CA2142546C - Apparatus and method for sounding the earth - Google Patents
Apparatus and method for sounding the earthInfo
- Publication number
- CA2142546C CA2142546C CA002142546A CA2142546A CA2142546C CA 2142546 C CA2142546 C CA 2142546C CA 002142546 A CA002142546 A CA 002142546A CA 2142546 A CA2142546 A CA 2142546A CA 2142546 C CA2142546 C CA 2142546C
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- receiver
- transmitter
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
- G01V3/10—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/15—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat
- G01V3/17—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat operating with electromagnetic waves
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- Physics & Mathematics (AREA)
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Remote Sensing (AREA)
- Electromagnetism (AREA)
- Environmental & Geological Engineering (AREA)
- Geology (AREA)
- General Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- Geophysics (AREA)
- Geophysics And Detection Of Objects (AREA)
Abstract
In electromagnetic instruments that continuously transmit and receive low-frequency fields in order to sound the earth, it is known to change the orientations of the transmitter and receiver to alter the depth of sounding. In this invention, a novel arrangement of one transmitter and two receivers enables simultaneous sounding of the earth to two distinct depths. The transmitter and receivers may be secured to an arcuate boom which, at the same time, allows a single operator to carry the instrument conveniently across the earth to be sounded, and positions the transmitter and receivers near the earth to increase the depths of sounding.
Description
~ ~ ~2 ~ ~ ~
This invention relates to a geophysical instrument that sounds the electrical conductivity ofthe earth ~imlllt~neously to two distinct depths. The instrument is of the type which supplies an alternating current at a fixed frequency to a llhlls~ coil, which consists of a bobbin-wound coil of wire. The Ll;~ er-coil ll~lSIlL~S an ",~ g ma~etiC field, or primary field, which induces electrical current in the earth.
The current in the earth genel~les a secondary m~gn~tic field which, in turn, induces current in the two receiver-coils ofthe instrument. Each receiver-coil consists of a bobbin-wound coil of wire located at some distance from the l,~l~slll;~ 1~1 -coil and held in a specific orient~tion relative to the l,~n~",;ll~r-coil.
The amplitude and phase ofthe current induced in each receiver-coil, relative to the current supplied to the transmitter-coil, provide inrullllalion about the electrical and m~gnp~tic char~ct~.ri.~tics ofthe earth to the effective depth of solm~l;ng The frequency ofthe alternating current supplied to the Lli1n~lll;ll~or, and the sp~rin~
between the llhl~ l and each receiver are such that the instrument satisfies the "low frequency appro~ ion" (Wait, J.R., "A Note on the Electrom~gn~,tic Response of a Stratified Earth," Geophysics. vol. XXVl:I, no. 3 (June, 1962), pp. 382-85), which is defined as:
o,u ~)mpl ~ 1/2 where i is the square-root of rninus-one, ~0 is the electric~l conductivity ofthe earth in siemens/metre, ,u is the ma~netic permeability ofthe earth in henrys/metre, a) is the angular frequency in radians/second ofthe ~ lll;lled field, and p is the spacing in metres between the l~ .r and either receiver.
4 ~
It is comm~ n in low-frequency-applv~c;.l.~liQn (LFA) instruments to use a single ler-coil and a single receiver-coil. The cQils are held at or slightly above thesurface ofthe earth at opposite ends of a linear boom. An illustrated description of such an instrument was published in 1966 (Howell, M.I., "A Soil Conductivity Meter,"
Archaeo~ lly~ vol. IX pp. 20-23). This instrument incorporates a ll~ .. 'ler- and receiver-coil in an arr~ng~.m~.nt that may be termed pe~ .l;c~ r aligned (PA), i.e. the coils are perpen-lic~ r, and the axis of one coil is aligned to inlel~e~;l the other coil. In 1975, another LFA instrument was described in which a tl;lllelll;ller- and receiver-coil are fixed at opposite ends of a linear boom (cf. Figure 1 of McNeill, James D., Black Gary G.A. and Bosnar, Miroslav, "Method and Apparatus for Measuring Terrain Resistivity," C~n~di~n Patent No. 1036221). The novel feature ofthis second instrument is its output meter, which is scaled to yield a direct reading ofterrain conductivity (or its inverse, resistivity). The second instrument incorporates a.";11er- and receiver-coil in an all~ nt that may be termed holi~o~ l coplallar (HCP), i.e. the coils (as defined by the orientation of a winding) lie in the same horizontal plane. An advantage ofthe HCP arrangement is that by rotating the instrument 90 degrees about the long axis of its linear boom, the coils are moved into the vertical coplallar (VCP) all~nge...~ l, wherein the coils lie in the same vertical plane. As shown later, the HCP- and VCP-~ gelll~ sound to distinct depths.
