Detailed investigation of preserved maar structures by combined geophysical surveys

Abstract

The detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history. Objective evidence for the existence of such structures can only be provided by geophysics and boreholes. The combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures. Their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density, and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character. Seismic measurements provide the most detailed information about the internal structure of the maar sediments. Zones of low seismic reflectivity and very low density represent sediments of the late maar-lake period. The early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors. The seismic sections clearly reveal the bowl-like structure of the maar. Outside this bowl-like structure, there are only a few reflections, which represent the basement. Taking into account the shape of the gravity anomaly, seismic information allows geometrical modelling of the maar structure. Optimal drilling sites can be selected based on the results of geophysical surveying. Comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern.

Deciphering lake and maar geometries from seismic refraction and reflection surveys in Laguna Potrok Aike (southern Patagonia, Argentina)

“…In the Baruth Maar, these sediments are underlain by a layer of slightly higher velocities (1800 to 2400 m s − 1 ), which was described as lacustrine sediments dominated by the irregular occurrence of turbidites. The same lithology was found in the Messel maar, but with a wider range of velocities (1600 m s − 1 up to 3500 m s − 1 ; Felder and Harms, 2004;Schulz et al, 2005). The Baruth Maar formed in granodioritic bedrock, while the Messel Pit cut through granodiorites, Fig.…”

“…Previous reflection seismic studies in other maar craters have already shown that the crater-wall discordance cannot always be easily detected, but often is marked only by a subtle facies change (Wiederhold, 2003;Schulz et al, 2005). Geophysical surveys at Messel and Baruth maars (Germany) were amongst the first attempts to investigate maar diatreme structures by high-resolution seismic methods (Wiederhold, 2003;Schulz et al, 2005;Buness et al, 2006).…”

“…Geophysical surveys at Messel and Baruth maars (Germany) were amongst the first attempts to investigate maar diatreme structures by high-resolution seismic methods (Wiederhold, 2003;Schulz et al, 2005;Buness et al, 2006). The Messel maar, a pit near Darmstadt, Germany, is an UNESCO World Heritage Site (since 1995) and mostly known for its fossil-rich oil shales, especially mammals and plants (e. g. Franzen, 1985).…”

“…This is further confirmed by different acoustic velocities inside and outside this discordance, as derived from the raytracing model. ; Schulz et al, 2005). In the Baruth Maar, these sediments are underlain by a layer of slightly higher velocities (1800 to 2400 m s − 1 ), which was described as lacustrine sediments dominated by the irregular occurrence of turbidites.…”

Three-dimensional potential field modelling of the subsurface morphology of complex maar volcanoes - Examples from the Newer Volcanics Province, Western Victoria

“…However subsurface volcanic structures are not exposed within the NVP and potential field modelling techniques are relied upon to image the volcanic conduit (Blaikie et al 2012). The application of geophysical modelling techniques to understand the subsurface architecture of volcanic systems is underutilised given its potential to reveal detailed information on the three-dimensional structure and morphology of a volcanoes underlying vent system, but has become increasingly common in recent years (Rout et al 1993;Brunner et al 1999;Schulz et al 2005;Blanco-Montenegro et al 2007;Cassidy et al 2007;Lopez Loera et al 2008;Mrlina et al 2009;Blaikie et al 2012).…”