We use the Bouguer anomaly data in gravity modelling and the total magnetic intensity anomaly in
our magnetic investigation. Both fields are represented by gridded data for the whole area of
Thuringia with the grid resolution of 500 m. The regional geological model of the study area is
We begin with a separation of sources in the gravity and magnetic anomalies into shallow (above 5
km), intermediate (between 5 and 20 km) and deep ones (below 20 km). For this purpose, we apply
our algorithm based on applying the subsequent upward and downward continuation procedures (cf.
Prutkin and Casten, 2009). The main goal of the algorithm is to extract the component of the field,
we can obtain the signal of sources located in the horizontal layer between the given depths
It is important to note that we deal with the depth to singularities of the corresponding component as
harmonic down to great depths.
Here we face a problem, which we call the problem of long wavelengths. We describe the problem in
the following way. High-frequency components decrease faster, therefore, if we have a signal
generated by deep objects, long wavelengths prevail. Equivalently, if we observe short wavelengths,
they should be generated by shallow objects. However, the converse implication does not hold,
meaning that if we observe long wavelengths, we cannot assert that they are caused by deep sources.
An example of an ambiguity in the interpretation of long wavelengths is the above-mentioned smooth
topography of a density interface.
The field of deep sources is shown in Fig. 1a. We calculated the gravitational effect of the TLUG
geological model. Then, we separated it in a similar way into the short, medium and long wavelengths.
The long wavelengths are shown in Fig. 1b. We obtained another example of the ambiguity in their
interpretation: a low-frequency anomaly occurs, although it is caused by density heterogeneities in
near-surface layers. After subtracting this field from the given data (i.e., long wavelengths) we observe
an increasing gravity signal from the south-west to the north-east (Fig. 1c), which can be explained by
a known Moho topography. Eventually, we interpret the low-frequency component of the given
gravity such that it includes a long-wavelength effect of the basin structure and the effect of Moho.
Medium wavelengths of the measured gravity values are shown in Fig. 2 (left). The negative anomalies
are explained in the next section by granitic intrusions located at the depth of several kilometres. We
deal again with the problem of the ambiguity in the interpretation of long wavelengths, meaning that
the same anomalies can be attributed to undulations of topography in shallow layers. To discriminate
sources of these anomalies, we performed the following investigations: For the area of the northern
negative anomaly, we inverted the anomaly for topography of two density interfaces: between
Muschelkalk (Middle Triassic shell-bearing limestone) and Buntsandstein (Lower Triassic bunter
sandstone) and between Buntsandstein and Zechstein (Permian rocks); the anomaly is shared in equal
parts (inversion algorithm is described in the next section). Then, we took a profile through existing
Fig. 1. Long wavelengths and their interpretation:
a – the low-frequency part of observed gravity, b – the
same for the effect of the geological model, c – the
residual field (effect of Moho). Gauss-Krüger
coordinates are used (in km).
a
b
c
boreholes and compared topography of density interfaces found by the inversion and their positions
according to information from the boreholes. Since the first and the third layers have higher mass
density than the second one, the negative anomaly is explained by thinning of the first layer and by a
depression of the second density interface. However, these undulations are not supported by data
from the boreholes, see Fig. 2 (right). In this case, sources of low-frequency anomalies cannot be
shifted upward, because this would contradict available geological information.
Dostları ilə paylaş: