Detecting and measuring the position of a break in solid formations by measuring the capacitance of an enlongated element embedded therein

This invention relates to an apparatus and method for detecting and measuring the position of a fracture in solid formations, such as for example a grout filled borehole. An electrical element is placed the length of the borehole prior to filling the hole with grout. When the hole is filled and the grout solidified, any fracture in the grout also breaks the electrical element. The element is constructed such that its electrical capacitance is a known function of its length. The element is constructed of easily frangible materials so that it is severed in close proximity to the grout fracture. The capacitance of the element after the fracture can be measured using suitable instrumentation. The position of the break is a function of the capacitance as measured after the break.

BACKGROUND OF THE INVENTION 
In many applications involving large solid structures it is desirable to 
detect internal disturbances. It is additionally valuable to be able to 
measure the position of such disturbances without further disturbing the 
integrity of the structure. In mining and construction it is often 
required to detect faults or disturbances and their positions in earth and 
rock formations. To detect such faults an electrical element can rigidly 
be embedded into the solid structure or earth. When a portion of the solid 
breaks or moves relative to the electrical elements it causes the 
electrical element to shear or break. Prior methods have used a ladder 
like arrangement of parallel resistors as taught by U.S. Pat. No. 
3,477,019. Such systems lack accuracy because of their use of discrete 
components. The accuracy of such system is directly proportional to the 
number of components and cost. Because these devices determine position by 
measuring resistance between parallel conductors, any shunting resistance 
such as moisture paths between the conductors results in an error in 
position determination. My invention overcomes these inherent faults by 
using capacitance as a measure of position and by using one continuous 
inexpensive element. 
An elongated electrical element is embedded in a solid formation. This 
element is made of easily shearable materials and has a capacitance that 
varies as a function of length. The element acts as an end-feed capacitor 
having accessible leads attached to the conductive surfaces at one end of 
the elongated element. When a meaningful disturbance occurs in the 
formation the frangible element breaks in the area of the disturbance 
effectively severing the element into at least two lengths and reducing 
the effective capacitance connected to the leads. The position of the 
break can be determined by measuring the capacitance at the leads and 
relating it to the length by the previously known function. 
It is often required to know the position of a disturbance in a solid 
formation such as earth, rock, or formed conglomerates. This information 
is especially valuable in excavations such as are found in mining and 
construction for example. In such applications boreholes are normally 
drilled to test or reinforce the strata or solids. In one application of 
my invention an elongated electrical element is securely embedded 
generally axially within these boreholes. The electrical element does not 
interfere with other objects such as for example, structural rods or bolts 
that may be included within the same borehole. A hard cementitous material 
such as for example concrete can be used to embed the electrical element 
within the borehole. If it is desired to detect a very slight earth 
movement or no additional strengthening of the earth formation is 
required, a weaker, more brittle material can be used as the embedding 
grout. The grout used need only be capable of transmitting the movement or 
force of the disturbance in sufficient amount to shear or break the 
electrical element. The electrical element is made of frangible materials 
that are easily severed by the forces present during a detectable 
disturbance. Firmly grouting the electrical element into the solid and 
constructing the element from easily shearable material, causes the 
element to sever at a point corresponding to the location of a disturbance 
or movement in the solid. 
The point at which the electrical element is severed can be calculated by 
comparing the electrical characteristics before the disturbance with those 
after. The electrical element is constructed so that the electrical 
characteristics are of a known function of the physical length of the 
elongated element. While any known function is sufficient, it will be 
desirable to use a continuous linear function so as to simplify the 
calculations. In certain applications it may be desirable to use a 
non-linear function which better suits the physical parameters of the 
disturbance; such as designing the element so that the electrical 
characteristics vary greater per unit length in the area where 
disturbances are anticipated so that the highest resolution and accuracy 
will be obtained in that area. 
Variation in the capacitance for non-linear elements may be made by varying 
the distance between electrical conductive surfaces, increasing or 
decreasing the area of the electrical conductive surface, or using 
electrical insulating material of varying dielectric constants between the 
conductive surfaces. 
