Note: Descriptions are shown in the official language in which they were submitted.
W091/17437 PCT/GB91!00721
2~8204~
.
APPARATUS ~ND METHOD FOR ASSESSING THE GEOPHYSICAL
CHARACTERISTICS OF A CORE SAMPLE
This invention is concerned with improvements in or
relating to an apparatus and method suitable for use in
assessing geophysical characteristics of a core sample.
Resistivity measurement has become an established
technique for mapping features under the earth's suxface
and equi~ment is mass produced for this purpose, for uqe
"in the fieldll where the geometry of the area does not
impose any restriction, see for example our copending
Patent Application No. 8924934.6.
Recent work has been carried out taking resistivity
measurements of core samples extracted from boreholes.
The size and shape of the samples render the usual
testing process time-consuming and unreliable.
Resistivity is measured in the ground by passing a
fixed current between two electrodes and sensing the
potential difference between two points: the voltage
be~ween these points will be proportional to the
resistivity of the ground between them. When voltage is
measured it is, in effect giving a reading for ground
between lines of equi-potential that pass through the two
WO91~17437 PCT/GB91/00721
2~82~4~ 2
points: these lines of equi-potential should ideally be
parallel planes and equally spaced over the region under
examination (assuming a homogenous material).
When a core sample is removed from the ground it
usually comes out in lengths of a metre or so, with a
diameter of around 10 cm. This is then transported to a
rock laboratory.
The moisture content of the samples has to be
preserved, and so it is cut into lengths of about 20 -
cm, covered in aluminium foil, dipped in wax and
stored in a cold room.
The lengths of the samples are dictated by practical
factors such as natural breaks during transit and the
maximum size that can be easily handled and fit in a wax
bath.
Using the known approach, the current distribution
in the core sample becomes relevant. Practically, a
length of core sample equal to the diameter of the core
cannot be used at either end of the sample because the
current flow in that region is not parallel (see Figure
l); thus approximately 20 cm of every core sample is not
useable and for samples less than 20 cm in length no
useful information can be gathered.
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WO91/17437 PC~/GB91!00721
20~20~
The cost of drilling out core samples is hundreds of
pounds per metre, and so it is desirable to make use of
as much of each individual sample as possible. To do this
the current would have to become parallel nearer to the
ends.
The applicants have discovered that considerable
improvement can be achieved using a multi-electrode
current source. However it is not possible to use simple
approaches, for example a disc electrode, or many
electrodes connected to one terminal of a conventional
current source since variations in contact resistance
result in uneven and unpredictable current distribution.
One of the various objects of the present invention
is to provide an improved apparatus suitable for use in
assessing geophysical characteristics of a core sample.
Another of the various objects of the present
invention is to provide an improved method of measuring
geophysical characteristics of a core sample.
The invention provides in one of its various aspects
apparatus suitable for use in assessing the geophysical
characteristics of a core sample comprising a first array
of first current electrodes, a second array of second
current electrodes, a constant current circuit adapted,
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WO91/17437 PCT/~B91/00721
2082~
in use, to pass a constant current between the electrodes
of said first and second arrays and co~prising first
control means adapted to supply substantially equal
current to each of said first electrodes and second
control means adapted to ensure that the current received
by the second array is substantially equally distributed
between each of saî~i second electrodes, the apparatus
further comprising a third array of third potential
electrodes which, in use, are positioned at spaced
positionS on a sample to be assessed and means for
measuring the potential of the third electrodes.
The invention provides in another of its various
aspects a method for assessing the geophysical
characteristics of a core sample comprising positionlng a
first array of first electrodes spaced from one another
on a first region of the sample, positioning a second
array of second electrodes spaced from one another on a
second region of the sample generally at the opposite
side of the sample to the first region, positioning a
third array of third electrodes at known spaced positions
on a third region of the sample, passing a constant
elec~rical current through the sample from the first
array to the second array, each of the first electrodes
supplying substantially equal currents and each of the
second electrodes receiving substantially equal currents,
and determining th0 potential at each third electrode.
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WO91/17437 PC~/GB91/00721
2~2~4~
The use of standard equipment to measure resistivity
is found to be quite time consuming which is undesirable
not only for conventional reasons but also as it is
detrimental to core samples which tand to dry out while
not sealed. The potential measuring electxodes
furthermore had to be placed in a sample, position
recorded, and readings taken and recorded by hand. Data
obtained was thus sparse in relation to time and effort
involved.
