Abstract-A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (-1.07 Ma) structure with a diameter of -10.5 km is exposed within early Proterozoic Birimian-Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high-resolution, low altitude (-70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, -12 km in diameter. The central-north part of the lake reveals a central negative magnetic anomaly with smaller positive side-anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact-melt rock or melt-rich suevite bodies.Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.

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Abstract-The estimates of the age of the Kaali impact structure

The current case study presents results of the ground-penetrating radar (GPR) profiling at one of the Saadjärve drumlin field interstitial troughs, the Rahivere bog, eastern Estonia. The study was conducted in order to identify the bog morphology, and the thickness and geometry of the peat body. The method was also used to describe the applicability of GPR in the evaluation of the peat deposit reserve as the Rahivere bog belongs among the officially registered peat reserves. Fourteen GPR profiles, ~ 100 m apart and oriented perpendicular to the long axis of the depression, covering the bog and its surrounding areas, were acquired. In order to verify the radar image interpretation as well as to evaluate the velocity of electromagnetic waves in peat, a common source configuration was utilized and thirteen boreholes were drilled on the GPR profiles. A mean value of 0.036 m ns–1 corresponding to relative dielectric permittivity of 69.7 was used for the time–depth conversion. Radar images reveal major reflection from the peat–soil interface up to a depth of about 4 m, whereas drillings showed a maximum thickness of 4.5 m of peat. Minor reflections appear from the upper peat and mineral soil. According to the borehole data, undecomposed peat is underlain by decomposed one, but identifying them by GPR is complicated. Mineral soil consists of glaciolimnic silty sand in the peripheral areas of the trough, overlain by limnic clay in the central part. The calculated peat volumes (1 200 000 m3) were found to exceed the earlier estimation (979 000 m3) that was based solely on drilling data. Ground-penetrating radar, as a method that allows mapping horizontal continuity of the sub-peat interface in a non-destructive way, was found to provide detailed information for evaluating peat depth and extent