modelling and survey results of in-mine electromagnetics
TRANSCRIPT
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).