Note: Descriptions are shown in the official language in which they were submitted.
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Characterisation of geological materials by thermally induced response
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for characterization
of geological materials. In particular, the invention relates to the
identification
of a constituent, such as a mineral or minerals, within a rock. More
particularly, the invention relates to a methods and apparatus for
identification
of a mineral or minerals within a rock that employ the application of low
level
electromagnetic energy to the rock followed by imaging of the thermal
response of the mineral(s) within the rock to the microwave energy.
The invention has particular, but non-exclusive application to the
identification
of minerals in drill-hole cores and geological samples, and also extends to
identification of minerals on interior wall surfaces of boreholes and/or
exposed
rock surfaces in-situ.
BACKGROUND TO THE INVENTION
Remote or non-contact sensing for the identification of minerals within rock
formations is of significant importance in the art of geological exploration
for
mineral deposits. Remote sensing is based on the study of the interaction of
geological material, particularly mineral deposits, with efectromagnetic
radiation. The radiation wavelengths traditionally considered cover visible
and
infra-red parts of the spectrum, generally within the range of from 0.35 to
40 m. For solid materials, electromagnetic radiation is absorbed or emitted as
a result of changes in the total energy content of the material. These
transitions within the material take place between specific energy levels, and
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may be between different electronic energy levels or between different
vibration levels.
In the former case transition appears in the visible or near infra-red (0.7-
2.5 m) part of the spectrum, while in the later case transition evidence
appears in the infra-red part of the spectrum (1.2-40 m). Vibration processes
are determined by the chemical composition, the geometry and the positions
of the constitutive atoms and the nature of inter-atomic forces. Therefore,
information available in the infra-red part of the spectrum is directly
related to
the bulk properties of rocks and minerals. Due to the nature of rocks, for
example ores, which are an ensemble of various minerals, the infra-red (IR)
spectral response of a particular rock is a composite of the spectral
responses
of the constitutive minerals.
Various approaches have been previously employed for passive remote
sensing of ground rock formations. Well-established techniques in multi-
spectral and hyper-spectral analysis may be coupled with new, state-of-the-art
imagery from space-borne, airborne and more recently ground based sensor
systems, enabling their direct and immediate application to geological
mapping.
Reflectance spectroscopy provides diagnostic information on the mineralogy
of the uppermost few microns of a rock surface. This'technique involves
measuring the spectrum of sunlight reflected from the rock surface, and is
therefore restricted to the wavelength range where the sun's flux is highest
and where the amount of energy reflected from the rock surface is greater
than the amount that is thermally emitted (the typical wavelength range is
from
0.3 to 3.5 m). Reflectance spectra reveal absorption features that are
characteristic of certain minerals. For example, the mineral pyroxene, a
common component of basaltic rocks on the Earth, can be detected remotely
by the measurement of diagnostic absorption features near 1.0 and 2.0 m.
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Variations in the abundances of Fe and Ca in the pyroxene can also be
inferred based on subtle shifts in the positions of these bands.
High-quality imaging at near-IR and mid-IR wavelengths has recently become
practical because of advances in infra-red-sensitive arrays. Specifically,
arrays constructed from indium and antimony (InSb) substrates have had
spectacular success in achieving high Signal to Noise Ratio (SNR) and high
dynamic range for telescopic and spacecraft imaging applications. Other IR
sensitive substrates, including silicon-arsenic (SiAs), germanium (Ge), and
1o indium-gai(ium-arsenic ((nGaAs), have also been used with good results. A
particular advantage of many of these arrays is their ability to operate
effectively with only modest cooling requirements.
Thermal infra-red (TIR) also provides diagnostic information on the mineralogy
of rock surfaces, as well as additional information on surface thermo physical
properties like temperature. Most of the major rock-forming minerals exhibit
their fundamental molecular vibration spectral features at mid-infra-red
wavelengths; typically from 3 to 25 m. In remote sensing practice water and
other gases in the atmosphere restricts. aerial systems to two wavelength
windows; 3 to 5 m and 8 to 15 m. Unlike reflectance spectra, thermal IR
spectra can exhibit features in both emission and absorption, depending on
the nature of the environment.
