Patent GB-1507748-A (GB)
Title: Apparatus for recording signals produced by telluric and magnetic field detectors
IPC: G01D9/28
Filed: 1975-05-12
Abstract: 1507748 Surveying electromagnetically SOC NATIONALE DES PETROLES D'AQUITAINE 12 May 1975 [21 May 1974] 19838/75 Heading G1N In magneto telluric prospecting the signals produced by the telluric detectors and magnetic field detectors are recorded automatically in digital form at the time of prospecting. Each telluric detector in the form of a pair of spaced probes inserted in the ground is connnected by a respective pair of lines (3a, 3a' or 3b, 3b') to the inputs of amplifying stages 2a, 2b which are each in the form of a differential amplifier Fig. 2 (not shown) the output of which is connected via either a high pass filter or directly to the input of a variable gain amplifier the output from which is obtained on line 6a or 6b after rejectin of frequencies of 50 or 60 Hz. The output is then passed through an appropriate filter 11a, b, 12a, b, 13a, b in accordance with the position of switch 8a, 8b, the filters passing 0À001 to 1 Hz; 0À3 to 20 Hz and 3 to 120 Hz, to the input of a respective binary gain amplifier 15a, 15b, Fig. 4 (not shown) and which comprises an amplifier with four selectable resistors in the feedback thereof so that the gain of the amplifier may be 1, 2, 4, 8 or 16; a bi-stable trigger circuit being connected to the resistors so that a signal representative of the binary gain may be obtained an lines 29a, 29b. The amplifier outputs on lines 17a, b are fed to the respective inputs of a multi-plexer 18 the outputs of which are connected to an A/D converter 19. The outputs from the converter 19 representing the address of the signal being sampled and the digital value of the sample are fed via lines 20, 21 to a gain adjusting circuit 22 and the digital value samples are also fed to a digital multi-plexer of a digital recorder 24 which also includes sizing module 36, buffer memory 28 and tape recorder 27. The magnetic field detectors are each connected a line 5a, 5b to the input of a respective amplifier 4a, 4b, Fig. 3 (not shown) having a gain which can be adjusted to one of three values depending on whether low, intermediate or high frequencies are to be recorded. The output is fed to lines 7a, 7b via a 50 or 60 Hz rejection filter which lines are connected via filter stage 10 and binary gain amplifiers 16 to the multi-plexer 18 as for the telluric detectors. Operation. Recording selector 35 is set in accordance with the selected recording mode, low, intermediate or high frequency e.g. if low frequency signals from the detectors are to be recorded the selector selects the sampling sequence of multi-plexer 18, selects via switches 8a, 8b, 8c, 8d the low frequency low pass filters 11a, 11b, 11c, 11d, the direct connection of the amplifier of amplifiers 2a, 2b and the low frequency gain of amplifiers 4a, 4b. In the first phase of the sampling sequence of the multi-plexer 18 with the gain of amplifiers 15a, b, c, d set to 1 the four analogue values taken by multi-plexer 18 are converted into digital samples and transmitted to the digital comparator of circuit 22 where they are compared with an appropriate reference value for obtaining a correct operation of the multi-plexer. The comparative output is fed to a rotatable memory to which is also applied via lead 21 the address of the channel from which the sample was taken. The memory then transmits via lines 23a, b, c, d the appropriate order to the feedback circuits of the binary gain amplifers to select the appropriate gain thereof as determined by the digital comparator. In the second phase of the sampling cycle after the gain amplifers have been adjusted to their optimum gain, the multi-plexer takes a second series of four samples and after conversion to digital values in 19 transmits them to the digital multi-plexer 25 of the recording system 24 which also receives via lines 29a, b, c, d the binary gain values corresponding to each one of the samples. The samples and values are then transmitted to sizing module 26 where they are arranged in the form of blocks having a selected length prior to being trasmitted to magnetic tape recorder 27 through buffer memory 28. Each operation may be labelled by means of module 30.