Folke Engelmark

The Effect of Resistivity Anisotropy on Transient Electromagnetic Earth Responses

Resistivity anisotropy is shown to have a dominant effect on earth electromagnetic responses. In particular we consider transverse anisotropy with a vertical axis of symmetry and show the effects of varying anisotropy on land and marine earth step and imp

Apparent Anisotropy Derived From Transient Electromagnetic Earth Responses

Summary 1D Earth electromagnetic responses to a transient source are shown to be greatly dependent on resistivity anisotropy. Isotropic inversion of synthetic data arising from an anisotropic model leads either to misleading results (incorrect target depth and poor accuracy in recovered resistivities for target and background) or to non-convergence. It is shown that when the correct anisotropy is included in inversion schemes the resulting model is well resolved. A practical method for obtaining a starting value of anisotropy through a newly defined apparent anisotropy is proposed.

Binary decomposition of sub-resolution three-term seismic inversion data

Summary Three-term seismic inversion creates data cubes representing P-impedance, Poisson’s ratio and density estimates. Due to the limited vertical resolution of seismic data, these volumes will often represent the average values of multiple lithological units. In the areas of hydrocarbon bearing sediments, the three elastic volumes represent an unknown net to gross (N/G) and an unknown water saturation in the reservoir rock. When hydrocarbons, especially gas and light oil, are involved, the effects of saturation are highly non-linear for P-impedance and Poisson’s ratio. However, we can take advantage of the fact that clastic depositional environments tend to be binary, i. e., consisting of alternating sand and shale. This study shows that if we have access to information regarding the elastic properties of the brine sands, shales, and the particular hydrocarbons involved, the elastic inversion products can be used to extract N/G, net water saturation (Sw) for the sand layers and total recoverable volumes given a particular production scenario.

Mud volcanoes and shear‐wave imaging — an example 4C test line in the Caspian Sea

The general description of a mud volcano is an injection or eruption of mud fluids from a highly pressurized zone to a shallower, lower-pressure zone or eruption to the surface. These features are often associated with gas and fluidized sediments forming complex structures. They are poorly resolved using conventional P-wave seismic imaging and they present a number of challenges for hydrocarbon exploration and development. The benefits from converted-wave methods include imaging through the gas, improved resolution, pore-pressure prediction, and better imaging of complex structures with wide-azimuth geometry.

Value of the Broadband Seismic for Interpreter and Reservoir Geophysics

Ideally, geoscientists would like seismic to provide clear, objective information about the subsurface in terms of: identification of the main geological features and stratigraphic sequences, structural elements, elastic/rock properties, potential prospects and lithology-fluid content of potential reservoirs. 3D seismic has offered the greatest benefits to seismic interpreters and reservoir geoscientists in the last few decades, but historically, seismic images have stopped short of delivering on these requirements, as the seismic bandwidth was limited due to the conventional streamer design and acquisition method. Over the last few years, starting in 2007 (Tenghamn et al. 2007) with the introduction of the dualsensor towed streamer technology, new acquisition methods and technologies have been made available with the aim of providing broader seismic bandwidth without any compromise in data quality or tradeoffs in acquisition efficiency. On one side, the combination of two sensors in the streamer cable itself enables an effective removal of the sea-surface ghost by wavefield separation, allowing us to capture the full bandwidth of the upcoming wavefield. More recently, a time and depth distributed source enables the removal of the sea surface ghost on the source side (Parkes, 2011) expanding further the frequency bandwidth. Thus, interpreters and reservoir geophysicists can now have ghost free seismic enabling a significant broadening of the seismic frequency bandwidth on the low and high side of the spectra. Some results of this latest development will be presented with an end-user perspective.