GB2180928A - Optical displacement sensor for timing geophones in seismic survey equipment - Google Patents

Abstract

An optical pattern 3 is interposed in a light path between a transmitter and a detector 9, movement of the optical pattern 3 causing a frequency to be generated by the detector, and this is amplified and converted to a voltage. When the optical pattern is fixed to the hydraulic ram 4, 5 of a seismic survey equipment, when the ram hits the ground the pattern stops and therefore so does the output. This event causes a monostable circuit to provide an output pulse indicative of the event, and this is used to synchronise the seismic geophones. <IMAGE>

Description

SPECIFICATION Measuring equipment This invention relates to a measuring equipment, particularly to such equipmentforproviding atiming signal for, but not exclusively for, seismic survey equipment.

It is known to carry out seismic surveys by impacting the ground with a large mass and one form ofthis equipment has a hydraulically operated ram which is vertically mounted and arranged to impactthe ground sending shockwaves through the earth. The subsequent reflected acoustic waves in the ground which are indicative ofthe nature of the substrate undersurvey, are picked up by sevies of geophones and analysed.

It is importantto determine the precise moment of impact of the ram to enable all the geophones data to be correctly synchronised, and the system described above uses an accelerometer to determine the time of impact. The accelerometer is secured to the ram and determines the point of maximum deceleration caused when the descent ofthe ram is halted as it impacts the ground.Atime signal is derived from the accelerometer to synchronise the geophone data.

Although the accelerometer operates satisfactorily it requires frequentchanging because it becomes damaged, due to the shock loads imposed, and this is expensive and inconvenient.

It is an object ofthe present invention to provide a measuring equipment which will overcomethisdis- advantage and is cheaper.

According to the present invention, there is provided a measuring equipment comprising an optical transmitter, an optical receiver for receiving the signal from the transmitter, and an optical pattern for location in the light path from the transmitter to the receiver, wherein relative movement between the light path and the pattern will cause an output signal from the receiver.

In orderthat the invention can be clearly under- stood reference will now be madeto the accompany- ing drawings, in which Figure 1 shows part of an hydraulically operated seismic ram according to an embodiment ofthe present invention, Figure 2 shows in greater detail the measuring equipment of Figure 1.

Figure 3 is a block schematic diagram of the circuitry of the measuring equipment, and Figure4, 4A, 4B, 4Cand4Dare graphs explaining the operation ofthe ram and measuring equipment.

Referring to Figure 1, the timing equipment is shown mounted on a mobile seismic survey ram, A reflective grating 3 (discussed in detail later) is mounted on the shaft 4 ofthe ram which has an impacting head 5. A casing 6 supports the ram and is mounted on a vehicle 7. A bracket 8 on the casing 6 supports an optical transmit/receive equipment 9.

Hydraulic hoses 10 drive the ram downwards tow ardsthe ground 11 so that the head 5 impacts the ground. As the shaft 4 of the ram travels downwards it carries the reflective grating 3 past the transmit/ receive equipment so that it is positioned in the optical beam path of the equipment. The ram impactsthe ground, with a repetition rate of up to 2 seconds, sending shock waves through the earth, and the precise moment of impact is determined by the grating 3 and the transmit/receive equipment 9.

Figure 2 ofthe drawings shows schematicallythe timing equipment in greater detail. The reflective grating 3 mounted onto the ram shaft 4 is illuminated bye beam 10Afrom an LED 10 (À 940nm),andthe reflected signal 1 OB detected by a suitably filtered photodiode 16,17. The signal frequencyfrom the detector 17 will be proportional to the velocity of the ram, and by use of a frequency to voltage converter 18 on the detector output V+ will give a waveform similarto Figure 40.

The required signal indicating the zero velocity point is a TTL pulse of predetermined width. The circuitry (Figure 3) to produce this will be resetbythe positive going output of the Fto V converter 18 after each stroke of the ram to prevent spurious trigger pulses. The pulse will be produced by a monostable 19, the triggerfor which will be the negative going edge of a schmittrigger, with reference levels set as in Figure 40. A second possible method would beto differentiate the output of the Fto V converter 18 and trigger on the zero crossing. This may prove neces spa wry if the output waveforms differwidelyforthevar- ious terrains likely to be encountered. This is represented graphically in Figure 4D.

In Figure 3 the output V+ from the photodetector 17 is applied to a rectifier D and the unipolara.c.

signal V1 appears across resistor R1 . This is amplified by an amplifierAMPandthefrequencytovoltage converter 18 output V0 is smoothed by resistor R2 and capacitor Cto provide a level d.c. inputto monostable 1 as shown in Figure 4C, as the grating 3 descends through the optical beam path 5A (Figure 2) which produces a frequency signal V+ from detector 17 in the audio range.

Figure 4A shows the displacement of the earth 11 impacted by the ram from time To when the head 5 just touches the ground, through the time T1 when maximum displacement has taken place, to timeT2 when the earth, under normal circumstances will revertto substantially the same level as before impact.

Figure 48 shows the corresponding velocity ofthe earth during the impact. Figure 4C shows the corresponding output V0 ofthefrequency-to-voltage converter during the impact. As impact commences the ram decelerates rapidly and so too therefore does the reflective grating 3 that the frequency of the output V+ from the photodetector decreases rapidly, thus rapidly reducing thevoltageVOthroughthe reference levels V1 and V2 ofthe Schmidt trigger as shown in Figure 4C, causing the timing pulse from the 0/P ofthe monostable 19 for synchronising the data from the geophones (not shown).

Referring backto Figure 2, the size of the reflective grating will be 60 cm long, sufficient to allowforthe variations in end position of 15 cm. The grating will have a 1:1 markto space ratio of 2mm each. This also gives a maximum frequency of 12 kHz. The grating can be produced by depositions of aluminium or silver on the rear face of a transparent material,followed by a black over coating. This will produce a contrast of better than 85%. The material used forthe grating will probably be "Lexan" (Trade Mark) poiy- carbonate. A coating is preferable which has very good abrasive resistance and resistance to solvents and fuels.

Both the transmitter and receiveraperturesizes will be commensuratewith the size andthegratingto produce the maximum signal contrast, and prevent any front surface reflection being detected. The power level of the transmitter will be matched to that of the receiver, such that even ifdirt causes a 90% loss in signal the system will still function correctly.

The receiver will be optical ivy filtered to remove ambientlighteffects in orderto maximisethe required signal to noise ratio and to enhance the performance in conditions of strong ambient light.

Preferably the reflective grating is mounted in an aluminium casing and botedtothe ram. The spacing between the reflective grating the Tx/Rx unit should be as small as possible, e.g. 2cms.

The system described gives an output proportional to the velocity of the hydraulic ram. This is achieved by detecting the reflected radiation from a grating reflector mounted on the ram. This velocity information isthen used to find thetime atwhich maximum ram extensions occurs, i.e. zero velocity.

The system requires no additional moving parts and there is no contact between the ram and the detection system. The ram-mounted reflector is the only component required on the ram and will be a very rugged device needing no servicing except perhaps a daily "wipe clean" before operation. The rest of the system will contain semiconductor devices.

Claims (1)

1. CLAIM

   1. A measuring equipment comprising an optical transmitter, an optical receiver for receiving the signal from the transmitter, and an optical pattern for location in the light path from the transmitter to the receiver, wherein relative movement between the light path and the pattern will cause an output signal from the receiver.