Apparatus in accoxdance with the in~ention therefore
utilises an array of potential measuring third electrodes
and preferably comprises support mounting the potential
measuring electrodes equally spaced one from the next;
individual electrodes are then preferably multiplexed to
known voltage measuring circuits. Once the position of
the support is recorded, the positions of the individual
electrodes are at known positions.
Preferably apparatus in accordance with the
invention further comprises computer means, for example a
portable microcomputer, to read and store potential
measurements and also to carry out any necessary control
functions; the location of the electrode suppor~ can be
entered using this arrangement ~ia the computer keyboard
and the computer means can control the multiplex
arrangement and store data recorded on a disc to be
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WO91t17437 PCT/GB91/00721
2082~ 6
processed later. Preferably all electrode voltages are
meacured relative to the same reference point, for
example earth, so that data will be in the form of a
potential field. Resistivity cannot be melasured using
continuous D.C. in only one direction because of both
polarisation and voltaic effects between the potential
electrodes and the surface of the sample; switched
direction D.C. is therefore preferably used, see Figure
3.
A typical potential wave form between first and
second arrays Cl and C2 of current electrodes is shown in
Figure 3b. The rise and fall shapes are due to electro-
chemical effects. Similar processes cause a drifting D.C.
potential between Pl and P2 which is always present, Ref.
2. The use of switched D.C. means that the 'signal'
voltage can be easily extracted from this back ground
level.
Filter circuits to convert this type of signal into
a form suitable for analog~le to digital conversion are
already known.
In appara~us in accordance ~ith the invention the
current source must operate such that the desired overall
current passes between the curren~ electrodes and this
current must be stable and constant. However it is not
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WO9l/17437 PCT/CB91/00721
2~2~
essential that each individual electrode pass precisely
the same current (although this is of course desira~le).
The size of the current is not important as such provided
that it produces sufficiently large voltages to be
measured but does not saturate the current source. There
is room for error between nominal and actual current but
once the actual value has been measured it must remain
constant.
Various methods providing a substantially constant
current at each individual slectrode may be used, for
exampie individual constant currents sources and sinks
for each individual electrode each having the necessary
accuracy to provide the overall accuracy re~uired for the
total current - however this is an extremely expensive
option and unnecessary because only the total curren~
through the core sample must be carefully controlled. An
alternative is current division (accepting minor
variations in current from electrode to electrode) - the
point at which measurements become possible on a core
sample will hardl~ be affected by this provided total
current flow is constant. Transistors might be usable but
difficulty arising in selecting transistors with
suffici~ntly good matching in ~he necessary current
range.
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WO91/17437 PC~/GB91/00721
2~82~ 8
A preferred method provides a current source and
sink of a simi.lar type provided for the overall current
control, for each current alectrode. ~n adva.ntage of such
a circuit is ~hat because each current elecl:rode has its
own high gain operational amplifier with chain feedback,
virtually any transistor (even ones of different types in
some circumstances) might be used. A current sink on this
principle is shown in Figure 4.
The. same reference voltage, Vref~, is fed to each
electrode circuit, and if the Rf' resistors are 0.1%
tolerance which are reasonably cheap, the output currents
must be very similar. If the op-amps are assumed to be
ideal then by inspection,
Rf'Io/N - Vref-Vref'
where Vref-Vref~ is identical for each electrode. Thus
the spread of Io/N will be the same magnitude as the
spread of Rf'.
The use of individual feedback for each electrode
also overcomes the early intercept voltage problem, which
could be significant where the load has such vari~tion in
contact resistance and electro-chemical effects.
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W091/17437 PCT/GB91/0~721
208204 b~
The total current is exactly the same as for one
current source using a precision voltage reference and Rf
resi~tor because as far as the overall circuit is
concerned, all the electrode current sources together act
just the same as one transistor.
The main advantage of this circuit is of course that
one precision reference voltage and resistor, control the
current through all electrodes and the splitting circuit
uses cheap components, but still provides acceptable
matching.
The circuit of Figure 4 is in fact a current sink,
however, it can be transposed to act as a current source,
working in the positive half of the voltage supply for
the final multi-electrode sink-source circuit.
Obviously the sink and source would have to be as
closely matched as possible, but perfect matching is very
difficult.
In a preferred apparatus the current sink is made to
match the current source by using feedback and error
detection. To this end, the core sample is preferably
provided with an earth electrode fitted to the current
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WO91/17437 PCT~GB91/00721
~0~20~
sink. Any error in matching produces a current to or from
earth which is sensed and used to control the current
sink (see Figure 5).
