Geophysics | 2019
The shape of things to come — Development and testing of a new marine vibrator source
Abstract
Ten years ago, CGG launched a project to develop a new concept of marine vibrator (MV) technology. We present our work, concluding with the successful acquisition of a seismic image using an ocean-bottom-node 2D survey. The expectation for MV technology is that it could reduce ocean exposure to seismic source sound, enable new acquisition solutions, and improve seismic data quality. After consideration of our objectives in terms of imaging, productivity, acoustic efficiency, and operational risk, we developed two spectrally complementary prototypes to cover the seismic bandwidth. In practice, an array composed of several MV units is needed for images of comparable quality to those produced from air-gun data sets. Because coupling to the water is invariant, MV signals tend to be repeatable. Since far-field pressure is directly proportional to piston volumetric acceleration, the far-field radiation can be well controlled through accurate piston motion control. These features allow us to shape signals to match precisely a desired spectrum while observing equipment constraints. Over the last few years, an intensive validation process was conducted at our dedicated test facility. The MV units were exposed to 2000 hours of in-sea testing with only minor technical issues. Introduction Please do not be taken aback by our title: “The shape of things to come.” We do not intend to predict a world such as that described in H. G. Wells’ 1933 book of the same title. Our aim is to describe our work of updating an old technology — marine vibrator (MV) technology — to provide a versatile source option that can deliver a shaped spectral output capable of meeting our industry’s future needs. The successful introduction of marine air-gun technology by Bolt Associates in 1964 displaced other sources such as dynamite, gas gun, and sparker that were in widespread use earlier. (Stephen Chelminski received the SEG Virgil Kauffman Gold Medal in 1975 for his invention of the marine air gun.) Since that time, seismic marine sources have consisted primarily of arrays of several synchronized air guns that constructively deliver a sharp and highly energetic pulse (Caldwell and Dragoset, 2000). This impulsive source signature becomes the propagating wavelet, which enables subsurface illumination. Lack of source signature repeatability/controllability, the inability to produce ultra-low frequencies for image accuracy and resolution (Dellinger et al., 2016), and any possible impact on marine life (Hovem et al., 2012) are sufficient reasons to develop new technologies that are more effective in finding oil and gas deposits and/or tracking reservoir changes over time. Benoît Teyssandier1 and John J. Sallas2 MV technology is a viable candidate to meet those needs. More than 15,000 miles of commercial seismic surveys were collected using hydraulic MVs prior to 1967 (Robinson, 1967). Over the years, there have been attempts to reintroduce MVs, but there was no major industry interest during the 1970s and 1980s (Broding et al., 1971; Baeten et al., 1988; Bouyoucos and Nelson, 1988). These attempts suffered from reliability problems and lack of low-frequency (LF) energy. The recent introduction of complementary and new technologies renewed interest in MV technology (Tenghamn, 2009; Jenkerson et al., 2013; Dellinger et al., 2016; Rassenfoss, 2016; Mougenot et al., 2017). The MV is a versatile source that can emit a wide variety of signals such as a long tone with changing frequency (called a sweep) or a band-limited pseudorandom signal. For most situations, multiple MV units can be configured to operate as source arrays to increase the overall acoustic output. The MV provides capabilities of a controlled bandwidth and low radiated instantaneous acoustic pressure (the emitted energy is spread out over time) compared to an air-gun energy pulse, which has a peak amplitude and frequencies emitted beyond the desired seismic range. The additional benefit of providing a repeatable source signature is important for data quality and efficiency. The MV opens the door to continuous illumination techniques (Sallas, 2014; Hegna et al., 2018), gains in productivity thanks to simultaneous shooting, complementary illuminations, and field reconstruction techniques such as those numerically simulated by Laws et al. (2018). First, we present our modeling effort leading to the predictability of source signature and validation testing of important features such as linearity and repeatability. Next, design considerations for tailored excitation signals, both pseudorandom signals and sweeps (swept sine wave signal, linear, or nonlinear), are discussed. Finally, we benchmark several techniques used to estimate the far-field radiation acquired during the 2D line ocean-bottomnode (OBN) survey and share the main results from our last sea trial — our first seismic image obtained using our MV source. Marine vibrator principles and development First conceived and developed by CGG in 2009, a small MV prototype was built to confirm our understanding of the ability of MVs to radiate efficiently at very low frequencies. After success with our first prototype, two full-size prototypes were built in 2012 followed by characterization, validation, and reliability tests. The full-size LF unit nominally operates at a depth of 15–35 m, while the high-frequency (HF) unit operates at 3–10 m depths. The LF and HF units are shown in Figure 1a and together form an optimized and efficient multiband system to cover the seismic 1CGG, Massy, France. E-mail: [email protected]. 2GeoMagic, Plano, Texas, USA. E-mail: [email protected]. https://doi.org/10.1190/tle38090680.1.