Study On The Time-Frequency Characteristics Of The Array Acoustic Logging Signal

Time-frequency analysis is an effective method to process array acoustic logging data,but different time-frequency analysis methods have different characteristics.The array acoustic logging signal is decomposed into finite intrinsic mode functions(IMFs)by empirical mode decomposition(EMD).The Short time Fourier transform,Choi-Williams distribution,smoothed pseudo Wigner-Ville distribution and reassigned spectrum were used to process the original array acoustic logging data,as well as IMF1,IMF2 and IMF3.These four methods were compared in terms of time resolution,frequency resolution and cross term interference.The results showed that the IMF1 mainly contains the information of P-wave and S-wave,the IMF2 mainly contains the information of high-frequency Stoneley wave and pseudo Rayleigh wave,and the IMF3 mainly contains the information of low-frequency Stoneley wave.None of the four methods has the problem of cross term interference affecting the pattern wave recognition,while the time-frequency resolution and frequency resolution of reassigned spectrum are the highest.In order to further verify the advantages of the rearrangement spectrum,the edge characteristics of time-frequency distribution was used to process the array acoustic logging signal of reservoirs with different fluid properties and different types of fractured formations.Because the time-frequency edge is calculated by accxmiulating the time-frequency distribution value,it is disadvantageous to identify the P-wave with low energy.Therefore,the time slice and frequency slice of the rearrangement spectrum are introduced to process the IMF1,IMF2,IMF3 of reservoirs with different fluid properties and different types of fractured formations.Compared with the time-frequency edge,the comprehensive analysis of different time slices and frequency slices can get the information that can,t be displayed in the time-frequency edge feature map.The conclusions are as follow:1?Based on the dry layer wave train,the energy of the P-wave and S-wave of the water layer is obviously attenuated,and the energy of the high frequency Stoneley wave and low frequency Stoneley wave are all attenuated to a certain extent.The energy of the P-wave and S-wave of the oil-water layer is between the water layer and the oil layer,and the energy of the low frequency Stoneley wave is attenuated Serious.The energy of P-wave and S-wave in the oil layer also attenuates,but the attenuation degree is not as large as that of the water layer,and the energy attenuation of high and low frequency Stoneley wave is very serious.2?The time edge shows that the peak time of P-wave and S-wave in the water layer lags behind to some extent,and the peak time of P-wave and S-wave in the oil-water layer lags behind,while the peak time of P-wave and S-wave in the oil layer lags behind,and the peak time of high and low frequency Stoneley wave in the water layer and oil layer does not change obviously.The frequency edge map shows that the main frequency of P-wave and S-wave in water layer has no significant change,while the main frequency of S-wave in oil layer reduce,and the main frequency of high and low frequency Stoneley wave peak in water layer and oil layer has no significant change.It can be seen that,compared with the dry layer,the water layer,oil-water layer and oil layer have different time-frequency edge characteristics,so the method in this paper can be used to distinguish the reservoirs with different fluid properties.3?Compared with the tight formation,the time lag of P-wave peak is obvious,the change of main frequency is not obvious,and the energy is decreased in the formation with low angle fracture;the time delay of S-wave peak is obvious,the main frequency is reduced,and the energy is decreased;the time of high and low frequency Stoneley wave peak is basically no delay,the main frequency is basically unchanged,and the energy is slightly reduced.4n Compared with the tight formation,the peak time of P-wave and S-wave in the formation with high angle fracture is delayed to some extent,but it is not obvious in the formation with low angle fracture.There is no obvious change in the main frequency of P-wave and the main frequency of S-wave decreases.The energy of P-wave and S-wave decreased.After the peak time of high-frequency Stoneley wave is delayed,the main frequency has no obvious change,and the degree of energy reduction is higher than that of medium and low angle fracture formation.The peak time of low-frequency Stoneley wave is basically not delayed,the main frequency is basically unchanged,and the energy change is not obvious.It can be seen that the high angle fracture has more influence on the high frequency component of Stoneley wave.