Gravity Surveying Explained

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gravity surveyGravity surveying is the process of measuring the local gravitational field of the Earth. Airborne gravity is the process of measuring and mapping these changes in the gravity field with an airborne gravimeter on a helicopter or an aircraft. It involves using ultra-sensitive equipment, called a gravimeter, to look at the structure density of rock in the subsurface of the Earth. The idea is based on a "Causative Body”, which produces a gravity anomaly. These anomalies are analyzed by geologists who can assess the depth, size and shapes of the subsurface anomalies.
Gravity surveying is based on the theory of Newton's Law of gravitation, which says that "the force of attraction between two bodies of known mass is directly proportional to the product of the two masses and inversely proportional to the square of the distance between them”. Essentially, denser formations in the subsurface have a slightly higher gravitational pull than normal formations.
The measurement of these anomalies is done with a gravimeter. A traditional gravimeter works by using a very sensitive spring and mass system. These instruments have a weight attached to a beam and also to a spring. As gravity increases, the weight is pulled downward also pulling on the spring. As gravity normalizes once past the increased gravitational force, the weight and spring return to their original locations. Now, newer systems are attempting to use lasers to achieve even more accurate results.
Once these measurements have been taken, corrections need to be made on the gravity survey results. The main reason being the system is so sensitive it is influenced by other factors. Airborne gravity surveys have four factors to correct for;
  1. Drift; A minor correction for stretching in the spring throughout the survey.
  2. Coriolis; A correction for the Coriolis Effect due to the rotation of the earth.
  3. Elevation; Correction for being above sea level.
  4. Latitude; Correction for the location on Earth, due to its shape.
The past decade has seen the constant development of new and improved gravimeters from a number of different manufacturers measuring a very high degree of accuracy. Accuracy is determined by analysing the differenced at cross-over points from the survey data. The resolution of the gravity data along line in a function of the speed of the vehicle/vessel and the filter length applied to the data, typically from 3.5 to 5 km of resolution. (Source; CMG)
New generation gravimeters back out the movement of the aircraft from the data, providing a more accurate measurement. Once corrections are made to the data, critical data information can be derived for mapping purposes. Looking at the variation in gravity, one can interpret the different densities beneath the surface. It can detect the size, shape and depth of the formation causing the change in gravity. The most common applications for gravity surveying are petroleum and natural gas, but also lead, zinc, copper, gold, uranium, and iron.
Airborne Gravity Survey Specifications;
Although results vary from system to system, superior gravity data is generally acquired at night when survey aircraft can fly in less turbulent conditions. Gravity systems cannot operate will in conditions above 15 knot, or on more than a 20 degree bank of the aircraft. Therefore, aircraft are selected that can provide the most stable flight path possible at a relatively slow rate of speed, typically 200-240 km/h and stay airborne for as long as possible.
Gravity surveys are typically flown at ~1,000 ft elevation and with traverse lines of not less than 250 m line spacing.
Gravity surveys can be flown with in combination with magnetics and radiometrics instruments to increase the survey benefits. Airborne gravity provides an economical alternative to ground and gravity technology and therefore an excellent starting point for your exploration program.
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