Gravity Method PDF

# Gravity Method ## Gravity Method - Gravity prospecting involves using passive potential fields of the Earth. - In gravity surveying, subsurface geology is investigated on the basis of variations in the Earth's gravitational field arising from differences of density between subsurface rocks. - A c...

# Gravity Method ## Gravity Method - Gravity prospecting involves using passive potential fields of the Earth. - In gravity surveying, subsurface geology is investigated on the basis of variations in the Earth's gravitational field arising from differences of density between subsurface rocks. - A causative body represents a subsurface zone of anomalous mass and causes a localized perturbation in the gravitational field known as a gravity anomaly. - **On a small scale:** buried relief on a bedrock surface, such as a buried valley, can give rise to measurable anomalies. - **On a larger scale:** small negative anomalies are associated with salt domes. On a larger scale still, major gravity anomalies are generated by granite plutons or sedimentary basins. - Micro-gravity surveys are those conducted to a very small scale of the order of hundreds square meters such as detecting buried cavity. ## Gravity Method Applications - Determine shape of the Earth - Hydrocarbon exploration - Regional geological studies - Iso-static compensation determination - Detection of sub-surface cavities (microgravity) - Location of buried rock-valleys - Determination of glacier thickness - Tidal oscillations - Basin Geometry **Image description:** There is a diagram of a salt dome. The diagram shows a wellpath drilling into the salt dome. It is labeled with the density of the salt layers and a density model. There are also three plots showing different gravity measurements (observed and modeled). ## Basic Theory - The basis of the gravity survey method is Newton's Law of Gravitation. - Newton's Law states that the force of attraction F between two masses m1 and m2 whose dimensions are small with respect to the distance r between them, is given by: $F = Gm_1. m_2 / r^2$ - The constant G, called the **Universal Gravitational Constant**, was experimentally determined and found to be of the value: - $G = 6.673 × 10^{-8}$ (cm³.gm-1.sec-2) - $G = 6.673 × 10^{-11}$ (m³.kg-1.sec-2) **Image description:** A cartoon of Isaac Newton sitting under an apple tree. ## Basic Theory **Image description:** A diagram illustrating the relationship between the gravitational force, two masses, and the distance between the masses. ## Basic Theory - Units of gravity - The mean value of gravity at the Earth's surface is about 9.8 m s-2 - Variations in gravity caused by density variations in the subsurface are of the order of 100 μms-2 - This unit of the micrometre per second per second is referred to as the gravity unit (gu). - In gravity surveys on land an accuracy of ±0.1 gu is readily attainable, corresponding to about one hundred millionth of the normal gravitational field. - At sea the accuracy obtainable is considerably less, about ±10 gu. - The c.g.s. unit of gravity is the milligal (1 mgal = 10-3 gal = 10-3cm s-2). ## Basic Theory - The Earth gravitational field in space is not constant, but varies from one point to another. - There are several factors that cause gravity changes on and above the Earth surface. - At an observation point, the measured gravity force (or acceleration) represents the vector sum of the various gravity components generated from different sources. - The earth is not homogeneous and it is neither stationary nor perfectly spherical. - The earth is an ellipsoid of revolution rotating about its polar axis. - These factors are the main causes for disturbing the uniformity of the gravitational field. **Image description:** A diagram showing the cross-section of Earth, with an ellipse encompassing its surface, and the sphere that it is modeled after. ## Gravity Variation - The rotating flattened earth causes the gravity value to change according to latitude position. - The equatorial radius (a) = 6378.160 km - The polar radius (b) = 6356.775 km - The difference (a-b) = 21.385 km - Beside this uniform global variation of the earth gravity there is another type of variations of local origins. **Image description:** A diagram showing the cross-section of Earth, with an ellipse encompassing its surface, and the sphere that it is modeled after. ## Normal Gravity - The Normal Gravity (gn) is expressed as a mathematical function of latitude (Ф), that is gn (Ф). It describes the global gravity variation which is attributed to both of flattening and rotation of the Earth. - $gn (Φ) = 978.031846 (1 + 0.005278895 sin^2 Ф + 0.000023462 sin^4 Φ)$ **Image description:** A plot of the normal gravity curve, in which gravity in Gals is plotted vs latitude in degrees. ## The Earth Gravity Variation - Gravity variations - Global variations - Shape of Planet Earth - Rotation of Planet Earth - Local variations - Space-Variant changes terrain and geological effects - Time-Variant changes tidal and instrumental-drift effects **Image description:** A tree diagram with the following branches: gravity variations, global variations, and local variations. Underneath each branch are examples, such as "shape of planet earth" or "space-variant changes." There is also a table containing the density ranges of different types of rocks.