Because each rock is generally a combination of several minerals, spectral
features in the composite spectrum are not normally well-defined. Generally,
the spectrum appears smeared and in some cases the contributions from
minor constituents may dominate the spectrum and completely mask the
presence of the mineral which is in fact the target for mineral exploration.
The present invention, at least in certain embodiments, proposes a method
and apparatus that employ active remote sensing and. that make use of IR
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sensors that detect responses from objects that have been irradiated from an
artificially-generated energy source. The proposed method and apparatus
may advantageously have mineral detection capabilities that are superior to
the presently used methodology and instruments.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method for the
identification of a constituent of or within a rock including:
applying low level electromagnetic energy to the rock thereby inducing
a thermal response from the constituent;
imaging the thermal response from the constituent to obtain thermal
images within a plurality of distinctive bands of IR spectra; and
interpreting the thermal images to identify the constituent.
As used herein, the term "constituent" is intended to mean any element or
component making up or forming part of a rock, a rock body, a core sample,
geological or rock formation and so on. The term also extends to deposits,
such as organic deposits, oil and gas, oil shale, oil sand, located within
rocks,
rock formations and so on.
The applied low level electromagnetic energy is preferably applied microwave
energy or applied radiowave energy.
The low level electromagnetic energy may be applied as desired at a
continuous power density, or as pulsed low level electromagnetic energy. It
has been found that resolution of the resultant images is improved if the low
level electromagnetic energy is applied as pulsed low level electromagnetic
energy. As such, it is preferred that the low level electromagnetic energy
applied is pulsed microwave energy or pulsed radiowave energy.
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In the instance when the applied low level electromagnetic energy is
microwave energy, the power density of the microwave energy applied to the
rock is not particularly limited. Preferably, the microwave energy is applied
at
a power density of less than 1000 MW/m3, more preferably from 10 to 100
MW/m3.
Similarly, the frequency of the applied microwave energy is not particularly
limited. Preferably, however, the microwave energy is applied at a microwave
frequency of from 895MHz to 245GHz, more preferably from 895 to 3500MHz,
and even more preferably from 895 to 950MHz. Suitably, the microwave
energy is applied at a microwave frequency of from 895 to 915MHz.
When the applied low level electromagnetic energy is radiowave energy, the
frequency of the applied radiowave energy, whilst not particularly limited, is
preferably at a radiowave frequency from 13.6MHz to 895MHz. More
preferably the applied radiowave energy is a radiowave frequency from
400MHz to 895MHz.
More suitably the applied low level electromagnetic energy may be an applied
radiowave energy having a radiowave frequency of 433.92MHz or an applied
microwave energy having a microwave frequency selected from 895MHz,
915MHz, 2450MHz, 5800MHz or 24.125GHz.
Imaging of the constituent is conducted to image the thermal response of the
constituent to the applied low level electromagnetic energy. This will provide
a signature response for immediate or later consideration and analysis.
Preferably imaging of the thermal response of the constituent includes infra-
red imaging in the spectral range of from 0.7 to 2.5 m, 3 to 5 m and/or 8 to
15gm. It has been found that the combined use of short wave IR (SWIR) and
thermal infra-red (TIR) spectral ranges allows for the identification of a
wide
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range of minerals. These spectral ranges are characterised with maximum
variability in terms of IR responses (emissivity) between minerals and minimal
absorption of IR energy in the atmosphere.
As will be dealt with in more detail below, the present invention facilitates
an
improved ability to distinguish various constituents within a rock as compared
with prior art methods. That is, the images that may be obtained following the
application of low level electromagnetic energy to the rock are substantially
more distinct than those obtained without the application of microwave
1o energy. IR images will be obtained over a plurality of distinctive bands of
IR
spectra. This facilitates more specific identification of the constituents
within
the rock, generally through comparison with a pre-established library of IR
spectra or spectral data located on a computer database. It should be noted
that the described imaging over a plurality of distinctive bands may include
continuous imaging over an entire range covering these bands. For example,
imaging over the range 3 to 5 m will include a number of distinctive infra-red
bands. That is, reference to imaging of distinctive bands should not be taken
to mean exclusive imaging of those bands, but rather inclusive imaging of
those bands.
