Background of the Invention When drilling boreholes, it is highly desirable for the person drilling the borehole to know where the drill bit is and the angle and pitch of the drill bit. This is done by having telemetry apparatus located close to the end of the drill sting adjacent the drill bit. Typically such telemetry equipment may comprise a three axis accelerometer and a triaxial magnetometer. Position and orientation information provided by the telemetry equipment is then transmitted in a signal through the drill string to the surface where the signal can be analysed by computer and the iformation displayed on a suitable display screen.

There are obvious logistical problems in transmitting infonnation from the distal end of a drill string along a bore hole, which might be 1500no long or more, to the surface or open end of the bore. One known system kiown as the "Mecca" system utilises a special drill stung, having a steel outer casing made up of a series of interconnectable steel tubular elements.

A coaxial cable runs along the central axis of each element and each element of the drill strung has a male cable connector at one end and a female cable connector at the other end so that when the drill string is assembled a coaxial cable running through the drill string is also assembled. This cable is used to transmit data from the telemetry device to the open end of the borehole.

One problem with such apparatus, is that the apparatus is expensive since each element of the drilling string lsas to include a length of coaxial cable and connectors at either end. Typically, such a drill string will cost AU $30.00 a metre. Hence, the cost for a 1500m drill string would be close to $50,000.00. The components are also relatively complex and expensive to maintain, particularly since they are used in a hostile environment. The system is also semi-permanent and if a core is to be drilled in the bore hole, it is necessary to have a second string for performing coring since the "Mecca" drill string carries a coaxial cable through its centre and thus cannot be used for coring. This adds to the expense of using the system and the substitution of the "Mecca" drill string with a coring drill stung is also logistically difficult, time consuming and hence expensive.

Other systems have been used including methods such as radio transmission and acoustic transmission but these are unreliable and tend to be unable to reliably achieve transmission of telemetry data over distances of greater than 1 kilometre. Most systems have problems with noise and leakage and often bore line data transmission systems require repeaters to boost the signal to achieve the necessary distance. Such transmission systems are energy intensive, expensive to make, run and maintain.

It is thus an object of the present invention to provide an improved borehole data transmission system and method which alleviates at least some of the problems of the prior art discussed above.

Summary of the Invention Thus in a first aspect of the present invention there is provided a method of receiving data from a sensor associated with a drill stung, the sensor and drill string, being located in a bore bole or the like, the method comprising: supplying an electrical signal to a drill string the signal including at least two distinct frequencies; providing a selective a.c. electrical conducing signal path in the drill string incorporating a frequency selective switch permitting transmission of electrical signals having selected frequencies; supplying a sensor data signal from the sensor to the frequency selective switch which periodically alters the frequency transmission characteristics of the switch and thus modulates the electrical signal transmitted through the drill string beyond the selective a.c. electrical conducting section; and measuring the electrical signal transmitted through the ground from the drill string to a point out of the hole, typically on ground level near an open end of the borehole, and analysing the measured electrical signal to extract the information carried in the sensor data signal from the measured electrical signal.

In a related aspect of the present invention there is provided a borehole data transmission system for transmitting a signal from a sensor associated with a drill string located in the borehole to the surface comprising:

means for transmitting an electrical signal including at least two distinct frequency components F1 and F2 along the bore hole, along the metal casing of the drill string; sensor means for providing sensor information relating to the drill stung, the sensor means, typically being located towards the distal end of the drill string; a selectively a.c. electrically conductive signal path disposed in the drill string associated with a frequency selective switch disposed adjacent the sensor means; means for switching the frequency selective switch to alter the transmission characteristics of the switch for preferential transmission of one or other of the frequencies F1 or F2 or both: means for measuring electrical signals transmitted from the drill string casing to a point, typically on the surface near the open end of the borehole: and means for analysing the measured signal and determining from the proportions of the frequencies F1 and F2 present in the signal the sensor information transmitted to the frequency selective switch.

The sensor device may be a telemetry device which provides information about the location and/or orientation of the drill stung. Typical telemetry devices include three axis accelerometers, triaxial magnetometers, electronic compasses, and callipers, although other telemetry devices could be used.

The present invention has a number of advantages over the existing prior art. First, the main signal transmission apparatus is located above ground. This means that the power requirements for the components located in the borehole (the "down hole componciiis''j are very low. All that is required is sufficient power to operate whatever telemetry or sensor device is being operated down hole, plus sufficient power to transmit telemetry or sensor information to the frequency selective switch and switch the same in response to the telemetry information. The downhole system could easily operate on a single battery cell for over a year under normal use. Also because the down hole components are virtually unpowered, they are also intrinsically very safe.