Ol~ fillg both HCP- and VCP-soundings with an instrument that incorporates a single l,ilne.-.;l ler and receiver is an inefficient three-step process, which enco~r~eees ." ?~-soun-li~ with the coils in one arrangement, rotating the instrument for the alternate all;1ng-P.."r.~l, and sounding for a second time.
The linear boom used in many LFA instruments adds further inefficiencies. To increase the depth of soun(ling into the earth for an instrument that uses a linear boom, the operator stops wherever a sounding is to be made, lowers the instrument to the ground, records a soun~in~, rotates the instrument and records the ~ltPrn~te so ~n~ lg, raises the instrument, and proceeds to the next location of interest.
A harness may be used for suspending the instrument slightly above the surface ofthe ground, which enables contin~lol~ls traversing of an area at a walking pace;
however, when a harness is used, it is rare that both HCP- and Vcp-scllnl1:ngs are made during a single pass, as the operator must stop, take a sollntling~ reposition the instrument, and take the second sounding. Furthermore, when the instrument is suspended near the lower legs ofthe operator, the linear boom must be aligned with the direction ofthe traverse, which ~ .es the width ofthe swath of ground that is sounded about the traverse line.
The present invention provides an LFA instrument which o~/el~iollles these inPfficiP.ncies by utilizing a novel arr~n~p~m~pnt of a single ~ n~ll,;llel-coil and two receiver-coils, which may be fixed to an arcuate boom to increase depth of so~ln-ling into the earth and allow for efficient traversing with the instrument.
In drawings which illustrate embo~limPMt~ ofthe invention, Figure 1 is a s~ ic elevation showing the al~ gP~ ofthe ~li.ll~lllill~l-coil and the receiver-coils ofthe invention, Figure 2 is a graph showing the depths-of-solm(ling ofthe -invention when ope~ lg near the lower limit ofthe LFA, Figure 3 is a graph showing the depths-of-sounding ofthe invention when opel~ling near the upper limit ofthe LFA, and Figure 4 is an elevation showing an embodiment ofthe invention that can be carried conveniently by a single surveyor.
The novel coil-arr~ng~,m~,nt sounds the earth ~iml Ih~neo~l~ly to two distinct depths. The ~ 1 is illustrated in Figure 1, which shows the coils in profile above a holi~olll~l earth (1). The ll~ r-coil (2) has holi~onlal windings, and is 4a connected to a l,iln.~i.,-;ll~o.r (3) which generates an electrical signal. The windings (4) of one of the receiver-coils are horizontal as well. This receiver-coil is held in a~p~ ely the same holi~o"kl plane as the ~ pr-coil~ and thus forms an HCP-arr~ngP.mPnt with the ~ l ";I IPI -coil. The other receiver-coil (5) has vertical windings; thus, the axis of these windings is horizontal. This receiver-coil is held such that the axis of its windings intersects the point at the centre ofthe windings ofthe ll;lll~lll;ll~r-coil~ placing these coils in a PA-arr~ng~.ment Both receiver-coils are held at known separations from the ~ n~ er-coil, and are conn~cted to a receiver (6) which processes electrical signals from the receiver-coils.
