Gravimetry
Gravimetry
Gravity, also referred to as gravitational acceleration, is the combined effect of the Earth’s gravitational attraction and the centrifugal force caused by the Earth’s rotation, and is generally considered constant. However, in reality, as expressed by the law of universal gravitation, gravitational force is proportional to the mass of an object and inversely proportional to the square of the distance. Therefore, gravity becomes slightly stronger near objects with higher density. Since the density of rocks varies depending on their type and porosity, it is possible to estimate subsurface structures by analyzing observed gravity distributions using various interpretation techniques to determine density structures.
Gravity Measurement
There are various types of gravimeters, but here we focus on the spring-type relative gravimeter that we commonly use. The measurement principle of this gravimeter is similar to that of a spring scale, consisting of a spring and a weight. The force acting on the spring is the product of the weight’s mass (m) and gravity (g). By measuring the difference in spring extension at different locations without changing the weight, we can detect the gravity difference between two points. Although the principle is simple, the gravity differences measured for subsurface exploration are less than one-millionth of the gravity we experience on the Earth’s surface. Therefore, gravimeters and the measurements they provide require high precision and stability. Other types of gravimeters include absolute gravimeters and superconducting gravimeters. In addition to land-based measurements, gravity observations are also conducted from ships, aircraft, and satellites, depending on the intended application.
Applications of Gravity Survey
The purposes of gravity surveying include estimating subsurface structures and tracking temporal changes in density (gravity monitoring). For subsurface structure estimation, gravity anomaly maps are created to represent the gravity distribution across the survey area. For example, in areas with faults, the spatial distribution of rocks changes abruptly, which can cause sudden changes in gravity. One application of gravity surveying is to extract such features from gravity anomaly maps to investigate fault distributions. Next, gravity monitoring involves repeated gravity measurements at the same location to detect temporal changes in density. Gravity monitoring is conducted in volcanic regions and Antarctica, aiming to capture precursory phenomena of volcanic eruptions or ice sheet melting.