If the elongated element is constructed to have a capacitance which is a 
linear function of the elongation, such as a parallel plate capacitor; 
then the position of the disturbance is directly proportional to the 
capacitance measured at the exposed leads. Additionally, the ends of the 
capacitor plates opposite the exposed leads can be shorted together to 
allow a continuity check from the exposed leads. A positive continuity 
check indicates an unbroken capacitor and no further capacitance 
measurement need be taken. 
Accordingly, one object of this invention is to provide a means to 
accurately and economically determine the position of a movement within a 
solid formation. 
Another object is to provide for the detection of breaks in the grout 
material as a function of the elongated frangible element. 
Another object is to provide an inexpensive electrical element that can 
readily be inserted into a borehole and easily fashioned to the exact 
depth of the borehole. 
Additional objects and features of the present invention will become 
apparent to those skilled in the art as the following description of 
certain present preferred embodiments thereof proceeds.

Shown in FIG. 1 is an embodiment of a parallel plate element that uses the 
characteristics of capacitance and continuity to detect a fault and 
determine the position of the disturbance. The element generally indicated 
by the reference 12, has been drawn broken to effectively show both ends 
of the elongated element. The element 12 can be of any length from a few 
feet to several hundred feet. The center portion of this element is 
composed of a dielectric strip or tape 13 which generally extends the 
length of the element. This dielectric strip may be, for example, a paper 
or plastic tape. While a dielectric material of any thickness will 
function in forming a capacitance in this parallel plate element, a 3 mil 
thick tape will tend to create a reasonable capacitance per linear foot of 
the elongated element and allow the element to be flexible during storage 
prior to installation. 
Firmly attached to each side of the dielectric strip 13 are parallel 
electrically conductive means or foils, 14a and 14b for storing electrical 
charge. The conductive foil strips or commonly referred to as tapes 14a 
and 14b have respective conducting surfaces 14c and 14d abutting the 
dielectric strip 13. The conductive foils and respective conducting 
surfaces are held in fixed relationship to the dielectric strip 13 by 
adhesive layers 17a, 17b. 
The conductive foils 14a and 14b can be made of any electrically conductive 
material that is frangible when subjected to the forces present in the 
particular disturbance desired to be detected. The conductive foil is made 
so as to sever generally transverse to the length of the element in close 
proximity to differences in forces along its length. These forces may be 
caused for example by a displacement in the grouting material resulting 
from a shift or fault movement in the solid. While only one conductive 
foil need be frangible it will usually be desirable to have both 
conductive foils made of similar materials. 
At one end of the element corresponding for example to the maximum depth in 
a borehole installation, the conductive foils 14a and 14b are electrically 
connected by shorting means, for example, shorting fold 16. While any 
means for electrically connecting these two conductive foils may be used, 
such as wiring, stapling or mechanically joining, it has been found that 
removing a portion of the dielectric allows one conductive foil strip such 
as 14a, to be folded on the other conductive foil strip such as 14b. 
At the end of the element 12 opposite the shorting fold 16, the two 
conductive foils 14a and 14b have been extended to produce connecting ends 
15a and 15b, respectively. These connecting ends 15a and 15b can be either 
directly connected to electrical instrumentation or connected to other 
conductors or terminals that provide for connection to electrical devices. 
In other embodiments provision can be made for connecting wires or 
terminals directly to the ends of foils 14a and 14b without extending the 
foils. In such embodiments, the object is to provide connective means for 
the respective ends of the conductive surfaces. This allows the conductive 
surfaces to be electrically charged or discharged through the connective 
means into instrumentation which measures the capacitance of the element. 
Such connective means also allows a test of the continuity through the 
path of series connected conductive foils 14a, shorting fold 16 and 
conductive foil 14b. 
FIG. 2 is a cross-section, of a parallel plate element similar to that 
shown in FIG. 1 taken transverse to the elongations showing the respective 
layers within the parallel plate element 12. Dielectric strip 13 is 
intermediate conductive foils 14a and 14b. Inner conductive surfaces 14c 
and 14d, of 14a and 14b respectively, are held in parallel arrangement 
with the dielectric strip 13 by the adhesive layers 17a and 17b. The 
capacitance characteristics of the parallel plate element is readily 
visible in the laminated arrangement of FIG. 2. The distance between 
conductive surfaces 14c and 14d correspond to the distance between 
conducting surfaces in a parallel plate capacitor. The capacitive 
dielectric is composed of the dielectric strip 13 and the adhesive layers 
17a and 17b. 