As shown later, the depths of sounding ofthe HCP- and PA-arrange",ell~s are more distinct than the depths for the HCP- and VCP-6,~;1n~ .ls~ making the present invention more effective than currently available LFA instruments at detecting layering of cond~lctivity in the earth, as well as more convenient to use. By using a single il ler-coil~ the novel ~ .nt Ill;~ es the number of coils required to e~lisl, the HCP- and PA-arran~.l,~..ls7 thus minimizing the additional cost and complexity ofthe instrument. Furthermore, the single ll~n~", ~ler-coil requires e~ 1ly the same power supply and signal-genel~ling means as a conventional LFA
instrument, thereby ".;ni.,.;~ the additional weight ofthe present invention.
The electroma~n~tic response ofthe earth to the HCP-, PA- and VCP-~l~n~ has been form~ d (Wait, J.R, "Mutual Ele j~lu~a~n~tic Coupling of Loops over a Homogeneous Ground," Geophysics~ vol. XX, no. 3 (July, 1955), pp.
630-37), and tabulated (Fri~.hn~r.ht, F.C., "Fields about an Osçillqtin~ Magnetic Dipole ~.
7 ~ ~ ~ 5 ~ ~
over a Two-Layer Earth, and Application to Ground and Airborne Electromagn~,tic Surveys," Quarterly ofthe Colorado School of Mines. vol. 62 (1967), pp. 1-326).
In general, the clc~ )nlagnetic response ofthe earth is colll~ . However, as shown by Wait (1962), the LFA simplifies the response greatly; most i~ )olL~lLly:
1. The compol~ of response in phase with the ~ ,;llçr can be ignored, as its ~mplitude is insignificant coll~pared to that ofthe component in quadrature;
This invention relates to a geophysical instrument that sounds the electrical conductivity ofthe earth ~imlllt~neously to two distinct depths. The instrument is of the type which supplies an alternating current at a fixed frequency to a llhlls~ coil, which consists of a bobbin-wound coil of wire. The Ll;~ er-coil ll~lSIlL~S an ",~ g ma~etiC field, or primary field, which induces electrical current in the earth.
The current in the earth genel~les a secondary m~gn~tic field which, in turn, induces current in the two receiver-coils ofthe instrument. Each receiver-coil consists of a bobbin-wound coil of wire located at some distance from the l,~l~slll;~ 1~1 -coil and held in a specific orient~tion relative to the l,~n~",;ll~r-coil.
The amplitude and phase ofthe current induced in each receiver-coil, relative to the current supplied to the transmitter-coil, provide inrullllalion about the electrical and m~gnp~tic char~ct~.ri.~tics ofthe earth to the effective depth of solm~l;ng The frequency ofthe alternating current supplied to the Lli1n~lll;ll~or, and the sp~rin~
between the llhl~ l and each receiver are such that the instrument satisfies the "low frequency appro~ ion" (Wait, J.R., "A Note on the Electrom~gn~,tic Response of a Stratified Earth," Geophysics. vol. XXVl:I, no. 3 (June, 1962), pp. 382-85), which is defined as:
o,u ~)mpl ~ 1/2 where i is the square-root of rninus-one, ~0 is the electric~l conductivity ofthe earth in siemens/metre, ,u is the ma~netic permeability ofthe earth in henrys/metre, a) is the angular frequency in radians/second ofthe ~ lll;lled field, and p is the spacing in metres between the l~ .r and either receiver.
