GB2104665A

GB2104665A - Borehole sensing device

Info

Publication number: GB2104665A
Application number: GB08222228A
Authority: GB
Inventors: Arsenio Negro; Figueiredo Alinor F De
Original assignee: FIGUEIREDO ALINOR F DE; SOIL INSTR Ltd; SOIL INSTRUMENTS Ltd
Current assignee: FIGUEIREDO ALINOR F DE; SOIL INSTR Ltd; SOIL INSTRUMENTS Ltd
Priority date: 1981-08-03
Filing date: 1982-08-02
Publication date: 1983-03-09

Abstract

A probe for lowering down a borehole which is water filled and which includes magnets spaced along its length whereby extension of the borehole may be determined. The probe comprising a body 1 housing a battery 5 and electrical audio oscillator 3 activated by reed switch 4 as same passes a magnet in the borehole. The oscillator feeds a signal through a conductive measuring tape 7 to a surface station where the signal is detected via a return path established through the soil and the probe body to soil capacitance. This obviates the need for separate signal cables to the surface station. <IMAGE>

Description

SPECIFICATION A sensing device for borehole extensometers This invention relates to a sensing device for borehole extensometers. The borehole extensometer is an instrument which is simple, reliable, accurate and relatively inexpensive but known devices have the disadvantage that it is necessary to lower the sensor with a separate tape and cable attached. For deeper boreholes this arrangement is troublesome as tape and cable can become twisted causing jamming in the guide tube and inaccurate readings may ensue. In addition at least two operators are needed for deep boreholes to handle the separate cable and tape.

The known system comprises a series of magnets installed adjacent to a plastic or nonferous access tube and a probe containing a reed switch is passed through the tube. The reed switch closes on entering the magnet field. The known system employs a twin conductor cable and a graduated steel tape.

An object of this invention is to provide a sensing device which does not require a separate electrical cable to be lowered into the borehole along with the measurement tape.

According to this invention a sensing device forming a borehole extensometer comprises a metallic probe housing an electrical oscillator, a power source and a magnetically operated switch which, when closed energises the oscillator, the oscillator output being connected between the probe body and one end of an electrically conductive tape, the other end of which is connected to detector means responsive to the oscillator output signal, a return path for the oscillator signal being established, in use, through ground conductance and via the probe to ground capacitance.

Through use of the conductive tape and ground capacitance for the return path the separate cable is avoided. The oscillator preferably operates at a frequency of 1 kHz and by using fast rise and fall pulses high frequency harmonics are produced to obtain sufficient signal transmission.

If the borehole includes a conductive lining then connection may be made thereto whereby the lining becomes the equivalent of the aforesaid ground connection.

The basis of the invention in a preferred embodiment is to discard the cable and transmit an electrical signal controlled by a reed switch up the standard PVC coated measuring tape or other suitable conductor to an operator at the top of the borehole. The oscillator is a square wave generator which is electrically connected to the bottom of the tape. When the reed switch closes power from a battery contained in the probe is applied to the generator producing square waves.

The signal is picked up at the top of the borehole, amplified and applied to a small loudspeaker or made to operate a buzzer. The signal return travels via ground and the capacitance of the probe in the borehole. The return presents a high impedance to low frequencies whilst the probe is surrounded by air in the borehole but, although the oscillator is set to run at about 1 kHz, the fast rise and fail times ensure that there are sufficient high fourier frequencies for good reception to be obtained at the top of the tape. When the probe is immersed in water the signal return impedance decreases significantly giving better reception. Use of an audio amplifier as a receiver gives the incoming frequencies an envelope which generates an audio tone on the loudspeaker or, rectified, can operate a buzzer switch.

Preferably the square wave generator is an integrated circuit and power is supplied by a stack of silver oxide cells, six for example being used.

