CA2187952A1 - Apparatus for airborne electromagnetic surveying - Google Patents

Abstract

In systems carried by aircraft that transmit and receive electromagnetic fields for geophysical exploration, it is known to mount transmitting antennae on an aircraft or other rigid structure attached to or towed beneath an aircraft. In every case, the area of the transmitting antenna is limited by the dimensions of the rigid structure, which limits the dipole-moment of the antenna and thus the strength of the signal received by the system. This limitation is particularly severe for systems using antennae in towed structures. The present invention consists of a system that incorporates an airborne electromagnetic transmitting antenna with flexible area. Before take-off and during landing, the area of the antenna may be collapsed for safety and convenience. In flight, the antenna assumes a large and stable area, which induces strong electromagnetic responses from conductive features in the earth.

Description

21~795~
This invention relates to an apparatus, or airborne clccllu",a~netic system, for geophysical map~ing, sounding or exploration. The system incorporates a collapsible l,;.n~.";~ ntenn~ The antenna is made up of ligid portions that " ,~ its appruAill~e area and shape during flight, and flexible portions that permit the antenna to collapse for take-off, landing, storage and transportation. When connP~ed to the rest of an airborne el~~ a~netic system, the transmitting antenna lr~l~llliL~ a repetitive m~gn~,tic field, or primary field, which induces electrical current in the earth. The current in the earth genel~es a secondary ma~netic field which, in turn, induces currents in the receiving ~nt~.nn~e ofthe system.
At present, all airborne ele~ olllagnetic systems use one oftwo types of ~ g l~nAe, both of which have been in use for more than 40 years. The most effective type of antenna consists of several turns of wire looped horizontally around an aircra~ (cf. C~n~ n Patent 564 361 C~n~i n Patent 1 168 308), and fixed at several points to the aircraft. The abbreviation FL, for Fixed Loop, is used in the following ~ s~iQn for this type of antenna and system with which it is used. For mo$ in~t~ tions, the area ~.n~ sed by a turn of an FL
antenna ranges from 180 m2 to 240 m2. The area is limited by the length and wil~ of aircraft that can be used economically for surveys.
For FL systems, the ~ loop of several turns of wire is fixed to booms on the nose, wing-tips and tail of a STOL aircraft. Signal-genel~ing means, signal processing means and a winch are mounted in the aircraft. Receiving ~ nl~e are housed in a structure called a bird which is towed at the end of a flexible signal-cable about 120 m behind and 60 m below the aircraft. During take-offand landing, the cable is wound on the drum ofthe winch, so that 2187~52 the bird is held against the fi1s~lage ofthe aircraft. Aircraft carrying FL systems are fiown at a nominal speed of 200 kmth and ground cl~ce of 120 m.
To enclose the largest area that can be borne by the aircraft, the turns of wire in an FL
antenna are fixed to the nose, wing-tips and tail. As a result, the turns of an FL antenna are applox;l,lA~ely holi~oll~l. An antenna ofthis ori~nt~tion couples most effectively with holi~ullt~l features in the earth. In general, this is disadvantageous with regard to mineral exploration as the horizontal regolith c~ g, for ~,~ll?'~, clay, ionic groundwater, or wealll~lt;d bedrock is typically more conductive than the underlying bedrock which hosts mineral deposits. Thus, the holi~ollli~l FL antenna is most s~lsceplil~lc to ele~illomaglletic , ofthe bedrock by the regolith. Ful~ llllole, many mineral deposits, such as massive sulfides in shield-terrain and kimberlite diatremes, tend to be oriented vertically and thus couple ly with the holi~ l FL ~nt~.nn~
Systems with FL ~ enl~e tend to be uneconomic for many ofthe surveys specified for mining exploration and geological mapping. Tl~ ion of such a system in an aircraft is a complex and protracted process. Extensive modifications to the aircraft are required to create air-worthy points of ~tt~çhm~nt for the ~nt~nn~ and numerous test-flights and adj-l~tm~.nt~ are needed to identify and ~i;lll;llA~e aircraft-related noise from the response ofthe system. Once in~t~lle~l, FL systems are seldom (if ever) removed and the modified aircraft cannot be used ecollo", ~ally for any other purpose. As a result, a large portion ofthe cost of surveying with FL systems represents fixed inve~lm~ in a sper,i~li7~d aircraft, and the ~ ~oll~lion around the world ofthe system by the survey aircraft. Fixed inve~ routinely causes economic difficulty for airborne electrom~g,n~.tic surveyors, as the demand for such surveys fl-lctuAtes 2187t~2 greatly from year-to-year and, in accordance with prices for base-metals, may remain depressed for several con~ec~ltive years. Transporting an electromagnetic system by means ofthe survey aircraft is in~ffi~ient as these aircraft are chosen for their manoeuvrability at slow speed and low altitude, rather than their efficiency at hauling cargo over long di~tanr,es The second type of airborne electroma~n~ic ~ n~" ~ antenna in general use is a fixed coil of wire. For a few systems, the coil is mounted on a wing-tip or nose of a fixed-wing aircraft (cf. C~n~ n Patent 653,286); more col~ lollly, the coil is in~talled in a fibre-leil~lced plastic bird, which is slung beneath a helicopter (cf. C~n~llian Patent 680,143). The abbreviation FC, for Fixed Coil, is used in the following discussion for this type of antenna and system with which it is used. An FC antenna is abobbin-wound coil of wire, encased in plastic, that fits within the .1;AI~eler ofthe bird, which is typically 0.5 m. To increase ll~-s".;1~ed moment, an FC antenna may be wound with several hundred turns, to obtain an effective area of in the order of 50 m2.

A typical helicopter-borne FC system is slung on a cable 30 m in length from the cargo-hook of a helicopter. The cable tows an assellll)ly that provides aerodynamic stability and suspension for a cylindrical bird that houses the FC ~ ",;~ E and receiving ant~nnae Con~ ctors in the cable carry power from signal-gene~ g means in the helicopter to the lli.,l~lll;ll;ilg ~ and signal from the receiving ~nt~.nn~e to signal-processing means in the helicopter. The bird is flown at a nominal speed of 100 km/h and ground clearance of 30 m.
Ceteris paribus, lower ground-clearance enables more detailed meas~lél~lelll~ to be made of ~llollger elecllu~ t;l;c responses from the earth. However, the smaller dipole-mom~nt of the FC antenna results in significantly weaker responses from the earth, despite the lower 2187~2 ground-clearance at which helicopter-borne FC systems are flown. Nevertheless, FC systems are widely used because they can be transported less expensively than FL systems, and used t~lnpol~ily in locally available helicopters.
The component of an FC system that is most difficult to transport is the bird which is approAilll~h~ly is 8.5 m in length, 0.5 m in .l;~ lt?r~ and weighs about 100 kg. Components of this scale can be ll~olled by light trucks with special fittings, or by aircraft that carry cargo longer than the standard 200 inches (5 m). Although such transportation is more econo -~l than ll~pol l~lion by survey aircraft, ~l~ipping large and heavy birds remains costly and inconvenient. Indeed, many surveys carmot be performed economic~lly, where they are required for remote areas where there is no infrastructure of roads or airports that service large cargo.
As airborne electroma~netic surveys have been relatively expensive, they have been flown in rather limited areas, as colllpaled to other airborne geophysical surveys such as ma~netiCS or gamma-ray ~e~ ulll~tly. This has retarded the discovery of many ore-deposits which have been found to respond to airborne electroma~etic surveys, subsequent to discovery by slower and, llhim~t~qly~ more expensive means. A recent c~ll~)le in Canada is the nickel-copper-cobalt discovery at Voisey Bay, NF (c~ The Northern Miner, April 17, 1995, p.3).
The present invention overcomes the small-ll;~ l";llel disadvantage of FC systems, and the inconvenience of FL systems, by providing an airborne electroma~n~tic system which incoll,oldes a ll~n~",~ antenna which can be expanded to enclose a large area to induce ~187952 strong responses from the earth, yet which can be collapsed for ease oftake-off, landing, lhly and ~I,ipmel".
The ~ antenna ofthe system consists of one-or-more turns of wire, slung from the cargo-hook of a helicopter or deployed through ports in the belly of an aircraft. The antenna contains flexible portions to allow the antenna to expand and collapse, and rigid portions to stabilize the shape and area of the ~.xp~nded ~nt~nn~ The flexible portions consist of wire and, where necess~ry, flexible in~ tion or ~w~ ent. The rigid portions consist of wire held by supports. Both the flexible and rigid portions of the antenna tli~ mhle into pieces with the dimensions of standard cargo. Devices may be ~tt~h~d to the antenna to support receiving ~nt~nnae or other components of the airborne system.
Tests of aerodynamic behaviour have been made with scale models of several versions ofthe ~ntenn~ The tests indicate that, without additional aerodynamic devices ~tt~ched to the ~nt.onn~ the antenna tends to align itself so that the rigid portions are either perpen~iclll~r to or aligned with the flow of air past the ~nt~.nn~; as the direction of air-flow ch~n~ec, the antenna may spontaneously realign to the alternate position of relative stability. Aerodynamic devices such as stays, vanes and skirts may be attached to the antenna to create a unique ~lignm~nt of stability; the flexible and rigid portions ofthe antenna may be deei~o.d and arranged for the same purpose. The pl~;~elled ~ljgnm~nt has the rigid portions aligned with the flow of air (i.e.
with the flight path). Aligned thus, the antenna couples optimally with conductive features in the sparsely sall"~led earth between flight-lines.

