About the causal relation between gravity and well-log data: Fundament for the joint interpretation of gravity and seismic data

In frontier exploration the important role of Gravity and Magnetic methods are established as a global screening tool especially in areas of limited seismic access in view of global data sets, lower costs and lower environmental impact.<br>The ability to integrate between different data sets is another advantage: older available seismic profiles, 3D seismics and well log-data can be used for the establishment of a 3D model from seismic key surfaces. This 3D model provides the initial geometry of the geological units. By assigning appropriate densities/susceptibilities to these bodies the gravity and magnetic effect can be calculated by forward modelling. Focusing on a joint interpretation of gravity and seismic data we meet the inherent ambiguity that the wanted information about a geological reality differs from what is measured or observed, respectively.<br>For example, seismic travel times are observed, in order to gain information about saturation, depths of geological interfaces and density changes. Seismic velocities are observed from velocity analysis of seismic surveys in general, and from velocity logs at well locations in particular. However, seismic velocity interpretation is dependent on the rock physics model used, setting velocity into relation to porosity via compaction curves. Thereby the rock physics model acts as a filter of the seismic observation, being a description of geological units that percolates the information about the reality of the nature. If an observation A is a function f (B), and if the parameter B can be derived by a different independent observation, then both observations A and B are in causal relation with each other. This means it exists a causal relation between the gravity observations and well log data, because the gravity signal is a function of density, and this parameter is observed independently via density log at a well location.