Studies Of Cross-well Seismic Attenuation Tomography Method Based On Amplitude And Frequency

When the seismic wave propagating in the rock, the viscoelastic properties of therock results in attenuation of seismic wave energy. This part of the attenuation energyis converted into heat by way of friction. Q commonly used to represent the strength ofthe layer medium’s internal friction. Q relates to porosity, pressures, pore fluid propertiesand pore fluid saturation of the layer medium, Fractures and lithology’s changes are alsolargely affect the Q value. Therefore, the studies on Q, can deepen their understanding ofthe formation, It is also very promising to explore the oil and gas through the Q value.Currently, there is many methods to calculate the attenuation factor, such as the peakamplitude of the direct wave changes, the rise time of the waveform changes, the ratio ofthe frequency-domain’s spectral fitting and centroid frequency offset. in practical,theirapplications have certain limitations. Through the research of the centroid frequencymethod and spectral ratio method, this paper proposes a joint amplitude and frequency ofthe inversion method.Attenuation tomography is performed based on the velocity tomography. Aftergetting the speed through tomographic inversion in the use of first break, it computes theGMM wave-front based on Fermat’s principle and ray-tracing, and unlike theconventional method of calculating wave-front, GMM algorithm has high accuracy andhigh efficiency. In the inversion, it uses the damped least squares algorithm, and theintroduction of priori constraint and smoothness constraints, as well as a wide constraint.This inversion algorithm improves noise immunity, reducing the multiple solutions.For the equations based on the amplitude and frequency of the information, it’scombined by the least squares criterion, and optimized the inversion effect by setting theweighting coefficients.A Cross-well Seismic Attenuation Tomography Software has been developed. The software has functions of first-break tomography, attenuation tomography, direct waveextraction and waveform processing. The attenuation tomography module contains thecentroid frequency shift method, spectral ratio and the joint amplitude and frequencymethod.Their applicability are introduced through some models, and the actual data is alsocalculated and analyzed. The results suggest that the joint amplitude and frequency ofattenuation inversion method inherits stability centroid frequency method, and theinversion effect is better than using a single information retrieval results.Firstly, twosynthetic models were applied to the centroid frequency shift method, spectral ratiomethod and the joint amplitude and frequency of attenuation inversion method, due tothe theoretical data without interference, three methods have shown good inversioneffect. Just for a theoretical model by wave equation, the picked direct wave presencesabnormalities in the high frequency spectrum, which resulted centroid frequency offseterror after processing. This error largely impacts on the high-Q region, and results thepartial anomaly of the centroid frequency shift method in inversion model. This error hadno effect on the information of spectrum ratio, so the inversion result of the jointamplitude and frequency method is well. Then, through the synthetic model addedhigh-frequency noise and random noise, this paper analyzes the advantages anddisadvantages of three methods, the centroid frequency shift method has certaincapabilities on noise suppression, and high correlation of spectrum ratio data contains thehigh-quality information of the attenuation factor, it will get a high resolution andhigh-credibility Q value by combining the information of amplitude attenuation andfrequency shift together. Finally, the joint amplitude and frequency method is alsoapplied to the actual data, the inversion results also has higher local resolution than theresult of centroid frequency shift inversion method, and the area of abnormal Q valuedue to data errors is also much less.