Exploration Geophysics - Airborne Surveys and Monitoring of the Earth - Application to the Mitigation of Natural and Anthropogenic Hazards, 2014

Variations of soil moisture content caused by precipitation often complicate the interpretation of airborne gamma-ray spectrometry data. This is particularly the case in repeated surveys designed to monitor the change of near surface abundances of radioactive elements or in large and time-consuming surveys.

To counter this precipitation effect we propose a correction method based on repeated survey flights over a monitoring profile. Assuming that the weather and the soil conditions at the monitoring profile are representative for the survey area, the weather dependent effect of soil moisture can be observed and sufficiently corrected.

Three-dimensional (3D) imaging techniques using the conjugate gradient solution are discussed for magnetic anomaly source reconstruction, especially for areas of rugged terrain, such as those of volcanoes. The analysis model configuration permits surface undulation and variable depth slicing. First, primitive source model data are put into the analysis to confirm the characteristic behaviour of the regularisation methods and parameter scaling. The compact regularisation method is then applied to synthetic geologic models to understand the resolving power of the method given the problem of the inherent weakness of a non-unique solution. Finally, the field data of a helicopter-borne magnetic survey of the Otoge cauldron are put into the 3D imaging analysis with the method. The analysis reveals a quite reasonable subsurface image of the magma reservoir and the Otoge cone sheets and Otoge stocks of post-cauldron activity, as inferred from geological studies.

Usu Volcano, Hokkaido, Japan consists mainly of dacitic volcanic rocks underlain by basaltic somma lava and Pliocene–Pleistocene andesitic volcanic rocks, and erupts every 20–30 years. The most recent eruption, in 2000, was the first since 1978. We conducted a helicopter-borne high-resolution aeromagnetic survey almost three months after the start of this eruption. We calculated magnetic anomalies on a smoothed observation surface using a reduction method, assuming equivalent anomalies below the actual observation surface. We conducted three-dimensional (3D) imaging of magnetic anomalies to constrain the subsurface structure. Our model indicates that there are magnetisation highs in the main edifice of Usu Volcano, which may reflect the subsurface distribution of the Usu somma lava. Meanwhile, magnetisation lows lie north-west of the Nishi-Yama Craters Area and on Higashi-Maruyama Cryptodome, where nearby Pliocene and Pleistocene volcanic rocks, respectively, are found. The reverse magnetisation observed at outcrops close to these sites could explain the magnetisation lows.

Although the survey improved our understanding of the surface and subsurface distribution of volcanic rocks in the edifice and basement of Usu Volcano, some limitations remain. No information about the magmas intruded during the recent eruptions in 1977–1978 and 2000 was obtained by the survey, though some of these intrusions were revealed by other geophysical data. The small magnetic contrast between the intruded magmas and their host rocks is the most probable reason. Perhaps the intruded magmas (in particular, those of the most recent eruption) had not cooled enough to become strongly magnetised by the time the survey was conducted.

Unmanned aerial vehicles (UAVs) have recently received attention in various research fields for their ability to perform measurements, surveillance, and operations in hazardous areas. Our application is volcano surveillance, in which we used an unmanned autonomous helicopter to conduct a dense low-altitude aeromagnetic survey over Tarumae Volcano, northern Japan.

In autonomous flight, we demonstrated positioning control with an accuracy of ~10 m, which would be difficult for an ordinary crewed vehicle. In contrast to ground-based magnetic measurement, which is highly susceptible to local anomalies, the field gradient in the air with a terrain clearance of 100 to 300 m was fairly small at 1 nT/m. This result suggests that detection of temporal changes of an order of 10 nT may be feasible through a direct comparison of magnetic data between separate surveys by means of such a system, rather than that obtained by upward continuation to a common reduction surface. We assessed the temporal magnetic changes in the air, assuming the same remagnetising source within the volcano that was recently determined through ground surveys. We conclude that these expected temporal changes would reach a detection level in several years through a future survey in the air with the same autonomous vehicle.

Grounded electrical-source airborne transient electromagnetics (GREATEM), a type of semi-airborne electromagnetics, was used to examine Aso Volcano in south-west Japan, to verify its applicability to surveying deep subsurface resistivity structures. Comparison of the GREATEM resistivity values with those of ground-based transient electromagnetics (TEM) data, repeated GREATEM survey results at the same and different flight heights, and lithologic descriptions indicated that GREATEM can successfully identify underground structures as deep as ~800 m in rugged mountainous areas. An active volcanic region (Naka-Dake crater) was mapped as a low-resistivity zone from the surface to a depth of 100 m. This low-resistivity zone extended to the west-north-west, implying future volcanic activity in this area. Therefore, the GREATEM method is useful for surveying deep structures in large, inaccessible areas, such as volcanic provinces, in a quick, cost-effective way.

An airborne electromagnetic (AEM) survey using the grounded electrical-source airborne transient electromagnetic (GREATEM) system was conducted over the Nojima Fault on Awaji Island, south-east Japan, to assess GREATEM survey applicability for studying coastal areas with complex topographic features. To obtain high-quality data with an optimised signal-to-noise ratio, a series of data processing techniques was used to acquire the final transient response curves from the field survey data.

The 1D inversion results were feasible in that the horizontal resistivity contrast was not much higher than the true contrast, but they were not reasonable in that the horizontal resistivity values were greatly changed. To circumvent this problem, we performed numerical forward modelling using a finite-difference staggered-grid method (Fomenko and Mogi, 2002) adding a finite-length electrical dipole source routine to generate a three-dimensional (3D) resistivity structure model from GREATEM survey data of the Nojima Fault area. The 3D model was based on an initial model consisting of two adjacent onshore and offshore layers of different conductivity such that, a highly conductive sea of depth (10–40 m) is placed on top of a uniform half-space, assuming the presence of topographic features on the inland side. We examined the fit of the magnetic transient responses between field data and 3D forward-model computed data, the latter were convolved with the measured system response of the corresponding dataset. The inverted 3D resistivity structures showed that the GREATEM system has the capability to map underground resistivity structures as deep as 500 m onshore and offshore. The GREATEM survey delineated how seawater intrudes on the landside of the fault and indicated that the fault is a barrier to seawater invasion.