Traces of impact craters in the geoid

Abstract

A well-known method for studying the impact craters on the Earth’s surface is gravity measurements. For practical studies, the gravity measured on the surface must first be reduced to a particular standard. In a simpIe procedure, after subtracting the latitude-dependent normal gravity, the topographic masses outside the geoid are either completely removed to infinity or shifted to the mean sea level. The gravity station is then lowered from the Earth’s surface to the geoid. Resulting values are Bouguer gravity anomalies, if the topographic masses are completely removed to infinity, or free air gravity anomalies, if these masses are shifted to the geoid. Since the Bouguer gravity anomalies indicate the effect of disturbing masses, they are often used by geophysicists in their search for shaIlow features in the upper part of the Earth’s crust. In Finland, these anomalies have been used to study impact structures, for example, those associated with Lake Lappaj~~i. According to Elo (1976) and Elo et al. (this issue) there is a negative Bouguer anomaly field with a diameter of 17 km and a relative minimum of 10 mGa1 at Lake Lappajarvi. Free air gravity anomalies are traditionally applied to geoid computation. The number of details in the geoid calculated depends on the density of the gravimetric net. The highest resolution