Gravity sensors or gravimeters, are specialized devices designed to detect minute localized changes in Earth's gravitational force. These changes can be caused by variations in underground materials' density, such as rocks or different distances from the Earth's core (altitudes). ### Operating Principles Gravity sensors function based on one of two main operating principles: 1. **Measuring the flight time of a free-falling object** over a specified distance. 2. **Measuring the amount of stretch caused by a set weight** hanging on a spring [[1](https://www.azosensors.com/article.aspx?ArticleID=2482)]. ### Types and Evolution 1. **Traditional Gravimeters**: These are typically expensive and bulky devices, often the size and weight of a car battery or larger. They have been sensitive enough to track Earth tides, which are movements of the planet's surface due to changing distances between the Earth and celestial bodies like the sun and moon. 2. **Micro-sized Gravimeters (2016)**: These devices are made from silicon and are the size of an SD card. They contain a 25 mg piece of silicon suspended by stiff fibrous structures. The silicon piece bobs up and down in response to gravitational changes, and this movement is measured using a light detector. 3. **Quantum Gravity Sensors**: A recent advancement in gravity sensors has been the development of quantum gravity sensing devices. These devices drop extremely cold clouds of rubidium atoms and measure the time it takes them to fall. Quantum sensing is achieved with laser pulses driving the atoms into a state of superposition, allowing the atoms to fall along different trajectories simultaneously. The patterns of interference between these trajectories change when over different gravitational forces, enabling precise measurements [[1](https://www.azosensors.com/article.aspx?ArticleID=2482)]. ### Applications Gravity sensors have found various applications in both research and industry: 1. **Geological Mapping**: They can be used to map large-scale geological activities like glaciers melting or changes to the planet's crust caused by earthquakes. 2. **Volcano Monitoring**: Micro-sized gravimeters could be deployed in networks to sense magma's motion in volcano zones. 3. **Exploration and Search & Rescue**: Drone-mounted gravimeters can find underground cavities. 4. **Civil Engineering and Construction**: Quantum gravity meters are utilized in construction, archaeology, and utility infrastructure, providing critical insights into the underground. They offer a faster and more accurate assessment compared to non-quantum counterparts [[1](https://www.azosensors.com/article.aspx?ArticleID=2482)], [[3](https://physicsworld.com/a/sensing-gravity-the-quantum-way/)]. 5. **Gravity Cartography**: Quantum gravity gradient sensors are used in geophysics applications, such as engineering and climate research, mapping aquifers, and evaluating impacts on the water table [[2](https://www.nature.com/articles/s41586-021-04315-3)]. [[Gravimeter Image Models]]