Ideally, in the steady state Ierror=O and so Ve=O.
The ou~put of Al has assumed a level such that
(Vref'/Rf') x N = Is.
Assu~e now that the current sink starts to take too
little current. The current source is still sending out
its fixed current and so the excess flows to earth. The
current has to flow through Re and if this is large, say
lMQ, then a sizeable voltage across it is produced. The
direction of this voltage sends the output of Al upwards.
This turns the current sink harder on, negating the
initial drop in current and therefore reducing the error
current to earth. -
Similarly, if the circuit begins to sink too much
current its drive is reduced by the action of the
feedback.
Again, as with the current source, cheap components
which provide quite acceptable matching can be used,
while the total current is kept accurate by making Ierror
approach zero.
W091/17437 PCT/GB91/00721
2082~
11 . .
If Re is IMQ and a moderate performance op-amp is
used where Vos =lOmV then
Ierror = lOmV/lMQ = lOnA.
The major source of error is the input bias currents
of the op-amp, say lOOnA.
The constan~ current through ~he load is likely to
be over lmA, this produces an overall error due to
current leakage of
(llOnA/lmA) x 100~ = O.011%
Conveniently the support for the potential
electrodes is arranged to contain 64 electrodes (although
more may be used if desired); suitably these are arranged
in a 4 x 16 grid. In the case of the large grid it is
simpler to measure the voltage at each electrode with
reference to a common point for example earth; voltage
between any two electrodes can ~hen be calculated la*er
using Kirchoff's laws. Only one multiplexor may therefore
be required to send signals to the analogue signal
processing circuit (see Figure 6 for example).
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WO91/17437 PCT/GB91/00721
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20~2~'15 12
As the preferred apparatus requires alternating D.C.
current flow, a preferred apparatus comprises switching
means. Any suitable switching means may be used to switch
the current in direc~ions between the first and second
array of current electrodes. Apparatus in accordance
wi~h the invention is not liXely to require voltages
exceeding + 30 volts and currents will normally be less
than ~0 mA. Either relays or analogue switches can be
used without difficulty: the switching frequency is
likely to be a few hundred Hertz - slower switching will
increase time taXen to carry out measurementc~
excessively. Analogue switches (which do not suffer from
contact bounce) are preferred at this frequency and also
because they can be switched directly.
A preferred apparatus also comprises circuits
monitoring each electrode. It is possible for an
individual first or second electrode to fall from the
core sample or to make poor contact. Preferably current
is monitored through each electrode, suitably by
monitoriny the voltage across each Rf' resistor (see
Figure 4); should any electrodes fail the same total
current will still flow but remaining electrodes will
carry a larger current. A suitable monitor circuit is
shown in Figure 7 using a multiplexor: when any address
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W0~1/17437 PC~B91!00721
13 - 2~82046
is sent to t.he circuit it outputs the Rf' voltages for
the appropriate electrodes of the same source onto
separate output channels.
A preferred apparatus comprises digit:al lines for
the D.C. switching, potential electrode multiplexing,
current source monitor and the analogue signal
multiplexing. All of the chips for the relevant functions
are available with latching inputs controlled by a wR
line. This means that the number of digital lines
required is that for the largest address (AO-A5 for 64-l
multiplexor) plus decode lines for the various wR~s. The
circuits which handle the analogue signal processing also
require a few digital lines.
The preferred computer has a digital I/O card with 3
x 8 input/output blocks which can all be outputs, so
there is no shortage of available lines.
There now follows a detailed description to be read
with reference to the accompanying drawings of apparatus
suitable for use in a assessing geophysical
charac~eristics and a method of using the apparatus. It
will be realised that this apparatus has been selected
for description to illustrate the invention by way of
example.
WO91/17437 PCT/GB91/0~721
2 0~2 ~ 4~ 14
In the accompanying drawings:-
Figure l is a diagrammatic view showing the use ofsingle current electrodes to investigate a core sample;
Figure 2 is a diagrammatic view showing multiple
electrodes; ~.`
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Figure 3 is a diagrammatic view showing various D.C.
switching wave forms;
Figure 4 is a circuit diagram showing a current
sink;
Figure 5 is a circuit diagram of a self balancing
current sink;
Figure 6 is a diagrammatic view of a multiplexor
circuit for use in monitoring potential electrodes of
apparatus ernbodying the invention;
Figure 7 is a diagrammatic view of a circuit for
monitoring current electrodes;
Figure 8 is a block diagram of the circuitry of the
illustrative apparatus;
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. W092/~7437 PCT/GB9l/00721
2~82~
Figure 9 is a circuit diagram of a current sink of
the illustrative apparatus;
Figure 10 is a circuit diagram of a current source
of the illustrative apparatus;
Figure 11 is a diagrammatic view of a first
experimental core test;
Figure 12 is a diagrammatic showing an alternative
arrangement of electrodes;
Figure 13 is an oscillocope trace showing voltages
between various electrodes in the use of the illustrative
apparatus; and
Figure 14 is a view showing a swtiching circuit of
the illustrative apparatus.
