EAGE Workshop on Broadband Seismic

Acquiring low frequency data using Vibroseis technology can be very beneficial for land seismic exploration. Unfortunately, because of physical limitations in vibrator mechanical and hydraulic systems, most conventional vibrators cannot produce sufficient ground force at low frequencies. To successfully push Vibroseis acquisition bandwidth into the lowest frequency range (< 5 Hz), the vibrator output force must be significantly increased. An improved design of vibrator actuator becomes necessary. This paper attempts to present a newly designed low frequency vibrator. Experimental results show with this new low frequency vibrator the vibrator ground force at low frequencies is significantly improved. Downhole data at the depth of 7500 ft (2288 m) demonstrate that a measurable force-energy from 0.5 Hz to 131 Hz (8 octaves) is achieved with this new generation low frequency vibrator.

In seismic land acquisition, harmonic-noise in vibrator ground-force has always been a major limitation in terms of data quality and productivity. In high productivity acquisition, vibrator distortion is usually prevented by waiting enough time between successive shots. Otherwise, it has to be reduced during seismic data processing.

The low-dwell sweeps that are now currently used in production can induce even more important distortion issues. Harmonics from low frequency content can be considerably more spread out over time after correlation. Consequently, the extensive use of the low-frequency bandwidth for vibro-seismic sources prompted the need for improved distortion control, especially at low frequency.

This paper describes a method to mitigate the harmonic distortion directly before emission. The output noise is measured and injected adaptively with opposite phase in the source input to converge towards an ideal output. The results show important noise reduction over the full bandwidth, with perfect low-frequency fidelity. A better source signal quality provides better seismic data that is easier to process, and opens new possibilities in terms of acquisition scenarios with possible productivity improvement.

Coil based geophones are proven to be a reliable technology that has been used in land data acquisition for a long time. However, to better acquire both low and high frequency data, it seems that the limitations of conventional geophones are reached. This paper attempts to present a new designed geophone accelerometer using closed-loop controlled dual-coil geophones. Field tests show that the new designed geophone accelerometer provides geophysical benefits in terms of frequency bandwidth, especially low frequency recording.

In the following case study we compare two different controlled phase processing approaches on a broadband, wide-azimuth, land dataset from Oman and evaluate their effect on the phase stability of the data. The first approach is “phase deterministic” and a single zero-phase filter, computed from the geophone response, was applied to the data. Gapped deconvolution operators were applied in order to preserve the wavelet. The second approach is “phase statistical” and zero-phasing spiking deconvolution operators were calculated to shape the wavelet to zero-phase. The phase of the wavelet was statistically estimated from the amplitude spectra of the data. Extracted wavelets from four wells; instantaneous phase attributes extracted over a smooth regional horizon; and pre-stack time migrated sections all provide evidence of the improvement in the phase stability when following a phase statistical processing route over a phase deterministic one. The improvement is especially notable in the low frequencies, which are important for seismic inversion.

Combined with recent receiver deghosting strategies, the use of multi-level sources can provide further uplift to the ever broadening bandwidth of seismic data. While multi-level sources help mitigate source notches in the output spectrum, the resulting emitted wavelet still exhibits residual ghosts, directivity, and bubble energy which must be handled in processing. We highlight the compatibility of Ziolkowski’s notional source method with multi-level source acquisition. We continue by showing how the directional signatures may be used for 3D directional designature and deghosting on shallow water towed streamer data. The results show a significant improvement in the level of ringing relating to source wavelet directivity effects.

Current research in the field of seismic depth imaging has identified a new approach to image with super-resolution fractured zones, fault edges, small scale faults, pinch-outs, reef edges, channel edges, salt flanks, reflector unconformities, injectites, fluid fronts, caves and karst, and in general any small scattering objects, by using Diffraction Imaging as a complement to the structural images produced by reflection imaging. Diffraction Imaging is the imaging of discontinuities in the earth. Diffractions are the seismic response of small elements (or diffractors) in the subsurface of the earth, such as small scale faults, fractures, near surface scattering objects and in general all objects which are small compared to the wavelength of seismic waves. We show results in different areas of the world, in fractured carbonates and unconventional shale reservoirs. Using Diffraction Imaging to identify areas with increased natural fracture density, which correlate with increased production, the reservoir engineers can design an optimal well placement program that targets the sweet spots and minimizes the total number of wells used for a prospective area.

Electromagnetic method is one of the important approaches to hydrocarbon prospecting. This paper describes an approach to invert resistivity and induced polarization (IP) using the combined constrain of seismic and electric logging data. As a result, the accuracy of hydrocarbon reservoir distribution prediction based on formation’s electrical properties is significantly improved. The paper presents an example illustrating that joint constrained inversion and integrated interpretation workflow of electromagnetic properties (resistivity and polarization), seismic traps as well as seismic attributes, can significantly reduce drilling risks for oil and gas exploration. The statistic number on practical projects also supports the conclusion. The paper also discussed how the joint inversion and interpretation workflow works at different stages of oil and gas exploration and production with very positive results.

Whilst the use of autonomous marine nodes for dense receiver - OBC style - geometries has been rapidly growing since its introduction in 2008, the majority of applications have been for appraisal and development purposes where so-called “development” quality data have been required to maintain or improve reservoir production. The use of blended sources has dramatically improved both survey performance and Ocean Bottom Seismic (OBS) productivity.

By combining the flexibility of receiver geometry that nodes afford with increasing numbers of blended sources the price/performance of full azimuth OBS node acquisition can be enhanced to enable the technology to be used for exploration objectives.

In this paper we will review the nodal technology whose unprecedented reliability allows the efficient use of multiple blended sources, describe how we have implemented the Delft University source blending/de-blending approach in practice and demonstrate how sparse receiver/high density blended sources can deliver cost effective full azimuth exploration surveys.

Due to the influences of rough conditions, complex steep-dip structure, cannot be well imaged with conventional source, geometry, and processing methods. The narrow-azimuth and low-fold geometry cannot provide high S/N data and cannot image the complex targets; the lack of low frequencies and the loss of high frequencies severely affect the resolution of seismic data and inversion quality. Nowadays, Wide Azimuth, Broadband & High Density seismic has become the technology for exploration because of the more benefits it offers such as fracture detection, inversion, and anti-aliasing & data regularization and imaging. The presentation consists of two case studies. One is in eastern edge of the Pre-Caspian basin, Kazakhstan in 2013. Another one is located in Junggar Basin, Western China in 2014.

During the last few years the importance of recording, processing and interpreting a wider range of frequencies has been highlighted with numerous field acquisition examples and case studies. A great effort has been dedicated in the recording of low as well as high frequencies for obtaining high resolution images. The ability to reduce the seismic wavelet’s side lobes by recording lower frequencies and at the same time the increase of bandwidth has been proven as the main advantage of recording broadband data. The land seismic data used in this paper was acquired in Saudi Arabia by using an acquisition configuration based on a variation of the dispersed source arrays concept ( Berkhout, 2012 ). During this seismic experiment we were sweeping three different frequency bands, namely, 1.5 to 8 Hz, 6.5 to 54 Hz and 50 to 87 Hz and with various sweep lengths, ( Kim and Tsingas, 2014 ). In order to increase productivity the data were continuously recorded in a blended mode. In this study we outline a novel seismic acquisition survey which aimed for the optimum and efficient recording of broadband data without sacrificing data quality and we demonstrate the methodology employed for optimum broadband processing in terms of deblending, Full Waveform Inversion (FWI) and Reverse Time Migration (RTM) technologies.