Interpretation of the thermal images to identify the constituent or
constituents
will generally be achieved by comparing the thermal images, or parameters
calculated from the thermal images, with a library of IR spectra or spectral
data. In a particular embodiment of the invention, given that the thermal
response of the constituent is imaged simultaneously within a plurality of
(i.e.
at least two) distinctive bands, a ratio of the two (or more) images may be
calculated to define a complete signature for the particular constituent.
In that regard, without wanting to be bound by theory, IR images of the
constituents will be affected by their temperature and their relative
emissivity
in comparison to black body emissivity at a given temperature. For thick,
solid
objects:
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emmisivity = 1 - reflectivity.
Reflectivity, and consequently emissivity, of a constituent varies as a
function
of the IR wavelength. This is illustrated in the figures, as discussed below.
In
order to improve detection of constituents with a similar ability to absorb
low
level electromagnetic energy, in particular microwaves and radiowaves, one
must use additional means to differentiate between those minerals. One such
means is to compare thermal energy coming from the particular constituents
1o in different parts of the IR spectra.
For instance, if one can measure the thermal energy coming from the
constituent in the spectral range 4-5 micrometers and in the range 8-9
micrometers, from the known spectra of minerals we can determine what the
ratio of these two IR bands will be for a given constituent at the same
temperature. Using IR images simultaneously recorded in several ranges of
IR spectra it is possible to calculate, for instance, the ratio of the two
images
to determine intensity of thermal radiation coming from each pixel of the
image. This will facilitate constituent identification, even in the case where
temperature of the constituents is practically the same. In the case where the
low level electromagnetic absorbing capacity of the constituents varies,
identification will be much easier due to the significantly higher temperature
of
the constituent having the higher absorbing capacity.
Therefore, in a particularly preferred embodiment of the invention a ratio of
thermal images obtained is calculated and compared with a library of IR
spectra.
In certain embodiments it may desirable to obtain a "blank" or reference image
of the rock prior to application of the low level electromagnetic energy. As
such, in some embodiments the method may include a preliminary step of
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imaging the rock to obtain a reference image prior to application of the low
level electromagnetic energy.
One particular application of the method of the invention will be in the
identification of constituents making up a length of an interior wall of a
borehole in situ. Such a method would provide advantages as would be
readily appreciated by those of skill in the art.
Therefore, according to a particular aspect of the present invention there is
1o provided a method of mapping the composition of a borehole including:
applying low level electromagnetic energy to a length of interior wall of
the borehole thereby inducing thermal responses from constituents making up
the length of interior wall;
imaging the thermal responses from the constituents to obtain a series
of thermal images within a plurality of distinctive bands of IR spectra;
interpreting the series of thermal images to identify the constituents;
and
thereby mapping the composition of the borehole.
In another particular application of the method of the invention, the remote
mapping of the composition of a rock formation is provided.
Therefore, according to a further aspect of the invention there is provided a
method of remotely mapping the composition of a rock formation including:
remotely applying low level electromagnetic energy to an exposed
surface of the rock formation thereby inducing thermal responses from
constituents making up the exposed surface;
remotely imaging the thermal responses from the constituents to obtain
a series of thermal images within a plurality of distinctive bands of IR
spectra;
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interpreting the series of thermal images to identify the constituents;
and
thereby mapping the composition of the rock formation.
The present invention also extends to various forms of apparatus that have
been developed for carrying out the methods as described above.