The illustrative apparatus comprises a cur~ent
source and sink, first and second arrays of first and
second current electrodes, each comprising seven
electrodes, a support for third potential electrodes
namely a potential pad/ and other circuits as outlined
above.
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WO91/17437 PCT/GB91/00721
2~820~ -
16
The potential pad is cQnstructed from pcb test point
probes se~ into a piece of vero board at 0.2'l spacing
(i.e. every other hole) and potted up. The potential
electrodes are connected in sets of eight via ribbon
cable to the multiplexing board.
Alternatively, single ribbon cable with ~uro card
type connector and purpose build pcb could be used. There
are preferably seven electrodes in each of the first and
second arrays, in each case six of the electrodes being
arranged at the apices of a hexagon with the seventh at a
central region of the hexagon. In use of the apparatus
the first array of electrodes is posit.ioned in contact
with one end face of the cylindrical core sample and the
other array of electrodes at the opposite end face.
In the preferred current source and sink the
reference voltage is provided by a lN827 6.2V temperature
compensated zen~r diode. The break down voltage is
extremely stable at lO ppm/C. The absolute accuracy is
5%. A lmA constant current FET J505, is used to minimise
effects from variation in supply voltage and a 4.7nF
capacitor put across the zener diode to smooth out high
frequency noise.
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WO91/17437 PCT/GB91/0072l
2082~4~
17
The op-amps providing overall control should be of
fairly high quality with a low offset voltage and an
1~301 was chosen. The other op-amps can be of a lower
standard as long as their bias currents do not become
significant compared with the current controlled by them,
or their offset voltages significant compared to the volt
drop across the Rf' resistors. 741 op-amps were chosen
due to their ready availability and cost.
The transistors used are VN46AF VMOS transistors
which can handle up to 2A and 15W. It might be expected
that if the sink circui~ uses N channel FETs then the
source circuit, being as it were a reflection, would use
P channel FETs. However, discrete P channel enhancement
mode transistors are fairly scarce compared to N channel
ones and to avoid any future complications, the source is
modified to work with N channel devices. This is simply
achieved by swapping connections to the + and - terminals
of each op-amp controlling a source ~ransistor.
The choice of Rf and Rf~ depends on the current
required. Initial tests were to be carried out passing
current through a vessel of saturated sand, and 5-lO mA
is suitable for ~his medium, thus
Rf = 6.2V/6.2mA = lKQ
.
WO91/17437 PCT/GB91/00721
2082~4~ -
18
If Rf' = Rf then each Rf~ drops
.
Vrf' - Vrf/No electrodes.
For seven elec~rodes,
Vrf' = 6.2/7 = 0.86 V.
~ ., :
This voltage is very much greater than the offset
voltage for 741 op-amps, and in itself would only give
rise to a mall spread in current between electrodes.
Thus for seven electrodes it is convenient to put Rf' =
Rf. Of course Rf is O.Ol~ and Rf' is O.l~ tolerance for a
final circuit, so they are only equa} in nominal
magnitude.
All the other circuits comprise analogue switches
and multiplexors.
The D.C. switching circuit (Figure l4) must be able
to pass a fairly large current in terms of analogue
i switches, which in general are used to feed voltage
signals into high impedance loads.
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W09~/17437 PCr/GB91/00721
2~2~6
19
i
The ADG221KB is a suitable chip, it has a latching
input, and contains 4 SPST switches. This means that one
chip can be used to do the switchiny for one electrode
pad. The chip can switch up to 30mA continuous D.C. at
44V, and is therefore within the demands.
The other multiplexors used are four 16-1 and three
8-1.
The criteria for choosing the chips was restricted
by the voltage that they could work up to. The supply
might ~e between + lOV to + 30V. The chips selec~ed are
DG 526 DJ 16-1 multiplexor and the DG 528 CJ 8-l
multiplexor.