4 ~
It is comm~ n in low-frequency-applv~c;.l.~liQn (LFA) instruments to use a single ler-coil and a single receiver-coil. The cQils are held at or slightly above thesurface ofthe earth at opposite ends of a linear boom. An illustrated description of such an instrument was published in 1966 (Howell, M.I., "A Soil Conductivity Meter,"
Archaeo~ lly~ vol. IX pp. 20-23). This instrument incorporates a ll~ .. 'ler- and receiver-coil in an arr~ng~.m~.nt that may be termed pe~ .l;c~ r aligned (PA), i.e. the coils are perpen-lic~ r, and the axis of one coil is aligned to inlel~e~;l the other coil. In 1975, another LFA instrument was described in which a tl;lllelll;ller- and receiver-coil are fixed at opposite ends of a linear boom (cf. Figure 1 of McNeill, James D., Black Gary G.A. and Bosnar, Miroslav, "Method and Apparatus for Measuring Terrain Resistivity," C~n~di~n Patent No. 1036221). The novel feature ofthis second instrument is its output meter, which is scaled to yield a direct reading ofterrain conductivity (or its inverse, resistivity). The second instrument incorporates a.";11er- and receiver-coil in an all~ nt that may be termed holi~o~ l coplallar (HCP), i.e. the coils (as defined by the orientation of a winding) lie in the same horizontal plane. An advantage ofthe HCP arrangement is that by rotating the instrument 90 degrees about the long axis of its linear boom, the coils are moved into the vertical coplallar (VCP) all~nge...~ l, wherein the coils lie in the same vertical plane. As shown later, the HCP- and VCP-~ gelll~ sound to distinct depths.
Ol~ fillg both HCP- and VCP-soundings with an instrument that incorporates a single l,ilne.-.;l ler and receiver is an inefficient three-step process, which enco~r~eees ." ?~-soun-li~ with the coils in one arrangement, rotating the instrument for the alternate all;1ng-P.."r.~l, and sounding for a second time.
The linear boom used in many LFA instruments adds further inefficiencies. To increase the depth of soun(ling into the earth for an instrument that uses a linear boom, the operator stops wherever a sounding is to be made, lowers the instrument to the ground, records a soun~in~, rotates the instrument and records the ~ltPrn~te so ~n~ lg, raises the instrument, and proceeds to the next location of interest.
A harness may be used for suspending the instrument slightly above the surface ofthe ground, which enables contin~lol~ls traversing of an area at a walking pace;
however, when a harness is used, it is rare that both HCP- and Vcp-scllnl1:ngs are made during a single pass, as the operator must stop, take a sollntling~ reposition the instrument, and take the second sounding. Furthermore, when the instrument is suspended near the lower legs ofthe operator, the linear boom must be aligned with the direction ofthe traverse, which ~ .es the width ofthe swath of ground that is sounded about the traverse line.
The present invention provides an LFA instrument which o~/el~iollles these inPfficiP.ncies by utilizing a novel arr~n~p~m~pnt of a single ~ n~ll,;llel-coil and two receiver-coils, which may be fixed to an arcuate boom to increase depth of so~ln-ling into the earth and allow for efficient traversing with the instrument.
In drawings which illustrate embo~limPMt~ ofthe invention, Figure 1 is a s~ ic elevation showing the al~ gP~ ofthe ~li.ll~lllill~l-coil and the receiver-coils ofthe invention, Figure 2 is a graph showing the depths-of-solm(ling ofthe -invention when ope~ lg near the lower limit ofthe LFA, Figure 3 is a graph showing the depths-of-sounding ofthe invention when opel~ling near the upper limit ofthe LFA, and Figure 4 is an elevation showing an embodiment ofthe invention that can be carried conveniently by a single surveyor.
The novel coil-arr~ng~,m~,nt sounds the earth ~iml Ih~neo~l~ly to two distinct depths. The ~ 1 is illustrated in Figure 1, which shows the coils in profile above a holi~olll~l earth (1). The ll~ r-coil (2) has holi~onlal windings, and is 4a connected to a l,iln.~i.,-;ll~o.r (3) which generates an electrical signal. The windings (4) of one of the receiver-coils are horizontal as well. This receiver-coil is held in a~p~ ely the same holi~o"kl plane as the ~ pr-coil~ and thus forms an HCP-arr~ngP.mPnt with the ~ l ";I IPI -coil. The other receiver-coil (5) has vertical windings; thus, the axis of these windings is horizontal. This receiver-coil is held such that the axis of its windings intersects the point at the centre ofthe windings ofthe ll;lll~lll;ll~r-coil~ placing these coils in a PA-arr~ng~.ment Both receiver-coils are held at known separations from the ~ n~ er-coil, and are conn~cted to a receiver (6) which processes electrical signals from the receiver-coils.
