SEISMIC SURVEYING 


A feature of BroadSeis is that the cable depth profile can be tuned for different water depths and targets


because it indicates the optimum horizontal drilling azimuth and offers the prospect of subsequent fracture stimulation as a solution to tap into existing natural fracture systems. A clear understanding of the geomechanical properties and their distribution explains the reservoir heterogeneity and thus the variation in economic ultimate recovery (EUR) between wells.


CGGVeritas derives a host of geomechanical properties from migrated CDP gathers, including Young’s Modulus, Poisson’s Ratio, and shear modulus, by first inverting the data for P- and S-wave velocities and density. With this information, fracture dimensions can be predicted and wells drilled in the most brittle rock. Linear Slip Teory for geomechanical


properties is used to calculate stress values. Generally, the stress state is anisotropic leading to the estimation of both the minimum and maximum horizontal stress. As the seismic data measure dynamic


stress, results are then calibrated to the static stress that is effectively borne by the reservoirs at depth, making it possible to predict the hoop stress and the closure stress as key elements defining the type and motion of fractures. At locations where the differential


horizontal stress ratio (DHSR – the ratio of the difference between the maximum and minimum horizontal stresses to the maximum horizontal stress) is low, tensile fractures will form in any direction, creating a fracture swarm. If the maximum horizontal


stress is much greater than the minimum, then fractures will form parallel to the direction of maximum horizontal stress.


Broadband marine solution Meanwhile, BroadSeis, the CGGVeritas broadband marine solution, continues to provide exceptional broadband images in different geological settings. Since its launch at the Barcelona EAGE


convention, BroadSeis has generated considerable interest. Eight projects have now been recorded in various locations around the world, from Australia to the Arctic. Tese include both 2D and 3D geometries in a variety of geological settings, water depths and target depths. Tese surveys are both proprietary and non-proprietary and clients include a number of major oil companies. Deepwater data examples from West


Africa and the Gulf of Mexico were shown at the recent SEG convention, providing exceptional bandwidth of 2.5 – 155Hz, almost 6 octaves of data. Tese results included true amplitude deghosted gathers, suitable for AVO analysis. BroadSeis acquires both low and high


frequencies simultaneously using Sercel Sentinel solid streamers with a variable depth profile (patent pending). Te cable ghost notch varies along the streamer with the cable depth and our new proprietary deghosting and imaging techniques (patents pending) use this notch diversity to produce a very sharp wavelet, with a superior signal-to-noise ratio and the maximum bandwidth. Te low-noise characteristics and


precise low-frequency response of Sentinel streamers have proved central to the success of BroadSeis. Sentinel is quieter and can be towed deeper than other streamers (up to 60m). Since Sentinel has an instrument low- cut of 2Hz, BroadSeis provides significantly better low frequencies, recording high-quality signal down to at least 2.5Hz. A key feature of BroadSeis is that the cable depth profile can be tuned for different water depths and targets so that the notch diversity and output spectra are optimised for each survey. Operational experience has shown that the control of solid streamers to variable depths is robust and stable for both 2D and 3D configurations, even down to 60m, especially when using Sercel Nautilus streamer control devices. Tis has been proven even in areas with variable currents such as the Gulf of Mexico and variable salinity conditions such as West Africa. Sentinel has retained its exceptional low-noise characteristics at all depths, with all streamer profiles and under all conditions. BroadSeis includes a new, true-amplitude,


fully-3D deghosting algorithm that makes no 2D assumptions. Tis gives it unrivalled suitability for 3D and all wide-azimuth acquisition configurations, as well as 2D data. Whilst the proprietary deghosting is a part of the BroadSeis technique, the raw field data is supplied to clients providing them with flexibility to apply any processing techniques desired. From shallow objectives to deep targets


beneath complex overburdens, BroadSeis is delivering data with exceptional bandwidths and excellent penetration. For clearer imaging of deep targets, as well as greater stability in seismic inversion, providing reduced exploration risk, BroadSeis is proving to be the Safer, Broader, Better choice for marine acquisition and processing. 


CGGVeritas’ headquarters is in Paris, France. www.cggveritas.com


CGGVeritas adds Lodestone to its North Sea Data Library C


GGVeritas, the geophysical exploration company, has annoinced the completed acquisition of the first phase of a new 3D program in and around the Quad 44 region of the Southern North Sea in association with GeoPartners. Te Lodestone survey complements the existing high- quality, recent vintage Cornerstone and


13 IHSS


Runestone surveys in the Central and Northern North Sea Basins. Te Lodestone survey provides state-


of-the-art long-offset data using Sentinel solid streamers which are essential when imaging the variable-quality and compartmentalised Permian and Carboniferous reservoirs lying beneath complex salt bodies, using high-end


processing techniques. Luc Schlumberger, Executive Vice


President, Multi-Client & New Ventures, CGGVeritas, said: “Given the increasing interest of the industry in the Southern North Sea, we believe Lodestone will offer a high-quality resource to unlock the full potential of this prolific basin.” 


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