Summary Interactive 3D potential field modelling has become feasible in the last years and has been used on a number of projects in the oil industry and academic research. It does not replace conventional inversion methods but is advantageous in cases where we quickly want to test scenarios based on our geological and geophysical understanding of the area. Integration with other methods becomes much easier when the modelling is done interactively. Based on experience with the current implementation of interactive 3D modelling in IGMAS+, some practical improvements have been implemented and will be presented.

Summary 3D interactive inversion of potential field data presents an interesting addition to the toolbox used to find subsurface models being consistent with the measured data and other information like seismic and geology. The concept was presented last year at the EAGE in London ( Alvers et al. 2013 ). In the meantime this concept has been implemented and applied to a realistic case, the SEAM model, representing a complex salt geometry in the GoM ( Pangman 2007 ). Results and learnings of this test will be presented.

M.R. Alvers, H.J. Götze, L. Barrio-Alvers, C. Plonka, S. Schmidt and B. Lahmeyer present a technique whereby triangulated facets and voxel-cubes are treated in parallel, allowing integrated models for seismic, magnetic and EM data. It is a commonly accepted truth in the oil industry that 'the easy oil has been found'. Finding the remaining hydrocarbons requires better technologies. Examples are exploration projects below salt and basalt, which are difficult to image with seismic. The main exploration method is still seismic but it has become more important to integrate seismic with other methods in order to improve imaging. In areas of strong lateral velocity and density changes, gravity modelling can help to improve velocity models used for seismic imaging. Efforts of joint interpretation of e.g., seismic, gravity and EM methods lead to more and more realistic and therefore more complex models.

We present improved 3D modeling techniques for potential field data. A new 3D-editing concept makes geometry changes in 3D easier, without the risk of creating inconsistent models with crossing boundaries or holes. Based on the new 3D editor the concept of interactive inversion is introduced. Also spherical modeling avoids incorrect calculations for big regional models. In order to guarantee that the improvements above are applicable in an interactive program we present strategies for big performance improvements. All the techniques described are currently tested and will be implemented in IGMAS+.

ABSTRACTModern geophysical interpretation requires an interdisciplinary approach and software capable of handling multiple geophysical data types such as seismic, full tensor gravity gradiometry, magnetics and magnetotellurics. We use the IGMAS+ (Interactive Gravity and Magnetic Application System) geo‐modelling software that is designed for 3D gravity, gravity gradient and magnetic modelling. This paper deals with a special aspect of potential field modelling, combining conventional triangulated model geometries (building polyhedrons) with voxel cubes.The hybrid modelling combines the advantages of both the vector and raster modelling system: both may be used alone (polyhedrons without voxels, voxels without polyhedrons) or simultaneously by superposition of both effects, which provides flexibility towards full interoperability. The key idea of our approach is, on the one hand to use two different, completely independent geometries (vector and raster) and give on the other hand the possibility to link both on demand for either editing the voxel model or to combine a large number of voxel cells under a common physical parameter function – which results in more reliable parameter inversion results.

In order to be successful in challenging depth imaging projects the integration of all available data is important. We present the interactive 3D gravity and magnetic modeling tool IGMAS+, its integration in the depth imaging workflow in Statoil and case histories from different geological settings.

BEB wants to maintain and extend its position as the leading gas-producing company in Germany. The application of the latest technologies in 3D seismic and drilling plays a key-role to achieve BEB's business target. There are still opportunities for new discoveries, but greater depths and increasingly complex reservoir conditions make an even greater technical challenge to find an economically viable hydrocarbon accumulation. The economically important gas-fields are located in Northern Germany. The geology of Northern Germany is characterised by two dominant tectonic features: inversion tectonics and salt diapirism. Many gas- fields are situated below either of these complex structures. Thus, there is a need for sub-salt imaging and imaging below complex structures in order to identify new hydrocarbon accumulations and to allow improved reserve estimates.

From 1982 to 1986, more than 2500 gravity data were taken in an Andean Geotraverse covering northern Chile and northwestern Argentina between 64° and 71°W and 20° and 26°S. Including 2100 reprocessed older data, there is now a data base of about 4600 gravity values available, which can be used together with other geophysical and geologicalin formation for an interdisciplinary interpretation to solve some problems of the structure and development of the Central Andes. The new gravimetric data base, which includes the Bouguer‐, free‐air‐ and isostatic residual anomaly, is presented together with a preliminary interpretation.

We would like to thank V. I. Starostenko for his interest in our paper and his historical remarks on the development of 3‐D modeling formulas in gravity and magnetics. The main objective of our paper was to publish a user‐friendly fast interpretation tool for 3‐D modeling including interactive computer graphics. Therefore, it does not deal with an extended derivation of the formulas used in potential field theory.

The calculation of Bouguer anomalies in high mountains requires a gravity field continuation from irregularly spaced gravity data on the topography to an equipotential surface. Least squares collocation (LSC) is successfully used for geodetic purposes and offers an elegant solution to this problem. A great advantage of this method is the possibility to give estimates of the accuracy of the approximated gravity field.

Three‐dimensional (3-D) interactive modeling permits integrated processing and interpretation of gravity and magnetic data, yielding an improved geologic interpretation. 3-D model bodies are constructed from polyhedra of suitable geometry and physical parameters (density and susceptibility), input on an interactive graphics terminal that is tied to a host computer. The method is especially designed for concurrent processing and interpretation in an interactive mode. The effect on gravity of a homogeneous polyhedron is calculated by transforming a volume integral into a sum of line integrals. Magnetic effects can be modeled by using either Poisson’s theorem or a slight modification of the formulas derived for gravity modeling. The interactive modeling program allows the user to change the geometry as well as the density and/or susceptibility of the elementary polyhedra and to observe results quickly during the course of processing. This capability enables the interpreter to decide immediately if and where a tentative geologic structure must be changed for the modeled effect to fit that of a field survey. He is able to drive the device‐dependent process by clear menu functions without any knowledge of the rather complicated data structure and the interaction between the main program and its many subroutines. In addition, application of this method requires considerably less computing time than conventional methods based on the direct evaluation of volume integrals.