As shown later, the depths of sounding ofthe HCP- and PA-arrange",ell~s are more distinct than the depths for the HCP- and VCP-6,~;1n~ .ls~ making the present invention more effective than currently available LFA instruments at detecting layering of cond~lctivity in the earth, as well as more convenient to use. By using a single il ler-coil~ the novel ~ .nt Ill;~ es the number of coils required to e~lisl, the HCP- and PA-arran~.l,~..ls7 thus minimizing the additional cost and complexity ofthe instrument. Furthermore, the single ll~n~", ~ler-coil requires e~ 1ly the same power supply and signal-genel~ling means as a conventional LFA
instrument, thereby ".;ni.,.;~ the additional weight ofthe present invention.
The electroma~n~tic response ofthe earth to the HCP-, PA- and VCP-~l~n~ has been form~ d (Wait, J.R, "Mutual Ele j~lu~a~n~tic Coupling of Loops over a Homogeneous Ground," Geophysics~ vol. XX, no. 3 (July, 1955), pp.
630-37), and tabulated (Fri~.hn~r.ht, F.C., "Fields about an Osçillqtin~ Magnetic Dipole ~.
7 ~ ~ ~ 5 ~ ~
over a Two-Layer Earth, and Application to Ground and Airborne Electromagn~,tic Surveys," Quarterly ofthe Colorado School of Mines. vol. 62 (1967), pp. 1-326).
In general, the clc~ )nlagnetic response ofthe earth is colll~ . However, as shown by Wait (1962), the LFA simplifies the response greatly; most i~ )olL~lLly:
1. The compol~ of response in phase with the ~ ,;llçr can be ignored, as its ~mplitude is insignificant coll~pared to that ofthe component in quadrature;
2. For soumlin~ a layered earth, using a fixed frequency and coil-spacing, the response of each layer is a simple fiunction of its depth, and total response is the sum ofthe layer-responses.
Fli~ch~ (ibid.) fi~rni~hes a table of le~ollses for the HCP-, PA- and VCP-arr~n~"~ raised to various heights above a homogeneous earth. The relative depths of sounding for the arr~n~ can be determined from this table.
Since air is highly resistive, essenti~lly no current is induced in the air by the transmiMer-coil, and the air makes no contribution to the le~ol1se measured by the receiver-coils. For an LFA instrument, total response is the sum of layer-re~ol1se~, since the layer-reponse ofthe air is zero, total response measured at a given height above the earth consists entirely ofthe response from the earth that lies beneath that height.
As this relates to soun~ling the LFA-response of the earth at a given height above the earth is equivalent to the portion of LFA-response at surface due to the earth that lies beneath a coll~spo~ depth in the earth. Thus, LFA-range l~ollses ,~
~ ~Di tabulated by Fli.~.~n~.ht (ibid.) indicate the relative depths of sounding for the HCP-, PA- and VCP-~~ g~ i. The r~ollses for:
¦(ic~o~ ~)mPI = 0 071 , i.e. near the lower limit ofthe LFA, are shown in Figure 2. For the figure, the unit of re~ollse is the amplitude of the quadrature-phase current received, expressed as a pel~e,llage ofthe ~ l-current amplitude. The unit of depth is the spacing between the ll~ er-coil and the receiver-coil. The symbols indicating the re~pollses of each coil-arr~n~.m~nt are plotted for the heights (or depths) t~b~ ed by Fri~l-n~.~t (ibid.). The values for-zero depth indicate the responses ofthe complete earth. Values for depths of 0.5, 1, 1.5, 2 and 3 units of lr~ ";l le~ -receiver coil-sp~-ingS indicate the responses due to the portions ofthe earth beneath these respective depths.
The responses for:
mpl = o 56 , i.e. slightly beyond the upper limit of the LFA, are shown in Figure 3. For this figure, the unit of depth remains the spacing between the ~ "~ coil and the receiver-coil.
However, instead of in~i~ting response by the ~rnp~ de of quadrature-phase current, responses have been norrn~li7ed such that the response ofthe whole earth (i.e. the earth beneath zero coil-sp~çi~s) is 100 percent.
