Method And Implementation Of Information Extraction For Pseudo-Random Coding TEM Method

With the domestic oil fields gradually entered into the late development stage,the problem that how to transport dynamic monitoring of oil gas water and remaining reserves so as to improve the effective and direct reference for oil recovery,is becoming more and more imminent.Because the differences in the resistivity parameters of oil gas and water are huge,and the electrical difference of electromagnetic exploration method to detect the target sensitive.Electromagnetic exploration method is the most direct and reliable method to quantitative evaluation of reservoir oil and gas.Compared with other electromagnetic exploration methods,time domain electromagnetic exploration method has the advantages of high resolution,strong anti-interference ability and high efficiency.It is the preferred method for reservoir dynamic monitoring.In this area there is a general oil pumping machine,oil tree,transformer substation.Because of the limitation of construction,environmental noise and other factors,time domain electromagnetic exploration method can’t get a good effect in the scene.In order to improve the signal to noise ratio(SNR)and the accuracy of measurement,two methods are used: increasing the transmit power and increasing the number of data stack.To improve the transmission power source,will cause the device to increase the weight.It is difficult to move fast.It will also increase the overall cost of equipment and technical difficulty,when reaching a certain power after the increase in power of the obtained improvement effect is not obvious.Increase the number of data stack,will pay more time for the acquisition,reducing the efficiency of the work,it will also lead into more unknown noise.Identification techniques which have been applied in many fields give the third choice.Due to the fact that the interference signal is not related to the transmitted signal,the power spectrum of the interference and noise in the signal frequency band can be greatly reduced by the correlation identification technology,so as to improve the output signal to noise ratio(SNR)of the system.The Wiener-Hopef equation has been how to identify the response of electromagnetic pulse through the cross-correlation function,only when meet the emission signal autocorrelation function has characteristics similar to the Delta function,equation was set up.Therefore,the focus of the problem is how to choose a reasonable transmission signal,and ensure that it has good autocorrelation characteristics.Although the white noise signal to meet the above conditions,the fact that the white noise signal is difficult to collect and response signal the required long time,indicating that the actual workingconditions greatly limits the white noise as the source signal correlation identification.If the pseudo-random signal instead of white noise,not only meeting the conditions of the equation,but also overcoming these two shortcomings.Pseudo random code is a kind of coding sequence which is similar to white noise,and the pseudo random code contains m sequence,reverse repeated m sequence,M sequence and Gold sequence.Therefore,the use of pseudo random code as transmitting signal,and then through the extraction method of magnetotelluric pulse response identification technology,it can reduce the influence of electromagnetic interference on the acquisition accuracy of formation information without additional power and acquisition time.Based on the above arguments,the authors propose a time-domain electromagnetic exploration method based on m sequence coding pulse source,call it Coding pulse-source TEM method,abbreviated as CPTEM.According to this method,a set of pseudo random electromagnetic exploration equipment,field construction process and information extraction method for oil and gas exploration and monitoring are designed.The m sequence signal has the advantages of wide frequency band,good self correlation and so on.It can be used to resist the strong electromagnetic interference in the work area to improve the exploration precision and exploration efficiency.The pulse source time domain electromagnetic exploration method has the advantages of high sensitivity and high resolution,and can be used to directly determine the oil and gas layers.Therefore,the combination of the two methods can be used as the first choice for reservoir dynamic monitoring.Firstly,by analyzing the generation process and related characteristics of several commonly used pseudo-random codes,and combining with the characteristics of time domain electromagnetic exploration methods,the M code which is the most easy to produce and self correlation is selected.By comparing the autocorrelation of the step signal coding and the pulse signal coding,it is proved that the use of pulse source coding will have better medium and shallow layer resolution.The coding scheme based on m sequence and pulse source is developed,and the selection rules of some important parameters are analyzed.Then the encoding scheme based on description of the relevant identification by extracting magnetotelluric impulse response function method.Taking the half space model as an example,to verify that the encoding scheme and relevant identification method can effectively extract the electro-magnetic pulse response function.According to the requirements of theoretical research and coding scheme,a hardware implementation scheme of time domain electromagnetic method for coded pulse sourceis proposed.The transmitting system consists of a pulsed electromagnetic field source,a timing controller,a current recorder,etc.The m sequence code controlled by the GPS in the timing controller is used to control the pulse electromagnetic field source to transmit the m sequence to encode the electromagnetic pulse signal.The receiving system adopts the V8 electromagnetic acquisition system of Canadian Phoenix company.The V8 system can be applied to the signal acquisition of the time domain electromagnetic signal of coded pulse source by designing the V8 field layout and working parameters.In accordance with the relevant identification principle,design the data source encoding pulse time domain electromagnetic method,including cutting data,data acquisition,conversion,lattice valued mean,correlation,amplitude correction and cycle interception and other steps,the final calculation of electromagnetic pulse response.According to the design scheme,the author completed the encoding method of time domain electromagnetic pulse source principle prototype,and on December 2016 in Tian Jin Da Gang oilfield were three sets of experiments were collected by the same system collocation and different emission sources.The emission sources of the three groups were conventional TD50 waveform emission,periodic pulse source emission and pseudo-random coded pulse source emission.Through the field data acquisition and information extraction process,by comparing the three groups of tests of the formation of impulse response curve,shown by pseudo random encoding pulse emission scheme can get the correct response information and has strong anti noise ability.Theoretical research and experimental results show that: according to the pulse type electromagnetic field source encoding pseudo random m sequence,and then use the impulse response method to extract land related identification method is feasible,the instrument has the advantages of small volume,strong noise resistance,and is compatible with the traditional software of electromagnetic instrument and traditional electromagnetic inversion,it makes up for the lack of the traditional electromagnetic method,has broad application prospects.