5. Summary and conclusions
We have applied a new method for a joint 3D inversion of the gravity and magnetic data at the study
area of the Thuringian Basin. The aim of the study was to investigate how geological constraints and
results of other geophysical methods can decrease the degree of non-uniqueness by means of
interpreting the potential field data, while assuming that we solve the geometry of a restricted body
with the same field as the given set of 3D line segments. The approximation based on segments
provided very stable estimates for the centre of mass coordinates. After introducing the spherical
coordinates with the origin in this point and employing the fixed density contrast value from
boreholes data, the solution of the inverse problem become unique. We also addressed the problem
of ambiguity in interpretation of long wavelengths.
The following conclusions are drawn from our study:
1. Based on applying the subsequent upward and downward continuation, we separated the
gravity and magnetic data in Thuringia into the long, medium and short wavelengths. Using
deep sources generated anomalies with prevailing long wavelengths, the converse implication
is not valid as low frequencies can also be caused by shallow objects. In the case of the
Thuringian basin structure contributes substantially to the long wavelengths of gravity along
with the Moho uplift. However, our numerical experiments with intermediate wavelengths
shown that it was not possible to explain negative anomalies with the topography of near-
surface layers. The upper part of the geological section is usually well investigated by
boreholes; therefore, it is not always achievable to shift sources upward, because this would
contradict available geological information.
2.
For each local anomaly, its interpretation includes several steps. We subtracted the model of
the regional field (2D harmonic function), then approximated the residuals with 3D line
segments and transformed a chosen set of line segments into a restricted object or a contact
surface with the same field. Each time, we solved a nonlinear integral equation related to the
function determining the geometry of an unknown object. Using this approach, we obtained a
model for medium wavelengths which included three restricted bodies (granitic intrusions) and
the mass density interface with the topography below them.
3. It is helpful to subtract the effect of the geological model beforehand. For short wavelengths,
this procedure pronounced the linear positive gravity anomalies correlated with known fault
zones. Another type of anomalies become also visible in magnetic data. For these anomalies,
we transformed magnetic data to the pseudo-gravity and compared it with the measured
gravity. We regard their correlation as an indication that both anomalies are allegedly caused
by the same object. Two main anomalies of this type were inverted for a 3D topography of
contact surface. We interpret these anomalies as caused by uplift of the crystalline basement.
4. For three local areas, we made detailed modelling for residual short wavelengths after
subtracting the effect of geological model. Our goal was to investigate salt tectonics. We
observed that, with some preliminary assumptions, it was possible to isolate the effect of a salt
deposit. We applied the modification of our inversion algorithm and obtained geometry of a
salt pillow which was supported by data from boreholes.
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