The plots indicate that the HCP-, PA- and VCP-arr~n~ have di~elen depths of solm~li~ For .q,Y~n~ple~ if the depth of sounding is defined as the depth ~: .
beyond which 30 % ofthe total le~,ollse is derived, this depth is apprv~in-d~ely 0.95 coil-spacing for the PA-arrangement, 1.1 coil-sp~cin~ for the VCP-~I~ and 2 coil-sp~in~s for the HCP-arrangement. The PA- and HCP-~I~ng~ have the greatest difference between their depths of sol~n(iin~ Therefore, PA- and HCP-solln-ling~ as colllpaled to VCP- and HCP-solm-ling~ will indicate more clearly any change in the el~ilrv".af~nP,tic cl~ eli~lics ofthe earth to a depth of 2 coil-sp~ing~
below the instrument.
A co,l~ison of Figures 2 and 3 shows that the depths of sounding are l~sol~ly cc)n;,~ over the entire LFA range. Using the same definition ofthe depth of sounding as the depth beyond which 30 % of the total response is derived, Figure 2 shows that this depth for the PA-al~ changes slightly to applv~ P,ly 1 coil-spacing; the depth increases to about 1.4 coil-~c.;l-gs for the VCP-~ g~ r~l and 2.8 coil-sp~in~s for the HCP-~ n~ ent The PA- and HCP-~ -g~ continue to show the greatest difference b~w~ll their depths of sounding.
A prt;rellt;d embodiment of the invention provides an instrument in which the Ll~ll~lll;llçr-coil and receiver-coils, described previously, are fixed to a novel arcuate boom. The arcuate boom enables a single op~ or to carry conveniently the 1l~ll~lll 'I~-coil, which is fixed near one end ofthe boom, and the receiver-coils, which are fixed near the other erld. The arcuate shape ofthe boom provides clearance ofthe vpe~lops hips at the centre ofthe boom, to f~ilit~te traversing with the instrument, and positioning ofthe ~ -coil and receiver-coils at the ends ofthe boom nearer to the earth, to increase depths of sounding.
4 ~
E?~en~ y embo~im~nt~ ofthe invention, which i~ ol~les the novel co",l)i~ n ofthe HCP- and PA-~l~ and the arcuate boom, are described further with reference to Figure 4, which is a dia~lll",i11;c profile ofthe 1ll~~
aspects ofthe form ofthe instrument that can be carried conveniently by a single opel~lol. A boom of arcuate shape (1), made of any light, rigid, non-conductive and non-m~n~tic m~t.qri~l, hol~lllaUy sep~les a horizontal ~ -coil (2) from a horizontal receiver-coil (3) and a vertical receiver-coil (4), the axis of which is aligned to intersect the Ll~".~i";~l~.r coil The lli1n~ r-coil and receiver-coils are f~ed rigidly to the boom. A signal-~,enel~ing means (5) is fixed to the boom near the ll~n~",;ll~r and a signal-processing means (6) is fo~ed to the boom near the receivers. Operating controls and monitors (7) are attached to the centre ofthe boom or at some other convenient location. Ancillary equipment, such as positioning systems and recording devices, may also be att~c~d or connected to the instrument. In use, an operator (8) grasps the instrument, which may be suspended from the operator's body, and carries it over the earth (9) to be sollnde(l The opel~lor may observe mollilol~ of l~ollse, which may be visual, aural, or a colllbillalion of both.
Fli~ch~ (ibid.) fi~rni~hes a table of le~ollses for the HCP-, PA- and VCP-arr~n~"~ raised to various heights above a homogeneous earth. The relative depths of sounding for the arr~n~ can be determined from this table.
Since air is highly resistive, essenti~lly no current is induced in the air by the transmiMer-coil, and the air makes no contribution to the le~ol1se measured by the receiver-coils. For an LFA instrument, total response is the sum of layer-re~ol1se~, since the layer-reponse ofthe air is zero, total response measured at a given height above the earth consists entirely ofthe response from the earth that lies beneath that height.