21879.S2 The antenna l~ lul~ a repetitive ma~nP.tic field, arising from a repetitive current supplied to the antenna by signal-generating means ofthe airborne electrom~gnP.tic system.
The system draws power from the auxiliary power-supply ofthe aircraft.
One-or-more receiving ~ntenn~p~ are suspended beneath the aircraft, and held 15 m or more from the ~ .";~ g antenna by flexible or rigid supports, so that the receiving ~ntP.nn~P

are not saturated by the primary field ofthe lli11~ ;1Ig ~ntpnn~
A p-~re -ed embodiment ofthe invention provides an airborne electrom~gnP,tic Ll~ antenna col1~;slillg of 1 turn of AWG#8 ~ mimlm wire. Flexible portions ofthe antenna consist of wire, which may in~ ted and/or ~einrolced Rigid portions ofthe antenna consist of wire supported by fibre-lei- rolced plastic In this embodiment, auxiliary supports are att~ched to the antenna such that the g antenna takes on the form of a 5-sided polygon, measuring about 58 m in height and up to 36 m in width, yielding an area of about 1500 m2.
Exe~ la~y embodiments ofthe invention are illustrated in Figure 1, which depicts a coll~psible 1~ ting antenna and ancillary components slung beneath a helicopter. Signal-generating means, signal-processing means, and ancillary instn mP.nt~tion similar to those used for ~ d~d FL systems (cf. C~n~ n Patent 662,184) are mounted in a helicopter (1).
Flexible portions (2) ofthe l~;.n~ antenna connect to the signal-generating means and suspend the antenna from the cargo-hook of the helicopter. A support (3) spreads and ~l~h;l;~ the centre ofthe ~ntPnn~ The support is made up of hollow rods made of fibre-~~lced plastic. The ~ "elp~r of each rod is reduced at one end, so that the end of a rod fitsinto the ~dj~cçnt rod in the support. A cable of aramid or other suitable fibre is threaded 218 7 9 .~ 2 through the rods, and each end the cable is clamped to the wire of the L. i~ ; i-g antenna to fix the support in place and keep the rods from sepa~ling. Lower flexible portions ofthe antenna (4) are clamped to the upper flexible portions (2), and extend to the rigid portion (5) which forms the bottom ofthe ~nt~nn~ The rigid bottom ofthe antenna is supported by rods similar to those used for the support (3). The rods at the bottom ofthe antenna are held together by the wire which forms the bottom part ofthe tl~ .";ll;"g loop. The wire is threaded through the rods and cla.-l~)ed to the lower Mexible portions (4). One-or-more receiving ~nt.q.nn~e (6) similar to those used in standard FL systems, which may incoll,o,~le signal-processing means, are held by support cables (7), (8) of aramid or other suitable fibre appl~x;."~l~o.ly 18 m from the wire ofthe l~ nt~nn~ A signal cable (9) carries the signal from the receiving ~nt~qnn~e to signal-processing means in the helicopter. A skirt for aerodynamic drag (10) is ~tt~çhed to the trailing edge ofthe ~1;.l,~lll;ll;ll~, ~nt~nn~
In altellldti\~e embodiment ofthe invention, the ~ antenna and ancillary components ofthe invention are deployed beneath a fixed-wing aircraft. For take-offand landing, the area ofthe antenna is collapsed by winding the fiexible portions ofthe antenna onto winches, or other suitable controllable devices in the aircraft. Before surveying, the area ofthe antenna is expanded by deploying the fiexible portions through ports in the belly ofthe aircraft, along with the receiving ~nt~Mn~e in a bird on a separate cable.

Claims (2)

1. An electromagnetic system borne by an aircraft, said system incorporating a transmitting antenna formed by one-or-more turns of wire, for which the area enclosed by said turns is able to expand during or after take-off of the aircraft, and to contract before or during landing of the aircraft.

2. A system as claimed in claim 1 in which the expansion or contraction of the area of transmitting antenna is effected by a controllable device.