A BROADBAND DIGITAL GEOPHYSICAL TELEMETRY SYSTEM

Abstract

A system has been developed to simultaneously sample and transmit digital data from five remote geophysical data receiver stations to a control station that processes, displays, and stores the data. A microprocessor in each remote station receives commands from the control station over a single telemetry channel. The commands adjust the sensing amplifier’s input voltage range (1 microvolt to 2 volts peak-to-peak), the number of samples (128 to 30,000), the sampling rate (256 to 32,768 samples/sec), and the number of signals (1 to 16) to average at the remote stations. A 12 bit analog-to-digital converter samples data with bandwidths of 100 Hz to 10 kHz for time periods selected from 100 seconds to 1 second, respectively. Each remote station begins sampling geophysical signals when it receives a synchronizing pulse relayed from the control station. Digitized geophysical data is transmitted to the control station over broadband (100 kHz bandwidth) UHF telemetry channels using standard asynchronous serial (19.2 kbaud) techniques and hardware dropout detection and recovery. The amount of data (480 kbits) and the maximum time to transmit data (30 secs) dictate using broadband telemetry (even though most geophysical telemetry is less than 10kHz in bandwidth). Header information (transmitted before the data) contains station number, digital sampling parameters, transmission block size, and checksum. This information is used by a computer program (in a PDP-11/23 (***) minicomputer) to maintain up to 5 simultaneous Direct Memory Access (DMA) transfers from the remote stations into 5 separate data buffers. Filled buffers are transfered (via an IEEE-488 bus) to an interactive analysis and display system (HP9845C (***)) for selecting data to be stored on magnetic media for subsequent laboratory analysis.