Good battery life is ensured by low power consumption and a low duty cycle being switched on only at the ring magnet positions in the borehole. The generator and cells form an assembly mounted immediately above the reed switch. The lower part of the probe may be of brass or stainless steel to give it weight and may be unscrewed to give access to the batteries. The square wave oscillator is earthed to the probe casing. The output lead is clamped firmly to the end of the tape, the clamp, tape end and output lead then being encased in an epoxy or other potting resin. This resin provides a moisture-proof seal and the attachment to the probe housing.

The input to the audio amplifier may be taken from the centre of the tape reel by means of a simple jack plug. The audio amplifier used is an integrated circuit, selected for high gain (75 db).

This is housed in a small box with batteries and a loudspeaker and mounted on the side of the winding reel.

This invention also thus provides an apparatus comprising the sensing device, tape reel and signal detecting means as well as a method of determining borehole extension using the sensing' device.

The accompanying Figure 1 and Figure 2 of the drawings shows two practical constructions of sensing device both in longitudinal section and by way of examples only.

As shown in Figure 1 the device comprises a stainless steel body 1 with a screw connected nose 2, the body housing an oscillator 3, magnetic reed switch 4 and battery stack 5. One side of the oscillator output is connected at 6 to the body 1 and the other side is connected to the one end of a graduated tape 7 by means of a clamp 8. The tape end and clamp 8 are embedded within epoxy resin 9 and thus electrically insulated from the body.

The oscillator produces a signal each time the reed switch 4 closes during passage past a magnet on travelling down the borehole tube.

This sensing device is associated with the detector at the top of the borehole as described previously.

Figure 2 shows a further embodiment wherein like parts are identified by like reference numerals.

In this embodiment the oscillator 3 and reed switch 4 are mounted on a printed circuit board 10 one end of which is connected to the tape 7 surrounded by an epoxy resin connector plug 9.

The earth connection 6 is formed directly by contact 11 at one end of the stack of batteries 5 connecting with the nose weight 2. The plug 9 may be an insulator other than epoxy resin and connected to the body 1 by screw threading 1 2.

Seals 13 and 14 may be included at the nose to body and the plug to body connections. The probe is typically 210 mm in length and 16 mm in diameter.

Claims

1. A borehole extensometer securing device comprising a metallic probe housing an electrical oscillator, a power source and a magnetically operated switch which, when closed energises the oscillator, the oscillator output being connected between the probe body and one end of an electrically conductive tape, the other end of which is connected to detector means responsive to the oscillator output signal, a return path for the oscillator signal being established, in use, through ground conductance and via the probe to ground capacitance.

2. A device in accordance with claim 1, wherein the ground is constituted by a borehole liner.

3. A device in accordance with claim 1 or 2, wherein the oscillator is a square wave audio frequency generator.

4. A device in accordance with claim 3, wherein the generator frequency is nominally 1 kHz.

5. A device in accordance with any preceding claim, wherein the probe body comprises a cylinder housing the oscillator, a battery forming the power source and a connection to the tape extending through an electrically insulating plug closing the one end of the cylinder, the other end of the cylinder being closed and carrying a nose weight.

6. A device in accordance with claim 5, wherein the nose weight is screw connected to the other end of the cylinder and forms an electrical connection with one end of the battery.

7. A device in accordance with claims 5 or 6, wherein the tape extends through a plug comprising an epoxy resin compound.

8. A device in accordance with any one of claims 5 to 7, wherein the oscillator comprises a printed circuit board mounted in the body with the plane thereof extending longitudinally of the probe.

9. A device in accordance with any preceding claim in combination with a reeling means about which the tape is wound and unwound, the reeling means having a detector responsive to the oscillator signal electrically connected to the tape and to the ground.

10. A method for the measurement of borehole extension using a device as claimed in any preceding c!aim and carried out as described herein.

1 A method as claimed in claim 10, wherein the borehole is filled with water.

12. A sensing device substantially as herein described with reference to and as shown in Figure 1 or Figure 2 of the drawings.