First Break - Volume 33, Issue 4, 2015

Modelling in 3D on a 2D screen is a difficult endeavour. The outcome depends mainly on how software implements modeling-tools. Tools used in the movie industry require a lot of time for user training. Usually geoscientists are specialized in their fields and they are not trained in computer-based interactive 3D modelling. They require simple-to-use modelling strategies, which can be used by geo-experts not familiar with sophisticated 3D manipulators and – most importantly – allow the creation and, most often, handling existing models (e.g., salt structures coming from seismic interpretation). Since models get more and more complex in geometry (e.g., complex multi-z geometries), modelling tools must be designed in a way that model-topology is conserved while constraints are respected, when such tools are applied. Thereby it is important that the tools must guarantee topology conservation, no matter what operations are performed to geo-models. Therefore tools for interactive 3D modelling of potential field data have been rethought and implemented in the IGMAS+ platform (Alvers et al., 2013; 2014, Lahmeyer et al., 2010). Responding to the question ‘Quo Vadis Inversion?’, this paper presents a new method (based on Alvers et al., 2013; 2014) for an automated interactive 3D-inversion. Thereby the methodology is, to let the user select a certain part of the model, thought to be wrong, by simply opening a box around it and start an automated inversion, which can be watched in (ideally) real time. The new approach is that now users can stop the automated inversion at any time, rewind if required, reset parameters (such as constraints) and rerun the process from there. The aim is to put the geo-expert in the driver’s seat to easily test geological and geophysical hypotheses. Based on a hybrid model (Schmidt et al., 2011) structure consisting of grids, voxel cubes and triangulated surfaces, complex models can be created and modified either interactively or by automated interactive inversion as described below.

For the past 15 years airborne gravity gradiometry has been used on a variety of petroleum and mineral exploration plays. As explorers focus on increasingly deeper targets with ever more subtle geophysical signatures, there is a growing need to accurately gauge the accuracy and resolution of airborne gravity gradiometer systems. In this article we present various noise estimates derived from surveys over the R.J. Smith Airborne Gravity Gradiometry Test Range in Western Australia.

Marine Controlled Source EM (CSEM) data has been used extensively to improve the chance of success in the search for hydrocarbons given that accumulations of oil and gas can be characterized by increased resistivity. CSEM data have been used mostly to derisk prospects. By using a Towed Streamer EM system it is possible to acquire CSEM data efficiently to determine the sub-surface resistivity at both regional and prospect scales. In addition, the simultaneous acquisition of both towed streamer seismic and EM data from the same vessel is enabled, with obvious efficiency benefits. By performing unconstrained inversion of the Towed Streamer EM data to determine the sub-surface resistivity we aim to extract the maximum possible amount of information from the EM data before considering any constraints on the solution. In any case, in a frontier exploration setting then geological knowledge may be limited and there may be relatively few wells. In addition, we wish to ensure that the resistivity models from CSEM data can be considered an independent piece of information so that, say, any correlation between acoustic and electromagnetic structure is unforced. We present case studies that demonstrate that the subsurface resistivity determined using unconstrained inversion of Towed Streamer EM data is consistent with the logged resistivity, and the regional geological background.

The representation of potential vector and tensor fields by 3 × 1 and 3 × 3 matrices of Cartesian components is ambiguous unless the coordinate directions are also specified, and in particular, whether the system is left- or right handed. In this paper we highlight the nature of the ambiguity, and suggest a unified approach to encompass any mixture of coordinate conventions. Failure to observe the correct conventions can lead to incorrect interpretation of the potential data, and the suggested protocols are a step towards data integrity. Tensors of rank one and of rank two, commonly referred to as vectors and tensors in geophysical contexts, are often recorded as data sets of 3 × 1 and 3 × 3 matrices of numerical Cartesian components. Surveys from different sources may, however, use different coordinate conventions, or the client may be unaware of the coordinate conventions assumed by the provider. In essence, vector and tensor array data are incomplete without also being accompanied by information stating the directions and ordering of the axes of the employed coordinate system. A commonly used system makes the use of North (N), East (E) and Down (D) coordinate directions. But is the ordering of the vector components END or NED? The first is for a lefthanded and the second for a right-handed co-ordinate system. The choice will affect the meaning of the vector and tensor component data. Moreover, standard vector operation software (in particular, cross product routines) assume right-handed reference systems, and can yield incorrect results for left-handed systems unless explicitly adjusted.

Monitoring of changes in brine chemistry (salinity and temperature), during water-flooding is important for injector optimisation, understanding efficiency, detecting early water breakthrough, locating bypassed hydrocarbons or detecting scaling in the heterogeneous reservoir. It is already known that water injection into the oil-leg of a hydrocarbon reservoir can be monitored by both seismic acquisition and also CSEM methods. The problem of an interfering pressure signal for seismic impacts quantitative evaluation for time-lapse analysis, while with EM there are the counterbalancing effects of salinity and temperature. CSEM becomes favoured when the pressure effects on seismic are dominant, or heavy oil is present with similar acoustic properties as the formation and injected waters. The quality of measurement for both methods is influenced by reservoir facies variations, acquisition repeatability and overburden heterogeneity. Time-lapse seismic is unlikely to detect brine distributions injected into the water-leg or aquifer, although it may detect associated pressure up effects. However, our calculations show that it may be possible for time-lapse CSEM to distinguish the inter-mixing of different brines in the subsurface hydrocarbon reservoir. Specifically, low salinity injection or injection into a highly saline formation can clearly be detected with this technique.

ADMA-OPCO undertook a large 3D-4C OBC (ocean bottom cable) survey over an oil and gas field offshore Abu Dhabi, UAE. The survey was designed to acquire a high-fold dataset having wide azimuth and long offset sampling. The survey utilized, for the first time, two simultaneous source techniques so-called DS3 (Distance Separated Simultaneous Shooting) and MSS (Managed Spread and Sources) to enhance survey productivity and minimize HSE exposure in the field. DS3 and MSS involve two source vessels operating simultaneously while maintaining predetermined separations among shots in time and space to produce records with minimal contamination of seismic interferences. Unlike land surveys, these two techniques had not previously been applied to an OBC survey. To ensure adequacy of the techniques, the acquisition started with 2D tests prior to full-field implementation. With DS3 and MSS, the 3D survey yielded remarkable enhancement of survey productivity as compared to conventional single source vessel configuration. DS3 and MSS along with proper temporal and spatial source vessel separations enable recorded data to be handled as if acquired by non-simultaneous source survey.