonian Dawson Bay Formation in the Saskatoon Potash Mining District, Saskatchewan. (Rep. No. 194).

Regina, SK: Saskatchewan Energy and Mines.Eso, R. A., & Oldenburg, D. W. (2006). Application of 3D electrical resistivity imaging in an underground potash mine. SEG Technical Program

Expanded Abstracts 2006. doi:10.1190/1.2370339Farquharson, C. (2006, June 1). Time Domain Inversion And Modelling Of Electromagnetic Data - Background for Program EM1DTM. Retrieved

November 16, 2017, from https://www.eoas.ubc.ca/ubcgif/iag/sftwrdocs/em1dtm/TheoreticalBackground.pdfGendzwill, D. J., & Pandit, B. I. (1980, December). A Computer study of electromagnetic sounding in a potash mine. Canadian Journal of Exploration

Geophysics.Gendzwill, D. J., & Stead, D. (1992). Rock mass characterization around Saskatchewan potash mine openings using geophysical techniques: a

review. Canadian Geotechnical Journal,29(4), 666-674. doi:10.1139/t92-073Gendzwill, D. J. (1967). Electromagnetic measurement of salt formation thickness. Saskatchewan Research Council - Physics Division.Gendzwill, D. J. (1978). Winnipegosis Mounds and Prairie Evaporite Formation of Saskatchewan--Seismic Study. AAPG Bulletin,62.

doi:10.1306/c1ea47f7-16c9-11d7-8645000102c1865dJeremic, M. L. (1994). Rock mechanics in salt mining. Rotterdam: A.A. Balkema.Kendall, A. C. (n.d.). Bedded Halites in the Souris River Formation (Devonian).Krivochieva, S., & Chouteau, M. (2002). Whole-space modeling of a layered earth in time-domain electromagnetic measurements. Journal of Applied

Geophysics,50(4), 375-391.Lane, D. M. (1959). Dawson Bay Formation in the Quill Lakes - Qu'Appelle Area Saskatchewan (Rep. No. 38). Regina, SK: Department of Mineral

Resources - Geological Sciences Branch.McNeill, J. D. (1994). Principles and Applications of Time Domain Electromagnetic Techniques for Resistivity Sounding(Tech. No. 27). Mississauga,

ON: Geonics Limited.Raiche, A. P., & Gallagher, R. G. (1985). Apparent resistivity and diffusion velocity. Geophysics,50(10), 1628-1633. doi:10.1190/1.1441852Unsworth, M., DR. (2009, January). Introduction to Electromagnetic exploration method[PDF]. University of Alberta.Wait, J. R. (1955). Mutual Electromagnetic Coupling Of Loops Over A Homogeneous Ground. Geophysics,20(3), 630-637. doi:10.1190/1.1438167

Fig.4: In-mine TEM

survey results. Left side

shows time slices of the

change in vertical

magnetic field and the

right side shows time

slices of the change in

radial magnetic field. The

bottom shows the plan

view of the survey. Data

courtesy of PotashCorp.

INC

REA

SIN

G D

EPTH

(u

pw

ard

)

INCREASING TIME

Fig.3: Showing the diffusion of

the magnetic field in vertical

cross-section in the both the

vertical and radial directions

using the 2D axisymmetric

module in COMSOL

Multiphysics.

Fig. 3 shows forward

computer TEM modelling

within COMSOL Multiphysics

of the propagation of a

magnetic signal through near

mine geological layers. The

colour contour in the top row

shows the intensity of the

vertical field over time, while

the bottom row shows the

radial field over time. The

black streamlines show the

direction of the field.

An in-mine time-domain electromagnetics survey was conducted by PotashCorp in the fall

of 2017 targeting an anomalous zone in the Burr carbonate detected via 3D surface seismic.

Two panels were surveyed, both facing east-west using a slingram style loop set-up. The time of

investigation was between 0.012 ms to 7.221 ms. Both the vertical and radial fields were

measured, as well as several TX-RX separations, including 40m, 60m, and 100m. Below in fig.4

is displayed the 60m TX-RX separation survey data at several time slices (0.13, 0.24 & 0.93ms,

from bottom to top). On the left is the vertical field measurements, and on the right is the

radial. The values that are contoured in fig.4 are the logarithm of the time derivative of the

magnetic field in µT/s.

NEAR-MINE GEOLOGY

Fig.2: Stratigraphy cross-section of the near-mine geology, including an idealized resistivity log.

Fig.2 shows an idealized resistivity log

of the geology. The solid line is the

normal conditions for the Burr, while

the dashed one is a hypothetical

anomalous Burr member.

The spread of the colour bars of each slice have been normalized to the slice with

the highest spread. This highlights the strong, consistent contrast in the radial field

response. The survey was conducted using a Geonics EM-57 transmitter and a GDD

InstrumentationNordic EM24 receiver.

A preliminary forward model decay curve comparison (fig.5) shows that a

conductive response from the Burr member is expected at around the same time

mark as it appears in the survey data. This gives some confidence to the proposition

that the conductive layer detected by the survey is the Burr carbonate member.

CONCLUSIONS AND FUTURE RESEARCH

Fig.5: Comparing data from the in-mine survey to preliminary time-domain EM forward models. Decay curves

responses are from the northern survey line; from both the far west station – “normal” conditions (lighter

curves) – and far east station – “anomalous” conditions (darker curves). The forward model on the left is a pre-

survey simulation using parameters for the geological layers based off of well-log data (these parameters are

shown in fig.2).