It is well known that the capacitance, C, for such a parallel plate 
capacitor is calculated by the formula C=KeA/D, where K is the dielectric 
constant, e is the permittivity constant, A is the surface area of one of 
the conductive surfaces and D is the distance between the parallel plates. 
If the electrical element is constructed as shown in FIG. 2 with uniform 
cross section throughout its length, then the equation for capacitance 
becomes C=(KeW/D)L wherein W is the width of the conductive surface and L 
is the length of the conductive surface. While any value of capacitance 
can be used values in the range of 1 to 100 picrofarads per inch are 
easily obtainable. Some installations may use such elements having much 
larger electrical capacitance per inch. Depending upon the length of the 
solid being monitored and the magnitude of the break or fracture desired 
to be detected, larger elements and corresponding larger capacitance could 
be used. In mining installations a convenient size element can be made 
using conductive foil less than 5 mil thick and less than one and one half 
inches wide, with a dielectric of similar thickness and less than two 
inches wide. 
In referring to FIG. 2 is should be noted that in cross-section the 
dielectric strip 13 has a larger width than the respective conductive foil 
41a and 14b. The wider dielectric strip in this embodiment acts as a 
protective barrier between the respective conducting surfaces 14a and 14d. 
In a one inch wide dielectric strip is used with one half inch wide 
conductive foils centered on the dielectric, a one quarter inch barrier 
exists to prevent the foils from shorting together along each edge of the 
element. 
It has been found that 3 mil hard aluminum foils as the conductive foils, 
and 3 mil paper tape as the dielectric strip with a 1 mil adhesive layer, 
produces a parallel plate element having good flexibility during insertion 
into the borehole and such strip is easily sheared by disturbances in the 
solid, for example earth disturbances in mining installations. 
FIG. 3 shows a cross-section of another embodiment in which a parallel 
plate arrangement is encased in an outer insulation. The dielectric slab 
51 is intermediate the two conductive plates 52a and 52b. An outer 
insulation covering 53 has been added. Such an insulation can be used to 
add additional rigidity to the element and/or simultaneously to protect 
the element from intrusion of water, acid, gas or other foreign materials. 
The element shown in FIG. 3 has a capacitance that can be calculated by 
the same equation as given for the element in FIG. 2. 
FIG. 3 is one example of an element that does not use an adhesive to 
maintain the proper spacing between the conductive means such as plates or 
foils for example. The adhesive layers may be omitted if the conductive 
means is bonded directly on the dielectric such as when a conductive metal 
coating is used as the conductive means. In some embodiments the 
dielectric itself is the adhesive as when the conductive foil is attached 
to a plastic dielectric, for example. 
Referring now to FIG. 4 there is shown a cross-section of an element having 
a coaxial arrangement. The coaxial element is composed of a center 
conductor 56 having a circular outer conducting surface 56a. Coaxially 
surrounding the center conductor 56 is a dielectric tube 57 having uniform 
wall thickness. Concentric with the dielectric tube is outer conductor 
sleeve 58 having an inner conducting surface 58a. The element is then 
encased in an outer protective covering 59. The protective covering could 
be omitted if the outer conductor 58 is made sufficiently durable for the 
specific application. While the embodiment shown in FIG. 4 does not have 
an adhesive layer shown, such a layer could be used. 
If the coaxial element has a uniform cross section throughout its length, 
then its capacitance will be a linear function of the length of the 
element. Such a coaxial element has a capacitance given by the equation 
C=2.pi.KeL/ln (a/b). Where a is the radius of the inner conductor; and L 
is the total length of one of the conductive surfaces; and b is the 
distance from the center of the inner conductor to the inner surface of 
the outer conductor; and K is the dielectric constant; and e is the 
permittivity constant. 
Similar to the element shown in FIG. 1 a coaxial element can have the 
conductive means or one end shorted so as to allow for a continuity check 
prior to capacitance measuring. This can be done by electrically 
connecting one end of the center conductor 56 to an adjacent surface on 
the outer conductor 58. 