In the current sink and source circuits it is
important to ensure that the primary operational
amplifier (LM301 in Figure 10) is sufficiently slow that
correcting action of the other operational amplifiers
(741 in Figures 9 and 10) is virtually instantaneous
compared to the change in the primary operational
amplifiers.
D.C. switching circuit of the illustra~ive apparatus
was controlled using a TTL signal generator; the output
square wave was passed through an invertor and both
levels spread to control lines.
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W091J17437 PCT/GB9llO0721
2 ~2 ~L~ 20
Using the circui~ shown in Fiqures 9 and 10 overall
current flow was set to about 6 mA using a lKQ ~f'
resistors and ~f resistors. Results were as ~Eollows:-
I(mA) I(mA)
Cl 1 0.87~ C2 1 -0.880
2 0.872 2 -0.878
3 0.876 ;3 -0.877
4 0.878 4 -0.872
0.876 5 -0.876
6 0.873 6 -0.872
7 0.877 7 -0.876
__
6.130 6.131
Spread from 0.880 to 0.872 = 0.917%
(Ignoring C21 the spread is 0.688%)
Ierror = 32.9nA (32.9mV across l.OOMQ
Ierror is 0.00054% of C1 or C2
The tes~ was carried out using saturated sand as a
load. Ierror was calculated by measuring the volt drop
across a l~Q resistor placed in series with the electrode
because the current i5 too small to measure directly on
normal equipment.
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WO91/17437 PC~/GB91/00721
~0820~
21 -
The measurements were taken with a 3~ d;git DVM.
The current distribution is within the ~olerance of
the Rf~ resistors at under 1%. The sum of the Cl and C2
electrodes is same to within l part in 6000. The
difference is most likely due to the inaccuracy of the
last digit on DVMs. The error current which will in fact
be the real difference between Cl and C2 gives rise to a
missmatch of 5.4 ppm.
The matching between electrodes on the same side is
quite acceptable, and the matching between Cl and C2 is
very good indeed. To get similar matching using separate
sources would require very well matched components indeed
and would be very expensive. The l~ resistors used for
this test cost 2p each.
In carrying out an illustrative method the firs~ and
second arrays Cl, C2 of current electrodes were pushed
into the ends of a cylindrical core sample in the
hexagonal arrangement referred to abo~e and the potential
electrode pad held in place. Electrode selection was
achieved by using the multiplexor under the control of
computer means which also operate~ the D.C. switching
operation. Outputs from each potential electrode were
recorded by the computer.
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W091/17437 PCT/GB91!0072]
20~34~ 22
A test on a core sample using a single position of
the pad was carried out. Visual inspection of the site
before hand showed a fine crack as in Figure 11.
The voltage field as measured directly shows the
crack to some extent but since the voltage drops
consistently along the core it is not obvious. The image
can be enhanced by taking initial and final values of
voltage along the core and using the straight line
equation
y = mx ~ c
between these points. Thus deviations of the potential
field from its ideal linear path are plotted in the
Suppressed Gradient map.
However, it is the elec~ric field or potential
gradient that is actually proportional to resistivity.
The component of electric field along the core can
be obtained by taking the difference between successive
readings. The resulting map clearly shows the small crack
in an otherwise fairly uniform f.ield.
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WO91t17437 PCT/GB91/00721
2~820~6
23
~ following test was carried out using multiple
adjacent positionings of the electrode pad to cover a
larger area (one of apparent uniformity). The same maps
were produced for this test as the first.
The resulting plots show an overall backyround level
with a feature running across near ~he middle. Th~s is
probably hair line cracks which have occurred in transit
or are due to drying out during testing.
One further test was carried out: the measurement
was taken on the end of the core, see Figure 1~, and
therefore measures horizontal rather than vertical
resisti.vity. This test has the potential to show the
orientation of the fabric of the core.
The various plots show the expected features and the
visible crack in the first test is readily identifiable.
The horizontal test shows roughly parallel lines of
potential as would be expected from a uniform current.
This makes interpretation a lot easier and could not be
achieved with a two electrode current source whatever the
length of the core.
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W09ltl7437 PCT/GB91!00721
2~82~ 24
The cost and complexity of the system has been kept
to a minimum without compromising its accuracy or
stability, largely due to the use of current division and
matching in the current sink-source by means of op-amp
circuits employing feedback. The resulting current sink-
source can, in principle, be extended to have any number
of outputs which, while maintaining individual matching
and balance overall, still use a single voltage and
resistance raference.
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