Crosshole seismic waveform tomography - I. Strategy for real data application

Abstract

S U M M A R YThe frequency-domain version of waveform tomography enables the use of distinct frequency components to adequately reconstruct the subsurface velocity field, and thereby dramatically reduces the input data quantity required for the inversion process. It makes waveform tomography a computationally tractable problem for production uses, but its applicability to real seismic data particularly in the petroleum exploration and development scale needs to be examined. As real data are often band limited with missing low frequencies, a good starting model is necessary for waveform tomography, to fill in the gap of low frequencies before the inversion of available frequencies. In the inversion stage, a group of frequencies should be used simultaneously at each iteration, to suppress the effect of data noise in the frequency domain. Meanwhile, a smoothness constraint on the model must be used in the inversion, to cope the effect of data noise, the effect of non-linearity of the problem, and the effect of strong sensitivities of short wavelength model variations. In this paper we use frequency-domain waveform tomography to provide quantitative velocity images of a crosshole target between boreholes 300 m apart. Due to the complexity of the local geology the velocity variations were extreme (between 3000 and 5500 m s −1 ), making the inversion problem highly non-linear. Nevertheless, the waveform tomography results correlate well with borehole logs, and provide realistic geological information that can be tracked between the boreholes with confidence.

Crosshole seismic tomography including the anisotropy effect

“…It is in contrast to a constant shrink factor used in the previous publication to include the anisotropy effect (Pratt and Shipp 1999). For the details of waveform tomography implemented in the frequency domain, please refer to Wang and Rao (2006).…”

“…Wang and Rao (2006) applied a weighting taper to the data, related to source-receiver (vertical) offsets, and effectively suppressed the common "X" type artefacts in the velocity image. Pratt and Shipp (1999) considered the anisotropy as a constant only in the background and preset it as a constant shrink factor in the whole research area during the numerical calculation.…”

Crosshole seismic tomography with cross-firing geometry

“…This frequency grouping strategy is for mitigating the effect of data noise, which is not necessarily white in the frequency domain (Pratt and Shipp, 1999;Wang and Rao, 2006). Simultaneously using neighboring frequencies from the same spatial imaging position in the inversion effectively increases the number of equations without changing the number of unknown model parameters; this means the inverse problem being much better determined.…”

Resolution of cross-well travel-time tomography

“…Тем не менее представляется целесо-образным рассмотреть некоторые общие закономер-ности влияния отдельных факторов на разрешающую способность, с тем чтобы сформулировать методичес-кие требования к соответствующим параметрам наб-людений и обработки их результатов для решения оп-ределенных практических задач. В качестве такой за-дачи нами рассматривается задача межскважинного просвечивания, активно применяемого при решении разведочных и, прежде всего, инженерно-геофизичес-ких задач [Иванссон, 1990;Wang, Rao, 2006;Бол гаров, Рослов, 2009]. Мы ограничиваемся рассмотрением двух основных факторов: частотного состава зондиру-ющего импульса и параметров системы наблюдений.…”