In particular, according to yet another aspect of the invention there is
provided
an apparatus for identification of a constituent within a rock including:
a low level electromagnetic generator/applicator for inducing a thermal
response from the constituent;
an infra-red imaging device for imaging the thermal responses induced
within a plurality of distinctive bands of IR spectra; and
a recording device for recording images produced by the infra-red
imaging device; and/or
a computing device for interpreting thermal images produced by the
infra-red imaging device to identify the constituent within the rock.
The low level electromagnetic generator/applicator may be a microwave
generator/applicator. The microwave generator/applicator may take any
suitable form. For example, this may be a microwave horn, or other
microwave generating device. Preferably, the microwave generator generates
microwave energy at a power density of less than 1000 MW/m3, more
preferably from 10 to 100 MW/m3.
Likewise, the microwave generator preferably generates microwaves at a
microwave frequency of from 895MHz to 245GHz, preferably from 895 to
3500MHz, more preferably from 895 to 950MHz. Suitably the microwave
energy is applied at a microwave frequency of from 895 to 915MHz.
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More suitably the microwave generator generates microwaves at a microwave
frequency selected from 895MHz, 915MHz, 2450MHz, 5800MHz or
24.125GHz.
The low level electromagnetic generator/applicator alternatively may be a
radiowave generator/applicator. The radiowave generator/applicator may take
any suitable form.
Likewise, the radiowave generator preferably generates radiowaves at a
radiowave frequency of from 13.6 to 895MHz, preferably from 400 to 895MHz,
more preferably a frequency of 433.92MHz.
The imaging device may also take any suitable form, for example this may be
any type of spectroscopic device. Preferably, however, the imaging device is
a high resolution infra-red camera with a number of band pass IR filters.
Taking the particular application of the invention to the identification of
constituents within a borehole, in one aspect the invention provides an
apparatus for mapping the composition of a borehole, the apparatus including:
a mapping sonde adapted to be lowered into the borehole;
a low level electromagnetic generator/applicator associated with the
mapping sonde for inducing thermal responses from constituents making up a
length of interior wall of the borehole;
an infra-red imaging device associated with the mapping sonde for
imaging the thermal responses induced within a plurality of distinctive bands
of IR spectra; and
a recording device for recording images produced by the infra-red
imaging device; and/or
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a computing device for interpreting thermal images produced by the
imaging device to identify the constituents making up the interior wall of the
borehole.
Each of the apparatus described above are provided with a recording device
and/or a computing device. It will be appreciated that in some instances
images may be recorded for later analysis at another location, in which case a
computing device for conducting the analysis on site will not be essential.
Likewise, it may be that the analysis is conducted on site in real time, in
which
case it may not be necessary to record the images. Rather the results of the
analysis using the computing device (i.e. the compositional mapping of the
rock, etc.) may be recorded.
DETAILED DESCIPTION OF PREFERRED EMBODIMENTS
A more detailed description of the invention will now be provided. It should
however be understood that the following description is provided for
exemplification only and should not be construed as limiting on the invention
in any way. In the following description reference will be made to the
drawings, in which:
Figure 1 illustrates a graph of rate of microwave induced heating of a
number of minerals;
Figure 2 illustrates a graph of rate of microwave induced heating of
some minerals having low microwave absorption;
Figure 3 illustrates an IR image of a microwave illuminated ore
fragment;
Figure 4 illustrates a TIR image of a number of drill core samples;
Figure 5 illustrates IR spectra for a chalcopyrite sample;
Figure 6 illustrates IR spectra for a pyrite sample;
Figure 7 illustrates IR spectra for an arsenopyrite sample;
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Figure 8 illustrates the combined IR spectra from Figures 5-7; and
Figure 9 illustrates an embodiment of an assembly of the invention.
The present invention relates to a method and apparatus for active remote
sensing, generally based on the short pulse illumination of drill-hole cores,
geological samples, rock surfaces within a borehole or exposed rock surfaces
in-situ, using a suitable microwave source and applicator. The proposed
method and apparatus do not deal with IR/microwave applications related to
the sorting of high and low grade metal ore or waste rock fragments for the
1o purpose of grade increase of ore that will be subject to further mineral
processing. Nor does the invention relate to upgrading oil recovery from oil
containing geological materials.
