S U M M A R YThe Ricker wavelet, which is often employed in seismic analysis, has a symmetrical form. Seismic wavelets observed from field data, however, are commonly asymmetric with respect to the time variation. In order to better represent seismic signals, asymmetrical wavelets are defined systematically as fractional derivatives of a Gaussian function in which the Ricker wavelet becomes just a special case with the integer derivative of order 2. The fractional value and a reference frequency are two key parameters in the generalization. Frequency characteristics, such as the central frequency, the bandwidth, the mean frequency and the deviation, may be expressed analytically in closed forms. In practice, once the statistical properties (the mean frequency and deviation) are numerically evaluated from the discrete Fourier spectra of seismic data, these analytical expressions can be used to uniquely determine the fractional value and the reference frequency, and subsequently to derive various frequency quantities needed for the wavelet analysis. It is demonstrated that field seismic signals, recorded at various depths in a vertical borehole, can be closely approximated by generalized wavelets, defined in terms of fractional values and reference frequencies.Key words: Time-series analysis; Numerical solutions; Computational seismology; Wave propagation.
I N T RO D U C T I O NThe Ricker wavelet is a well-known symmetrical waveform in the time domain (Ricker 1953). In order to better represent practically observed non-Ricker forms of seismic signals (Hosken 1988), the symmetric Ricker wavelet is generalized to be asymmetrical.While the Ricker wavelet is the second derivative of a Gaussian function, generalization is achieved by modifying the derivative order from the integer '2' to a fractional value. For mathematical convenience, the base function is the same Gaussian function, rather than other alternative forms. Therefore, generalized wavelets are systematically defined by fractional derivatives of a Gaussian function.Generalized wavelets have similar Fourier spectra because they are derived from the same Gaussian function. Their spectra differ from each other only in a frequency-related factor (iω) u , where ω is the angular frequency and u is the fractional order of the time derivative. Although there are possibly different definitions for field seismic wavelets, the current paper provides a systematic definition of non-Ricker wavelets and meanwhile can use the Ricker wavelet as a benchmark. This paper will prove that the power spectrum of a generalized wavelet is close to a Gaussian distribution, and thus the mean frequency is approximately equal to the central frequency. The degree of similarity between a generalized wavelet spectrum and a Gaussian distribution depends upon the fractional value.For a generalized wavelet with a variable fractional value u, the central frequency and the frequency band can be expressed analytically, using a special function, the Lambert W function (Lambert 1758(Lambert , 1772...