| The transient electromagnetic receiver on shallow seafloor has been designed. It is an application of Transient Electromagnetic Methods (TEM) to seafloor, and adopts a special measurement—towed measurement under the special environment of seafloor. TEM utilizes ungrounded loop-line (big loop-line or magnetic dipole source) or grounded electric source (electric dipole source) to send the first field and then measure the induced magnetic field (the second field) created by underground substance during the intermittent periods, and then it can determine the position of underground inhomogeneous substance as well as its scale and electric capability. The transient electromagnetic system on shallow seafloor consists of the upper and lower computer, and the lower computer is composed of receiver, transmitter and observation equipment. The high-powered receiver acquires the valid signal which is reflected by underground substance, and transmits it to upper computer to be processed, computed and plotted. TEM applied to seafloor can supply a gap of seismic exploration, and it can apply primely to such areas as hydrochloric acid reef, salt hill, permafrost on seafloor etc where seismic method has lower resolution. TEM on seafloor mainly explores pool, sulfide with metal, sandbar, especially sea engineering problem along the seacoast. Transient electromagnetic receiver on shallow seafloor mainly consists of master unit, data acquisition unit and data transmission. The features of TEM receiver are as follows: ①receiver acquires signal at the turn-off moment of transmitting pulse so it requires precise synchronization. ②As transient signal attenuates quickly during early period and with high amplitude (from dozens of μS to hundreds of μS at time and from volt degree to mV or even μV degree at amplitude), while attenuates slowly during middle and late period and with low amplitude (from hundreds ofuS to several mS even several S at time and from mV or μV degree to μV or nV degree at amplitude), in order to acquire the whole transient signal, such requirements as high sampling rate, wide dynamic range, measurable weak signal with low noise must be met. ③Transient signal is a kind of signal with wide frequency which energy mainly focuses on low frequency area, so the receiver has a low frequency range. ④Towed measurement can be realized on the flat seafloor. So it requires the miniaturization of the receiver and a series of functions included in the receiver such as auto-sampling and auto-communication etc. Based on the above features, floating-point amplification acquisition technique is adopted. It acquires the transient signal by exponent-code and trail-code, and exponent-code signal by 8-bit AD converter and trail-code by 16-bit AD converter with 5 μS conversion rate, and final reaches 24-bit floating-point acquisition which resolves both high sampling rate and high precision, that is wide dynamic range to measure weak signal. Preamplifier, signal accumulation and ambipolar acquisition in-phase methods are adopted to reject all kinds of noise. Signal accumulation method aims to reject white noise, and ambipolar acquisition in-phase method to reject the noise with low change, 50Hz noise and its harmonic. As GPS is non-effective on the seafloor, and high precision crystal synchronization requires the crystal be at a constant temperature and exists aging problem, so line synchronization manner is adopted on the condition that receiver and transmitter are placed in one towed body. In order to realize towed measurement, real-time display, and high rate & precision communication between upper and lower computer are required. Network transmission technique is adopted through integrated compare. The embedded CPU PC104 is used in the TEM receiver on shallow seafloor. PC104 is ISA of cutdown dimension, with compact construction, standard and mature system, high reliability, high assembly, modularization, series, flexibility in hardware. And it can totally compatible with the AT system structure of IBM PC. Any developing system can connect with the stack of PC/104 which greatly reduces the cost and period of hardware and software, so PC/104 module is an ideal core of reliable embedded system. This paper adopts PC104 with network card --HXL/P300 and HD in notebook PC installed with Windows 98. Program...
|
PDF Full Text Request