Youli Quan

Seismic attenuation tomography using the frequency shift method

We present a method for estimating seismic attenuation based on frequency shift data. In most natural materials, seismic attenuation increases with frequency. The high-frequency components of the seismic signal are attenuated more rapidly than the low-frequency components as waves propagate. As a result, the centroid of the signals spectrum experiences a downshift during propagation. Under the assumption of a frequency-independent Q model, this downshift is proportional to a path integral through the attenuation distribution and can be used as observed data to reconstruct the attenuation distribution tomographically. The frequency shift method is applicable in any seismic survey geometry where the signal bandwidth is broad enough and the attenuation is high enough to cause noticeable losses of high frequencies during propagation. In comparison to some other methods of estimating attenuation, our frequency shift method is relatively insensitive to geometric spreading, reflection and transmission effects, source and receiver coupling and radiation patterns, and instrument responses. Tests of crosswell attenuation tomography on 1-D and 2-D geological structures are presented.

Differential acoustical resonance spectroscopy: an experimental method for estimating acoustic attenuation of porous media

A new acoustic method of estimating attenuation in rocks is presented. This method, called Differential Acoustic Resonance Spectroscopy or DARS, uses the perturbation in the Q and resonant frequency of a fluid-filled cavity resonator caused by the introduction of a small sample of rock. The changes in the resonant frequency and the quality factor are used to characterize the velocity and attenuation properties of the rock sample. The frequency of DARS operation is determined by the size of the fluid-filled cavity, not the size of the rock sample; therefore, DARS can be used to measure an acoustically small sample in the laboratory at low frequencies. This paper describes the DARS concept, theory of operation, and some attenuation results for small porous samples at a frequency near 1 kHz.

Seismogram synthesis for radially layered media using the generalized reflection/transmission coefficients method: Theory and applications to acoustic logging

A new method based on generalized reflection and transmission coefficients is proposed to calculate the synthetic seismograms in radially multilayered media. This method can be used to efficiently simulate full waveform acoustic logs and crosswell seismic profiles in situations where we need to consider borehole effects. The new formulation is tested by comparing our numerical results with previous available work and shows excellent agreement. Because of the use of the normalized Hankel functions and the normalization factors, this new algorithm for computing seismograms is stable numerically even for high-frequency problems. To show the applicability of this new approach to full waveform sonic logging, we apply it to investigate the effects of complex invaded zones on the geometrical spreading and attenuation estimation for P-waves. We find that a damaged zone (its velocity is slower than the unperturbed formation velocity) exhibits a convergence effect on the P-waves, and a flushed zone (velocity is faster than the unperturbed formation velocity) exhibits a divergence effect on the P-waves.

Estimating Flow Properties of Porous Media With a Model For Dynamic Diffusion

We present an approach for estimating effective compressibility and permeability from Differential acoustic resonance spectroscopy (DARS) laboratory measurements. The effective compressibility of fluid-saturated porous medium, located in a harmonic pressure field, is a function of the loading frequency and the fluid in the pore space, as well as the permeability and porosity of the medium. The process is describable by a diffusion process that relates effective compressibility and permeability. DARS is used to measure the effective compressibility. Then the diffusion model is used to estimate the permeability. This method is tested with DARS lab data.

Stochastic seismic inversion using both waveform and traveltime data and its application to time‐lapse monitoring

A stochastic approach to seismic inversion using the ensemble Kalman filter (EnKF) is proposed. Seismic depth and time image data are used as the input for EnKF stochastic seismic inversion. The sonic log is used to estimate source wavelet and create initial models for the inversion, which provides an efficient integration of sonic log data and seismic data. We use both travel time and waveform data for the inversion and obtain the absolute seismic velocity instead of the relative impedance. EnKF can continuously update the model using time-lapse data. A synthetic example is used to demonstrate the possible application to seismic monitoring.

Enhanced Oil Recovery Monitoring Using P-wave Attenuation

Crosswell attenuation tomography is used to image reservoir zones that were flooded by carbon dioxide. The swept zones contain mixtures of oil, water and carbon dioxide. Monitoring the saturation of these fluids is an important part of a enhanced oil recovery process. We estimate the changes in P-wave attenuation caused by the injection process. Laboratory data on flooded cores are analyzed in support of the field data interpretation. We use a broad band amplitude spectra method and the centroid frequency shift method to estimate the attenuation. The methods are applied to seismic waveforms recorded before and after the injection.

Acoustic Attenuation Logging Using Centroid Frequency Shift And Amplitude Ratio Methods: A Numerical Study

The centroid frequency shift method is proposed to estimate seismic attenuation from full waveform acoustic logs. This approach along with the amplitude ratio method is applied to investigate the attenuation properties of the P head wave in fluid-filled boreholes. The generalized reflection and transmission coefficients method is used to perform forward modeling. The authors suggest an empirical formula to describe the frequency-dependent geometrical spreading of the P-wave in a borehole. They simulate a more realistic borehole by including a mudcake and an invaded zone which are modeled by a large number of radially symmetric thin layers. The numerical tests show that this invaded zone exhibits very strong influence on the attenuation measurement.

Image integration with learned dictionaries and application to seismic monitoring

Sparse coding can be applied to train an overcomplete dictionary on time-lapse seismic data or images. The learned dictionary generally consists of sparse representations of one or more images. We then use such sparse representations, along with L1-regularization techniques, to predict missing values in seismic images by solving an inverse problem. The practical outcome of the proposed methodology can be a significant reduction in field operational costs by requiring only sparse instead of dense surveys, and by integrating in the seismic images the information captured by the learned dictionary from previous time-lapse and baseline images. A synthetic example is presented to test the method.

SUPER RESOLUTION OF TIME-LAPSE SEISMIC IMAGES

We present results of an on-going project to assess the applicability in reflection seismology of emerging super resolution techniques pioneered in digital photography. Our approach involves: (1) construction of a forward model connecting low resolution seismic images to high resolution ones, and (2) solution of a Tikhonov-regularized ill conditioned optimization problem to construct a high resolution image from several lower resolution counterparts; the high and low resolution images derived, respectively, from dense and sparse seismic surveys.

Cross-well Seismic Monitoring of Coal Bed Methane (CBM) Production: A Case Study From the Powder River Basin of Wyoming

The primary mechanism for Coal Bed Methane (CBM) recovery in a typical biogenic, low-rank, shallow and subbituminous coal like that of the Powder River Basin (PRB) of Wyoming is by dewatering or depressurization of the coal beds. In this case history, cross-well seismic experiments were designed to image changes in seismic signatures emanating from the dewatering process. Quantitative estimation of these changes is vital to the technical viability of the CBM reservoir process optimization and monitoring. The preand post-CBM production cross-well seismic surveys were executed in July, 2002 and August, 2003 respectively at the PRB’s Big George coal beds, Wyoming.