While two specific types of elongated elements, parallel plate and coaxial, 
have been described it is to be understood that elements composed of 
variations of these or other known types of capacitor design are included 
within the scope of this invention. Such other embodiments would include 
elements wherein at least one of the conducting surfaces is a rigid 
support member such as for example an anchoring or roof bolt. In such 
systems only one of the conducting means in the element need be frangible 
or easily shearable. Depending upon the desired accuracy and dimensions 
some applications may use the actual grout material itself as a dielectric 
between two conducting surfaces. 
While an equation for the capacitance, such as the two stated previously, 
can be determined for any given geometric structure by an analysis of 
electric fields using Gauss's law. Such equations are not necessary to 
determine the capacitance as a function of length of the elongation as 
empirical methods can be used. After any element has been formed having 
uniform cross-sectional dimensions and materials, such an element will 
have a capacitance which is a linear function of the length of the 
elongation. Using instrumentation the capacitance of the complete element 
can be measured; such measurement may be taken after the element is 
inserted into the solid and prior to any disturbance. This measured total 
capacitance divided by the total length is a constant u. For linear 
varying capacitance this relationship can be written as C=F(x)=ux, where x 
is the length of an element. 
Since the constant u is the same before and after the break, this equation 
can be used to solve for the value of x after the break by dividing the 
capacitance measured after the break by the constant u. The new value for 
x will indicate the length of the element after the break. 
While any dielectric material can be used it may be desirable in some 
applications to use a highly breakable dielectric such as glass. Generally 
a somewhat flexible dielectric such as paper or plastic will result in 
easy storage prior to insertion of the element into a borehole. The use of 
materials such as aluminum foil, and plastic tape allow for easy 
transportation and cutting of laminated materials to the exact length at 
the insertion site during grouting operations. 
FIG. 5 shows a cross-section of an installation of a parallel plate element 
firmly embedded in a grout filled borehole. The borehole 21 is drilled 
into the face 26 of an earth formation 20. A parallel plate element 12 is 
inserted into the borehole 21 so that the element 21 extends generally 
axially within the borehole 21. When the element is in proper position the 
hole is next filled with the grout material 22, such as for example, 
concrete. The embodiment shown in FIG. 5 has provision for the borehole to 
be filled with grout material 22 by means of hollow grout pipe 24. A seal 
plug 27 is fitted into the mouth of the borehole and has provision for the 
grout pipe 24 and the connector 25 to extend through the seal plug 27. 
While many means can be used to provide electrical connection from the 
element 12 to a position outside the borehole at the face 26, the 
embodiment in FIG. 5 uses electrical connections 23a and 23b which are in 
electrical contact with the respective conductive foils of 14a and 14b and 
the grout pipe 24 and connector rod 25. When so connected an electrical 
current path exists through the series arrangement of grout pipe 24, 
electrical connection 23a, conductive foil 14a, shorting fold 16, 
conductive fold 14b, electrical connection 23b and connector rod 25. 
If a continuity detector is connected to the grout pipe 24 and the 
connector rod 25 as shown in FIG. 5, such continuity detector will show a 
very low resistance in the current path. This low resistance indicates 
that the element 12 is intact and no disturbance of the earth has 
occurred. In normal testing it would not be necessary to take further 
capacitive measurements; embodiment not using shorting fold 16 would 
indicate the total capacitance C, corresponding to an element of length L. 
The total capacitance, C of the element 12 is known either from calculation 
or from actual measurement prior to any break of the element. If an 
element having its capacitance varying as a linear function of length such 
as in FIG. 5 is used, then the capacitance per unit length, u, is C/L. 
FIG. 5 shows a typical installation with an element length L where the 
element is set back or recessed a distance S from the face 26. While the 
scope of this invention encompasses any distance L, experimentation has 
been done in which L varied from a few feet to several hundred feet. In 
installations where L is several hundred feet or where accuracy is not 
critical the dimension S may be neglected in calculating the position of a 
break. In other installations the end of the element 12 may be extended to 
the face so that the set back dimension S is zero. 
The installation shown in FIG. 5 is in a vertical borehole having an open 
bottom. Such an installation would be typical of a mine roof bolt hole. 
