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Basin Analysis: A Synthesis
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
An aeromagnetic survey, which provides the quickest method for obtaining the depth of the basin-floor by detecting the boundary between the magnetically susceptible, presedimentary, often metamorphosed basement and the non-magnetic sediments overlying it, is useful in detecting the gross thickness of sediments. The overall direction of thickening and thinning can also be determined by the same method. An aeromagnetic survey is generally followed by ground gravitymeter survey for obtaining information on density anomalies in the subsurface. The data obtained are sometimes useful although the inherent ambiguity in the interpretation of gravity data may not always provide a unique solution to problems.
Geophysical constraints on the structure and formation of Onepoto, Orakei, Pupuke and Tank Farm maar volcanoes, Auckland Volcanic Field
Published in New Zealand Journal of Geology and Geophysics, 2019
Alan G. Nunns, Manfred P. Hochstein
Németh et al. (2012) inferred an absence of either lava fill or a juvenile-clast rich diatreme in the crater, citing Cassidy et al. (2007) to the effect that no residual gravity or magnetic anomaly is preserved over Orakei Basin. It is true that the regional aeromagnetic survey discussed by Cassidy et al. (2007) does not show a distinct magnetic anomaly over the Orakei Basin volcano. However, the spacing (450 m) and location of the aeromagnetic survey lines over the northern part of Orakei Basin (S. Soengkono, pers. comm., 2014) were insufficient to detect the upward continuation of the significant magnetic anomaly measured at basin level. Our basin-level survey clearly established the presence of a substantial igneous body within the excavated crater, which is further supported by the residual positive gravity anomaly. We estimate that the volume of volcanic material remaining in the crater is about 1.0 × 107 m3, which is about three times greater than the estimated volume of magma that was explosively ejected outside the crater.