Further, the problems of leakage of a signal from the drill pipe to the ground do not exist in the present invention since it is effectively the leaked

signal that is measured. Further while existing borehole data transmission systems have problems with noise, in the present system the noise signal is a common element to both the Fl + F2 signal received from above the frequency selective switch and the modified signal received from the drill string below the frequency selective switch, and since the noise element is common to both signals, the system is effectively immune from noise.

Typically the electrical signal will comprise two sine waves of a frequency of 10-1000 Hz and be fairly closely spaced together.

Typically Fl may be 60 Hz and F2 may be 70 Hz. It is preferred that the two frequencies are not to dissimilar since this would create problems in terms of the conductance of the ground varying substantially with changes in frequency.

The frequency selective switch may be a simple tuning circuit incorporating an inductance and capacitance and a switch which when operated prevents transmission of one of the two frequency signals e.g. F2.

Brief Description of the Drawings A specific embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which; Figure 1 is a schematic view of a system embodying the present invention in use in a coal mine borehole; Figure 2 shows an exemplary schematic circuit layout for apparatus embodying the present invention; and Figure 3 shows an exemplary frequency selective switch circuit.

Detailed Description of the Invention Referring to the drawings, Figure 1 shows a borehole, generally indicated at 10, into which a drill string 12 has been inserted. The drill string includes an outer drill pipe comprising a series of sections 12a, 12b to 12n which are steel pipes whose ends are threaded and which screw together. At the distal end of the drill string there is a drill bit 14. Adjacent to that bit there is a telemetry device 16 and adjacent the telemetry device 16 there is a selective a.c. electrically conductive section incorporating a frequency selective switch 18. Neither the drill bit nor the section which carries the telemetry device are a.c. insulating. Out of the hole, in the mine tunnel 11, there is located a signal generating and processing means

generally indicated at 20 which will include a power source, a processor means and a display screen for displaying information to an operator.

A typical circuit diagram for the system 20 is shown in Figure 2. The system uses oscillators 22 and 24 to produced sinusoidal electrical signals at frequencies Fl, typically 60 Hz and F2, typically 70 Hz. Those signals are summed by an op amp 26 and then transformed down from about 15 volts to 1 volt by a transformer 28. The resultant signal is then transmitted to the steel casing of the drill pipe along wire 30. The signals F1 and F2 are then transmitted down the steel casing of the drill string (see Figure 1) where the signal leaks to ground along the drill string. The leaked signal is measured at a point in the ground say 40.

A current transformer 43 measures the current passing through the pipe to point 40 and the signal is trans1.rlitted along wire 46 to processor 44.

As illustrated in Figure 1, over much of the length of the drill pipe the leaked signal comprises equal components of frequency F1 and F2 as illustrated by the electrical conductance lines marked F1 and F2 in Figure 1.

However, at the distal end of the drill bit the insulating section and frequency selective switch 18 (see Figure 3) can be operated to prevent transmission of one or other of the frequencies to the conductive part of the drill string beyond that section.

Thus, if frequency selector switch 18 is set to prevent transmission of F2, electrical conductance from beyond the frequency selective switch 18 only include elements of frequency F1. Thus when the total signal F1 + AF2 received at 40 is analysed at the processing means 44, proportionately more of frequency F1 is present in the signal F1 + AF2 then in the originally transmitted signal F1 + F2. By switching the frequency selective switch say to allow transmission of F1 and F2 equally, the received signal will then comprise Fl and F2 in equal quantities. Thus the switch 18 modulates the signal received at 40. The received signal can be analysed and the "background transmission" F1 + F2 from the majority of the length of the drill string removed to effectively produce a binary (1 or 0) signal. The signal can be picked up at the surface without the need for wires or expensive communication systems since the ground itself is used as the signal transmission medium.

Thus, the telemetry signal can be converted to a digital signal and used to switch the frequency selective switch rapidly. Typically the system

will operate at 60 bits per second depending on the carrier frequency, and thus telemetry information can be transmitted back to the processor means through the ground at 60 bits per second.

Because the system effectively relies on leakage between the drill pipe and the earth, signal leakage which is a scourge of existing systems is not a problem. Also background noise which is a problem with existing systems being common to both the F1 + F2 signal and the F1 + AF2 signal tends to be cancelled out in the analysis of the signals and is thus not a significant problem.

Because the downhole apparatus need not be powered to transmit signals up the hole, the frequency selective switch can be comparatively cheap piece of equipment with the result that loss of the end of the drill string is not as expensive as compared with existing systems.

Although the device described above is used to transmit telemetry signals from a bore hole to the surface, it will be appreciated that the system is equally applicable to transmission of other information.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.