Ultra-Low Frequency Seismic: Benefits and Solutions
Authors: Long Andrew Samuel , Reiser Cyrille 
Published: March 2015

Ultra-low seismic frequencies less than about 7–10 Hz cannot be produced with significant amplitudes by conventional air gun arrays, for any configuration and for any towing depth used during marine seismic surveys. Removing the source-side and receiver-side ghost effects is now achievable, improving the recovery of available ultra-low frequency amplitudes, but improving ultra-low frequency output on the source side remains an unrealized dream.

We demonstrate that if 1–7 Hz amplitudes could be usefully injected into the earth it would be possible to facilitate much sharper seismic representation of geological contacts and internal features. Furthermore, seismic inversion would yield robust and precise predictions of reservoir lithology and fluid properties—without well control. The net result would be improved exploration and appraisal wells, reduced exploration and development risks, and optimized recovery of reserves. A complication now, however, is that seismic inversion has a traditional dependence upon building a low frequency model (LFM) to compensate for the missing ultra-low frequency amplitudes, usually by low-pass filtering available impedance logs and incorporating very smooth velocity model data.

After discussing the robustness of wavefield separated data from dual-sensor streamer data for inversion with limited LFM input, we describe novel towed marine source technologies that may eventually enable the injection of strong 2–7 Hz amplitudes into the earth. In the near term the dependence of LFMs upon log data may be relaxed by improving the structural and stratigraphic detail within (depth) velocity models. Full waveform inversion (FWI) makes the bold promise that raw seismic field gathers can be directly used to invert for extremely high resolution velocity models, almost without any human intervention. In its ultimate application, FWI will directly invert for the elastic properties of the earth, and may therefore represent a significant step towards automated and quantitatively robust quantitative interpretation (QI).

Introduction

As will be discussed later, conventional air gun source arrays used for towed streamer (marine) seismic surveys cannot produce significant amplitudes below about 7 Hz. This 0–7 Hz range is hereafter referred to as "ultra-low frequencies". Although it may be counter-intuitive, ultra-low frequency amplitudes can greatly improve vertical (temporal) seismic resolution, as demonstrated in Fig. 1. Assuming that equal amplitudes are recovered in processing for all frequencies of relevance, additional low frequency amplitudes will attenuate side lobe amplitudes on the zero phase wavelet representing elastic impedance boundaries in the earth. Overall, the interference between neighboring reflection events in vertical columns of thin stratified layers will be minimized, thus optimizing vertical resolution.

Ultra-low frequency amplitudes are also critical for quantitatively precise estimation of elastic reservoir properties. Fig. 2 is a comparison of 2D towed streamer results over the Møre Margin area of the Norwegian North Sea. The data on the left side of Fig. 2 were acquired with conventional source and streamer technology, and contain the effects of both the source-side and receiver-side ghost effects (refer also to Fig. 3). In contrast, the data on the right side of Fig. 2 were acquired with acquisition technologies that allow the full removal of both the source-side and receiver-side ghost effects. Note the significantly sharper and more detailed resolution on the right. Additional low frequency amplitudes recovered in the ghost-free data have contributed to reduced side lobe amplitudes. Both datasets were taken through pre-stack simultaneous seismic inversion, and the color scale represents the relative acoustic impedance; the product of interval velocity and bulk density. Note also how the acoustic impedance color scale also enables the improved delineation of different lithology classes in the right side of Fig. 2.