The invention does, however, have fields of application in mineral/rock type
detection, exploration and mapping and classification of other than metal
bearing ore concentrations within mines and associated mineral processing
plants. As such, hereafter particular reference will be made to the
identification of minerals within a rock, rock body, core drill, geological or
rock
formation and so on. Such references are not to be construed as limiting on
the invention.
The below detailed description of the invention describes the invention when
the low level electromagnetic energy is microwave energy. It will be
appreciated that similar methodologies and apparatus also apply when the
applied low level electromagnetic energy is radiowave energy.
During and immediately following the short pulse microwave illumination of the
rock, drill core, rock fragment or rock mass in situ, IR imaging of the
exposed
rock surface takes place. IR imaging is performed using a high-resolution IR
camera that operates over the spectral range of IR emissivity of the targeted
minerals or group of minerals. The most common spectral ranges will be from
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0.7-2.5 m, 3-5 m and 8-15 m. As noted above, the combined use of short
wave IR (SWIR) and thermal infra-red (TIR) spectral ranges allows for
identification of a wide range of minerals.
The rock surface, drill core, or borehole wall is subjected to short pulsed
microwave' irradiation of low to moderate power density to induce differential
heating which correlates with the presence of microwave absorbing minerals
within the rock surface, drill core etc. The thermal responses of minerals to
microwave illumination vary to a large extent. Experimental results show that
the highest microwave heating rate occurs for carbon (coal) and most metal
oxides. Most metal sulfides heat rapidly as well.
Gangue minerals such as quartz, calcite and feldspar heat relatively slowly
when exposed to microwave radiation. The proposed technique, however,
may, also provide the opportunity to differentiate between ranges of
ferromagnesian and felsic silicates. For example, due to the presence of
different amounts of metals, such as Fe, Cu, Pb etc, within particular
silicate
minerals, the method of the invention will be able to distinguish between
varieties of silicates.
Reference is made to Figures 1 and 2 that illustrate a plot of rate of
microwave induced heating of various minerals.
According to the Stefan-Boltzmann law, emissive infra-red power of a material
can be calculated as:
E=s6T4
Where E(W/m2) is emissive power, c is the emissivity constant of the material
at the particular wavelength and temperature, 6 is constant and T is absolute
temperature of the material. Based on this equation, any increase in the
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temperature of the material will significantly increase the amount of infra-
red
radiation that the material will radiate. Hence, even small variations in the
ability of minerals to absorb microwave energy and convert that energy into
heat will result in significant variation of total infra-red power emitted
from a
particular mineral or group of minerals.
The thermal images that are obtained contain regions of different brightness
(or equivalent false colours). Intensity of electromagnetic flux coming from
the
surface of the rock will be directly proportional to the temperature of the
rock
surface, multiplied by the emissivity of the particular rock or minerals
within
the rock. Based on that it is possible to use microwave induced selective
heating as a parameter for the identification of minerals within the rock. For
example, rocks containing a large proportion of quartz (such as granite) are
characterised by relatively low TIR emissivity (-0.75-0.8), while rocks with a
low content of Si02 (such as basalt and gabbro) are characterised with high
average TIR emissivity (> 0.9).
The infra-red emissivity spectrum of each mineral has a signature
characterised by the position of a number of maximums and minimums in the
spectrum (reference is made to Figures 5 to 8). With an increase in the
temperature of particular minerals, these spectral features (i.e. the position
in
the spectrum) will be preserved, but their intensity will be multiplied by a
factor
determined by the difference in temperature that exists between the specific
mineral phase and the IR sensor. Infra-red images are recorded over several
distinctive spectral bands within the thermal infra-red part of the spectrum.
The recorded information is compared with reference IR spectral data of
various minerals. It is noted that the IR spectra of a wide range of minerals
are
readily available from public domain sources.
