The Effect of Low Frequencies on Seismic Data in Theory and a West Texas Case Study
Mcdaniel, Nicholas Carter
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Low frequencies in seismic data are important as they contribute to better wavelet stability, resolution, penetration, and inversion. These contributions from low frequencies in the seismic frequency band are investigated in theoretical terms through synthetic modeling and in a practical application via analysis of field data. Modeling wavelet stability shows that attributes of the seismic wavelet, such as peak-to-trough ratio, central lobe width, and central-to-sidelobe energy ratio, have a complex relationship to the frequency band that is often not dependent upon the maximum or minimum frequencies independently. Modeling penetration reiterates the importance of low frequencies for long travel paths because of the inverse relationship of frequency to scattering and absorption. Modeling impedance inversion with varying frequency band demonstrates the importance of low frequencies in filling the gap between conventional seismic and well log data. Modeling of the Fresnel zone connects spatial resolution to frequency and introduces the possibility of sparse sampling for low frequencies. Comparison of field data from a 2D seismic line in West Texas with collocated conventional geophones (10Hz) and low-frequency geophones (5 Hz) investigates the additional coherent frequency content from use of low-frequency geophones and the value of the associated extension of the frequency band. This analysis was performed through a comparison of frequency spectra, comparison of filter panels, and calculation of magnitude squared coherence using the raw data and comparison of frequency spectra, filter panels, and extracted wavelets in the processed data. The low-frequency geophone dataset did not yield as much coherent low-frequency content as anticipated, likely due to a lack of strong low-frequency signal in the presence of significant low-frequency noise. In conclusion, synthetic modeling demonstrated the importance of low frequencies in seismic data, while the field data analysis did not yield as much coherent low-frequency content as anticipated, likely due to lack of strong low-frequency signal in the presence of significant low-frequency noise.