Other installation sites would include for example horizontal boreholes, 
vertical top opening holes, and cast concrete structures. 
Referring to FIG. 6, this shows the installation of FIG. 5 after a 
disturbance has occurred in the earth formation. The displaced earth 20b 
has caused the grout material to separate into three sections, a severed 
grout 22a, a displaced grout 22b and a remaining grout 22c. The displaced 
grout 22b has caused the element 12 to break into three portions. A 
portion of the element 12b has become displaced and severed; and a portion 
12a has become severed. The remaining portion 12c is intact and 
electrically connected to the connector rod 25 and grout pipe 24. Because 
the severed element 12a containing the shorting fold 16, is no longer in 
electrical connection with the conductive foils of element portion 12c, a 
complete current path does not exist between the grout pipe 24 and 
connector rod 25. When an ohmeter or other means for indicating continuity 
means is attached to the grout pipe 24 and connector rod 25, an open 
circuit is indicated by the high resistance measured. The continuity 
detector could be any known circuit such as for example, a lamp or voltage 
source in series, or an ammeter and battery in series. In normal 
installations a simple continuity test can be made to detect disturbances 
as a prerequisite to the more exacting capacitance measurement. A series 
of such elements may be electrically interconnecting so that an automatic 
monitor of continuity could indicate a disturbance in the system. 
To determine the position of the break in element 12 of FIG. 6 a 
capacitance measuring instrument is connected to the connector rod 25 and 
the grout pipe 24. The capacitance measured after an earth disturbance or 
fault, herein referred to as the break capacitance, is related to the 
length of the element portion 12c by the same function as previously 
calculated or measured for the unbroken element 12. The distance B can be 
found by substituting into that equation the break capacitance and solving 
for the length of the element which corresponds to B in FIG. 6. The sum of 
S and B will correspond to the position of the break. 
While the earth movement in FIG. 6 has resulted in a single stratified 
movement of earth 20b, actual earth disturbances may cause additional 
movements or be of such magnitude so as to displace both section 12a and 
12b of the element. The procedure previously described allows position 
determination of the disturbance in closest proximity to the face 26. 
Should the installation allow additional electrical connectors to the end 
of the element opposite the face such as on portion 12a in lieu of the 
shorting fold 16, then the position of the break area could be ascertained 
relative to both ends of the element by using the measured break 
capacitances of both portions 12a and 12c. Such additional electrical 
connector would be brought out of the solid in a direction opposite the 
face 26, so as not to be affected by the disturbance. Such additional 
connection would allow the portion 12a to be measured in the same way as 
12c. These additional electrical connections can be brought to the face 26 
or another measuring position in any manner such that they do not 
electrically disconnect during a disturbance. 
In the preferred embodiments specifically described the position of the 
disturbance is indicated by a reduction in the measured capacitance of the 
element. This reduction occurs when at least one of the conductive 
surfaces of the element is severed. In the drawings, both conductive foils 
and surfaces, and the dielectric have been shown as severed, but it is to 
be understood that only one surface need be broken to indicate the 
position of a disturbance. For this reason it may be desirable, where fine 
sensitivity is required to have one conductive surface made of a thin 
conducting metal coating which is easily broken. 
FIG. 7 shows a block diagram for a disturbance tester connected to an 
element similar to FIG. 6 after a break has occurred. The tester uses a 
switch or mode selector 43 to electrically connect one of three circuits 
to the elements by means of connections 23a and 23b. The element having 
original length L is shown severed having a remaining length B. The 
element is composed of a dielectric slab 13 intermediate two conductive 
foils 14a and 14b. At the end of the element opposite the electrical 
connections 23a and 23b is a shorting fold 16. 
Prior to the fault the capacitance and length of the element have been 
determined and recorded. In the normal sequence of operations the tester 
is periodically connected to the element preferably keeping all leads 
relatively short to avoid stray capacitance. The mode selector is placed 
in position "a" so as to connect the continuity detector 40 to the 
element. The detector 40 may be any known means for continuity indication 
such as for example an ohmeter. If the element is unbroken, a low 
resistance current path exists between conducting foils 14a and 14b 
through the shorting fold 16. If as shown in FIG. 7 the element is broken 
the detector 40 will show a high resistance path or open circuit 
indicating an earth disturbance. 