Following from the above, the IR images of the illuminated rocks or minerals
of different type will show a substantially improved differential compared
with
non-illuminated rocks. In the case of non-microwave illuminated rocks, the
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difference in IR emissivity between minerals varies in the range of 15-20%,
while in the case of microwave illuminated rocks the difference in IR
emissivity
between minerals is in the order of 50-100% or more. Therefore, the
invention provides for improved delineation between various mineral types.
A difference in the surface temperatures of a rock surface will be evident due
to different rates of absorption of microwave energy of the different minerals
making up the rock surface. Hence, in such a way the method of the invention
facilitates differentiation between minerals which, from the point of view of
1o classical IR sensing, are almost identical. Selective microwave energy
absorption further differentiates minerals, enhancing detection capabilities
of
the system. Using this approach it is advantageously possible to differentiate
among silicate minerals because of differences that exist in their ability to
absorb microwave energy. Reference is made to Figure 2.
Reference is also made to Figures 3 and 4 that provide thermal images of
microwave illuminated samples of various mineral types.
A schematic illustration of an embodiment of the invention is provided in
2o Figure 9. Figure 9 illustrates a mapping or geophysical sonde (1) which may
be used to map rock types and map mineral composition of rock intersected
with a borehole (2). For illustration purposes only the rock types and mineral
composition (3) at or near the borehole wall (4) are shown as being
stratified.
It will be appreciated that rock types and mineral compositions (3) will vary
significantly from location to location. In this embodiment, the borehole must
not be filled with water prior to analysis.
First, a reference IR image is taken over a plurality of bands of IR spectra
before microwave energy is applied to the borehole wall (4). The low level
electromagnetic energy, in this example is microwave energy is then applied,
by a microwave generator/applicator (5) to the borehole wall (4) and
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immediately after each microwave energy application an IR image is collected
using a suitable IR sensor or IR imaging device (6). The infra-red (IR)
imaging device (6) is preferably an IR imaging camera equipped with a
number of suitabie band pass filters
Images are collected within a plurality of distinctive spectral bands,
covering
the thermal infra-red part of the spectrum. Recorded images are sent via
cable into a central recording device (7) for processing and, if desired,
interpretation.
In certain embodiments the apparatus may take the form of a surface device,
handheld or mounted on vehicle, that will apply microwave energy onto the
surface of a rock formation in-situ, ore body outcrop, or rock wall in an
active
mine. The apparatus may simultaneously, and immediately after application
of the microwave energy, collect IR images using a suitable IR sensor. In such
a way the apparatus and method may be used for geological exploration and
delineation and the detection of mineralised zones either within or around
existing mines or at greenfield sites.
The apparatus of the invention, and consequently the method of the invention,
may also be embodied in the form of device for mapping and identification of
minerals in drill hole cores after they are removed to the surface. In such a
case, the drill core may be conveyed through, or against the microwave
generator. During passage through the microwave generator/applicator, short
microwave pulses transfer microwave energy into the rock, thereby inducing a
thermal response. The thermal responses are recorded using an IR imaging
sensor over a number of bands of IR spectra. Based on the recorded IR
responses within selected spectral bands, the minerals within the rock can be
classified. Classification will be performed by comparing the recorded IR
spectral responses with a library of IR spectra for various minerals.
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The apparatus of the invention may also be embodied in the form of device for
mineral identification within rock samples supplied to a laboratory. Rock can
be exposed to microwave illumination using a small scale microwave
applicator. Induced thermal response of the minerals within the rock can be
recorded using an IR imaging device. The recorded images can then be
analysed using a range of filters to extract characteristic spectral features
of
each mineral phase present in the rock. Due to differential heating, IR
spectral
features of particular minerals will be further enhanced. Consequently, the
minerals within the rock can be identified with greater confidence as
compared with current IR imaging techniques.
It will of course be realised that the above description is provided by way of
illustrative example only of the invention and that all such modifications and
variations thereto as would be apparent to persons skilled in the art are
deemed to fall within the broad scope and ambit of the invention as herein set
forth.
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