地震反演与层析成像1.ppt

* In gravity prospecting, we measure very small variations in the force of gravity from rocks within the earth. Different types of rocks have different densities, and the dense rocks have the greater gravitational attraction. To the left is a “gravimeter” which measures the force of gravity in the earth. Figure courtesy of Lacoste-Romberg Did you know … In oil exploration, we measure changes in gravity that may be only one-millionth or even one-ten millionth of the earths total gravity field. With a small kitchen scale, measure the weight of different rocks you find in your area. The heavier rocks have a greater gravitational pull than lightweight rocks. Pyrite is a heavy rock Sandstone is a lighter rock Figure courtesy of Scintrex, Ltd. In Magnetic prospecting we look for variations in the magnetic field of the earth. The magnetic field of sedimentary rocks is usually much smaller than igneous or metamorphic rocks. This let’s us measure the thickness of the sedimentary section of the earth’s crust. The instrument to the left is a “magnetometer” which let’s us measure the magnetic field of the earth. Many rocks such as magnetite are naturally magnetic. Compare a piece of magnetite with a piece of sandstone by holding a compass near each rock. Does the compass behave the same? Figures courtesy of Scintrex, Ltd. All rocks conduct electricity to varying degrees. The resistance to electrical current flow is called “resistivity”. Resistance is measured using electrodes that are implanted in the earth. Resistivity surveys are commonly used for groundwater studies. Fresh water is resistive, brackish water is conductive. With an Ohmmeter, check the resistance of different rocks in your area. Do you find that some rocks are more “resistive” than others? Pyrite has little resistance. It conducts electricity easily. Sandstone is very resistive. It does not conduct electricity very easily. Hold a slinky tightly in one hand and give a quick jerk of the slinky in the