The following chapters briefly describe the methods and data TERRASYS works with. The focus is on non-seismic techniques, but for a multi-physics interpretation approach all available geodata need to be integrated to achieve reliable and accurate results.
Depending on the density distribution underground, gravity varies at the earth’s surface. Even if we cannot feel these variations, gravity meters can detect them. In reservoir level studies, we are dealing with anomalies that are sometimes as small as 10 mGal – equivalent to 0.000 000 1 m/s2 in SI units. When we speak of gravity, we usually mean the vertical component of the gravity vector, which is the derivative of the gravity potential. Differentiation of the gravity vector leads to the Gravity Gradient Tensor (GGT), which is used to generate many qualitative analysis (or attribute) maps. The GGT can be measured with a gradiometer, often referred to as FTG (Full Tensor Gradient) sensor. Although the resolution of this data is significantly higher than that of gravity measurements, i.e. smaller sources can even be detected by airborne surveys, both are often required to obtain an improved database because the GGT is insensitive to long-wavelength signals.
Since gravity anomalies represent lateral density variations, the method is particularly successful for rather vertical structures, such as salt-sediment flanks, faults, synclines/anticlines, etc. In combination with other data, e.g. sparse 2D seismic lines, a 3D structural model can be interpreted.
The magnetic field measured at the earth’s surface depends on the susceptibility distribution in the subsurface and the inducing field, namely the earth’s magnetic field; in addition, rocks can have a remanent magnetisation that is independent of the induced field. Due to the dipole character of magnetism, a direct analysis of the measured Total Magnetic Intensity (TMI) is often difficult. Therefore, a Reduction to The Pole (RTP) is often performed first, assuming a vertical inducing field, but neglecting the remanence. Hence, anomalies appear above the sources, as in the case of gravity. TERRASYS’ RTP routines are based on advanced equivalent source techniques for several reasons, e.g. to overcome problems in areas of low inclination (close to the magnetic equator). If remanent magnetisation is present, it can sometimes be inferred directly from the measured field, or it is only found (or confirmed) during modelling. Although RTP is a very useful transformation for understanding the data, it must be used with caution. Modelling should always be done on the TMI itself, while qualitative analysis can be based on attributes that are known to be more stable in remanent cases.
Due to large susceptibility variations of rocks, magnetic surveys can be successfully applied to interpret geological structures that are due to different rock types, e.g. to locate basalts or to determine the depth of basement. On the other hand, even a thin highly magnetic layer can be misinterpreted as a larger source; therefore, magnetic data is particularly valuable in combination with other methods.
The MGT is the Magnetic Gradient Tensor, comparable to the GGT. It is particularly useful for near-surface applications.
EM & MT
Electromagnetic (EM) and Magnetotelluric (MT) methods respond to rock properties different from seismics, gravity or magnetics and are thus a valuable complement to the understanding of the subsurface. Induced electromagnetic fields, either from geophysical transmitters (EM, active) or natural sources (MT, passive), and their recorded responses (secondary fields) are used to determine the underground distribution of electrical resistivity (or conductivity).
Applications range from near-surface investigations, e.g. for construction sites, or marine reservoir-scale measurements, as for CCS programmes, to airborne reconnaissance surveys, e.g. to assess mineral prospectivity, or regional studies, either onshore or offshore, such as to better define specific geological structures.
However, similar to other methods, typically remaining ambiguities can be significantly reduced by adding data that are responsive to different rock parameters from complementary techniques and jointly interpreting them, e.g. in the context of the JIF, as previously described.
Potential field methods are inherently ambiguous if constraints, i.e. boundary conditions, are not available. Gravity and magnetics are particularly sensitive to lateral density and susceptibility contrasts, respectively, but are insensitive to vertical ones. Therefore, geological boundaries determined by seismics and wells are especially important to increase the reliability of interpretation. These supplementary data are used in TERRASYS’ integrative multi-physics approach. We usually work closely with the respective experts, and thus have broad experience with software interfaces and fast data exchange. The main types of constraining data are as follows.
Seismics is a valuable resource – if available – as it has the advantage of providing good resolution in the vertical direction, but less horizontally. This complements the resolution of gravity and magnetic field data. In addition, density-velocity relationships, either long established or regionally adapted (or even jointly inverted), help to build consistent models, and the optimised density distribution can be used as an update for the velocity model.
Depending on the degree of possible integration in our framework (JIF), the coupling of seismics with potential field methods can range from rather weak, e.g. via cross gradients, to strong, in the case of a full joint inversion setting.
Well Data Analysis
Borehole data is also an important constraint for gravity, magnetic and EM interpretation. In addition to calibrating model geometries in depth, also densities, susceptibilities and resistivities can be defined locally – either through direct downhole recordings or by relationships to other logged parameters. For example, if there are a sufficient number of wells with both density and sonic measurements in the study area, a locally valid velocity-density relationship can be established (see next section). The knowledge gained from borehole data can be used during modelling as well as guiding the inversion process.
Based on rock physics, laboratory data and/or published results, relationships between key parameters of the methods used are valuable constraints on the joint inversion process. These relations can be defined either directly, e.g. between velocity, density, susceptibility and resistivity, or via rock parameters such as porosity or lithological composition. Depending on data availability and distribution, such relationships can be locally, regionally or globally valid or can also be adjusted (inverted) during a joint inversion.
If geological data or reliable concepts are available, they need to be integrated into the model and thus are important constraints. However, sometimes they are the main objective of a study, in which case geophysical interpretation helps to find the most likely model of two (or more) equally viable scenarios.
All kinds of further information can constrain forward modelling, inversion and interpretation. Based on TERRASYS’ inversion concept, all constraining data also needs to be accompanied by a reliability assessment. This allows a detailed evaluation of the final inversion results.