As this relates to soun~ling the LFA-response of the earth at a given height above the earth is equivalent to the portion of LFA-response at surface due to the earth that lies beneath a coll~spo~ depth in the earth. Thus, LFA-range l~ollses ,~
~ ~Di tabulated by Fli.~.~n~.ht (ibid.) indicate the relative depths of sounding for the HCP-, PA- and VCP-~~ g~ i. The r~ollses for:
¦(ic~o~ ~)mPI = 0 071 , i.e. near the lower limit ofthe LFA, are shown in Figure 2. For the figure, the unit of re~ollse is the amplitude of the quadrature-phase current received, expressed as a pel~e,llage ofthe ~ l-current amplitude. The unit of depth is the spacing between the ll~ er-coil and the receiver-coil. The symbols indicating the re~pollses of each coil-arr~n~.m~nt are plotted for the heights (or depths) t~b~ ed by Fri~l-n~.~t (ibid.). The values for-zero depth indicate the responses ofthe complete earth. Values for depths of 0.5, 1, 1.5, 2 and 3 units of lr~ ";l le~ -receiver coil-sp~-ingS indicate the responses due to the portions ofthe earth beneath these respective depths.
The responses for:
mpl = o 56 , i.e. slightly beyond the upper limit of the LFA, are shown in Figure 3. For this figure, the unit of depth remains the spacing between the ~ "~ coil and the receiver-coil.
However, instead of in~i~ting response by the ~rnp~ de of quadrature-phase current, responses have been norrn~li7ed such that the response ofthe whole earth (i.e. the earth beneath zero coil-sp~çi~s) is 100 percent.
The plots indicate that the HCP-, PA- and VCP-arr~n~ have di~elen depths of solm~li~ For .q,Y~n~ple~ if the depth of sounding is defined as the depth ~: .
beyond which 30 % ofthe total le~,ollse is derived, this depth is apprv~in-d~ely 0.95 coil-spacing for the PA-arrangement, 1.1 coil-sp~cin~ for the VCP-~I~ and 2 coil-sp~in~s for the HCP-arrangement. The PA- and HCP-~I~ng~ have the greatest difference between their depths of sol~n(iin~ Therefore, PA- and HCP-solln-ling~ as colllpaled to VCP- and HCP-solm-ling~ will indicate more clearly any change in the el~ilrv".af~nP,tic cl~ eli~lics ofthe earth to a depth of 2 coil-sp~ing~
below the instrument.
A co,l~ison of Figures 2 and 3 shows that the depths of sounding are l~sol~ly cc)n;,~ over the entire LFA range. Using the same definition ofthe depth of sounding as the depth beyond which 30 % of the total response is derived, Figure 2 shows that this depth for the PA-al~ changes slightly to applv~ P,ly 1 coil-spacing; the depth increases to about 1.4 coil-~c.;l-gs for the VCP-~ g~ r~l and 2.8 coil-sp~in~s for the HCP-~ n~ ent The PA- and HCP-~ -g~ continue to show the greatest difference b~w~ll their depths of sounding.
A prt;rellt;d embodiment of the invention provides an instrument in which the Ll~ll~lll;llçr-coil and receiver-coils, described previously, are fixed to a novel arcuate boom. The arcuate boom enables a single op~ or to carry conveniently the 1l~ll~lll 'I~-coil, which is fixed near one end ofthe boom, and the receiver-coils, which are fixed near the other erld. The arcuate shape ofthe boom provides clearance ofthe vpe~lops hips at the centre ofthe boom, to f~ilit~te traversing with the instrument, and positioning ofthe ~ -coil and receiver-coils at the ends ofthe boom nearer to the earth, to increase depths of sounding.