If the detector 40 indicates an open circuit the operator changes the mode 
selector 43 to position "b" thereby connecting a capacitance measuring 
device 41 to the element. The capacitance measuring device can be of any 
known type and for ease of operation can be calibrated so that it reads 
directly in units of length. If the capacitance measuring device reads in 
units of capacitance, the length B can be calculated by the equation 
previously given. 
In some environments, especially where moisture is present, the dielectric 
strip 13 may develop leakage current paths, represented in FIG. 7 as RL. 
This leakage path shown as a leakage resistance RL, is often present when 
a moisture absorbant material such as paper is used for the dielectric. 
Leakage resistance should generally be larger than 20,000 ohms to 
facilitate accurate capacitance readings from measuring device 41. If the 
leakage resistance is low, indicating sizable current paths between 
conductive foils 14a and 14b, an electrical heating power source 42 may be 
connected to the element by position "c" on the mode selector 43. This 
source 42 provides current to produce I.sup.2 R, resistance heating within 
the element. 
FIG. 8a is a block diagram of a circuit for a capacitance measuring device 
which can be used as the device 41 in FIG. 7. The square wave generator or 
SWG 70 in the form shown is of the type with an output frequency which is 
a function of the connected capacitance C. The output pulses of the SWG 70 
are used to trigger a gating module GM, reference numeral, 73. The period 
of the output of SWG 70 is a known function, usually linear, of the 
capacitance C. This period is used as the "on" interval for the GM 73. The 
clock, 71 feeds a series of pulses to the scaling function generator or 
SFG 72 which scales the frequency so that the readout will be in proper 
engineering units of length. The output of the SFG 72 is permitted to pass 
through the gating module 73 during a period of the SWG. This string of 
pulses is proportional to C and are counted on the digital counter or DC 
74. This count can then be shown on the digit display or DD 75. The actual 
circuits used in each of the blocks 71 through 75 are well known in the 
art and a variety of known circuits can be used for any of the circuits 
represented by the blocks. 
The circuit diagram shown in 8b is an example of an embodiment of a 
capacitance measuring device which compares the output of two matched 
monostable multivibrators. The output of one monostable multivibrator 
MSMV-1, 81, is a fixed pulse having width T. The output of the other 
monostable multivibrator MSMV-2, 82, is a pulse having a width T+t where t 
is proportional to the capacitance C added in the external circuit. The 
values for C1 and R2 are fixed and may be chosen so as to enhance the 
relation of T to t so that desired accuracy can be achieved. While R1 will 
normally be equal to R2, R1 may be variable so as to provide a 
calibration. 
Both MSMV-1, 81 and MSMV-2, 82 are initiated simultaneously by the 
triggering unit or Tu, 80. The output pulses, as shown on FIG. 8b are fed 
to a pulse width comparator or PWC, 83. The PWC subtracts the output from 
MSMV-1 from the output of MSMV-2 and feeds the remaining signal to the 
amplifier or AMP, 84. 
The AMP, 84 amplifies the signal t which is proportional to C. The signal 
can also be scaled by the amplifier so that when it is fed into the 
indicator or I, 85, the units will read directly in units of length. If 
for example, I is a meter it can be calibrated to read in feet, meters or 
other units of length. The individual blocks of FIG. 8b are well known to 
those skilled in the art. While any capacitance measuring device can be 
used; it is desirable that the device be designed to operate accurately 
even when a leakage resistance is present. 
Referring now to FIG. 8c, which shows a circuit that can be used both as 
the continuity detector 40 and the heating power source 42. When the 
terminals OC and COM are used the battery V1 and the ammeter A are in 
series to function as an ohmeter and a means for indicating continuity. If 
the terminals OH and COM are used the circuit can function as a heating 
current source with the battery V1 and the variable resistance RV in 
series. 
While the specification has shown and described certain present preferred 
embodiments it is to be distinctly understood that the invention is not 
limited thereto but may be embodied in other alternatives, modifications 
and variations apparent to those skilled in the art. Accordingly, it is 
intended to embrace all such alternatives, modifications, and variations 
as fall within the spirit and scope of the appended claims.