Physical Geology - Gravity Quiz


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You live in Pasadena, Texas and have reason to believe a pipeline runs under your back yard. The density of the soils in your area is about 2.4 g/cc. The density of air is nearly 0.0 g/cc, the density of oil is 0.75 g/cc and the density of water is 1.0 g/cc. For this problem assume that the density of the pipe is the same as the density of the soils and that the pipe is 1 meter in diameter.

A geophysicist brings a gravimeter to your back yard and makes a series of measurements. He obtains the following results:

Last Four Digits of SSN :

  1. This is a negative gravity anomaly which means that the material in the pipe has a higher density than the enclosing material.

  2. Assume that the center of the pipe producing this anomaly is 5 meters below the surface. If it were 10 meters below the surface the anomaly would have a greater magnitude.

  3. Assume that the pipe producing this anomaly is full of water. If it is now filled with gas, the anomaly will have a greater magnitude

  4. If the pipe producing this anomaly is replaced with one at the same depth but 0.5 meters in diameter you would expect the anomaly to have a lower magnitude.

    Note that a number of assumptions have been made in this problem. Often, all you would have is the anomaly itself. You might not know the diameter of the pipe or how deep it is buried. This would cause some uncertainity in your interpretation.

  5. A smaller pipe that is shallow could give the same magnitude of anomaly as a bigger pipe that is deeper.

    Thus, in working with this type of data you have to accept the fact that you may end up with several possibilities that could create the same (or nearly the same) responses.

  6. Even though there is some uncertainity, you might be able to use gravity measurements to detect changes in the composition of the fluid being pumped through the pipe. For example, if a company were to start pumping heavy drilling mud (with a density of 4.0 g/cc) through the pipe you should detect a stronger negative gravity anomaly.

  7. If the pipeline were removed and its volume replaced with concrete (density of about 2.6 g/cc) and the soil replaced you would expect to detect a positive gravity anomaly.

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