4 ~
E?~en~ y embo~im~nt~ ofthe invention, which i~ ol~les the novel co",l)i~ n ofthe HCP- and PA-~l~ and the arcuate boom, are described further with reference to Figure 4, which is a dia~lll",i11;c profile ofthe 1ll~~
aspects ofthe form ofthe instrument that can be carried conveniently by a single opel~lol. A boom of arcuate shape (1), made of any light, rigid, non-conductive and non-m~n~tic m~t.qri~l, hol~lllaUy sep~les a horizontal ~ -coil (2) from a horizontal receiver-coil (3) and a vertical receiver-coil (4), the axis of which is aligned to intersect the Ll~".~i";~l~.r coil The lli1n~ r-coil and receiver-coils are f~ed rigidly to the boom. A signal-~,enel~ing means (5) is fixed to the boom near the ll~n~",;ll~r and a signal-processing means (6) is fo~ed to the boom near the receivers. Operating controls and monitors (7) are attached to the centre ofthe boom or at some other convenient location. Ancillary equipment, such as positioning systems and recording devices, may also be att~c~d or connected to the instrument. In use, an operator (8) grasps the instrument, which may be suspended from the operator's body, and carries it over the earth (9) to be sollnde(l The opel~lor may observe mollilol~ of l~ollse, which may be visual, aural, or a colllbillalion of both.
Claims (3)
1. An electromagnetic instrument for measuring the conductivity of the earth, comprising a bobbin-wound transmitter-coil with horizontal windings held at or above the surface of the earth, a bobbin-wound receiver-coil with horizontal windings spaced horizontally from said transmitter-coil, a bobbin-wound receiver-coil with vertical windings spaced horizontally from said transmitter-coil, the axis of which is aligned to intersect said transmitter-coil, signal-generating means which supplies an alternating current to said transmitter-coil to transmit an alternating magnetic field, signal-processing means connected to said signal-generating means and said receiver-coils, so as to compare the amplitude and phase of the alternating current in said transmitter-coil to the alternating currents induced in said receiver-coils, and to isolate any components of currents in said receiver-coils in quadrature with the current in said transmitter-coil, the spacing ~ in metres between the transmitter-coil and either receiver-coil and the frequency .omega. in radians/second of the alternating current being restricted to values such that the product ¦(i.delta.0µ .omega.)1/2~¦
does not substantially exceed 0.5, where i is the square-root of minus-one, .delta.0 is the maximum electrical conductivity in siemens/metre and µ is the maximum magnetic permeability in henrys/metre of the earth to be sounded.
does not substantially exceed 0.5, where i is the square-root of minus-one, .delta.0 is the maximum electrical conductivity in siemens/metre and µ is the maximum magnetic permeability in henrys/metre of the earth to be sounded.
2. An instrument as defined in claim 1, wherein said transmitter-coil is fixed at or near one end of an arcuate boom of non-magnetic and non-conductive material, and material, and said receiver-coils are fixed, in separation and orientation relative to said transmitter-coil, substantially at the opposite end of said arcuate boom.
3. A method of sounding the conductivity of the earth simultaneously to two distinct depths utilizing an instrument as defined in claim 1, and using the amplitude of the quadrature component from said receiver-coil with horizontal windings as a measure of the conductivity of the earth to a depth not exceeding 3 times the spacing between said transmitter-coil and said receiver-coil, and using the amplitude of the quadrature component from said receiver-coil with vertical windings as a measure of the conductivity of the earth to a depth not exceeding 1 times the spacing between said transmitter-coil and said receiver-coil.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002142546A CA2142546C (en) | 1995-02-15 | 1995-02-15 | Apparatus and method for sounding the earth |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002142546A CA2142546C (en) | 1995-02-15 | 1995-02-15 | Apparatus and method for sounding the earth |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2142546A1 CA2142546A1 (en) | 1996-08-16 |
| CA2142546C true CA2142546C (en) | 1999-04-13 |
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ID=4155245
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002142546A Expired - Lifetime CA2142546C (en) | 1995-02-15 | 1995-02-15 | Apparatus and method for sounding the earth |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2142546C (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102012008194A1 (en) | 2012-04-26 | 2013-10-31 | Forschungszentrum Jülich GmbH | Electromagnetic conductivity measuring system with improved drift correction |
-
1995
- 1995-02-15 CA CA002142546A patent/CA2142546C/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| CA2142546A1 (en) | 1996-08-16 |
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