Different Methods of Geophysical Surveys Used in Mineral Prospecting

Reflection Seismology

What it is

Reflection Seismology (called “seismic” in the minerals industry) is the practice of inducing a vibration in the Earth’s surface, and analyzing the reflection. The vibration is induced using a vibration machine or a series of dynamite blasts. This creates a seismic wave that travels to the Earth’s mantle, the ball of molten rock below are feet. The mantle reflects the wave back up to the surface, where they are measured.



What it is good for

• Hydrocarbons. Oil, gas, and coal occur in stratified sedimentary rock. This is because they were originally biomass, like forests or grasslands that were covered up in layers of rock. These layers are easy to spot with a seismic wave, allowing geologists to easily pinpoint any variations in the layers that might contain hydrocarbons.

What it is not good for

• Minerals tend to occur below the sedimentary layer, in igneous (solidified magma) or metamorphosed (heat-transformed) rock. Geologists call the parts of the crust where these rocks occur a “hard rock environment.” Hard rock environments are often extremely complex, and are much more difficult to analyze seismically. However, due to the popularity of seismic in the oil and gas industries, many geologists focus exclusively in it and do not apply more appropriate methods when exploring for minerals.
• The environment. The vibrations, especially when caused by dynamite, have many negative environmental effects.

Gravity

Gravity exploration is a passive exercise that has no environmental effects. Instead of creating a seismic wave, geologists measure variations in the Earth’s gravitational field, adjusting for polar flattening, the rotation of the Earth, and the gravitational impacts of the sun and moon. The remaining variations show dense formations in the hard rock.

What it is good for

• Oil. Gravity can find oil deposits with spectacular accuracy. Airborne gravity surveys can find likely areas for exploration, and ground surveys can find the perfect place to drill.
• Gems. Gravity is often used to find diamonds and other precious stones.

What it is not good for

• Certain mineral deposits. Some minerals occur in distributions with little resemblance to the patterns in the hard rock, and are hard to see by gravity.

Magnetism

Magnetic exploration is much more complicated and precise than gravity. Geologists use variations in the Earth’s magnetic field to detect certain magnetized ores and precipitated minerals.

What it is good for

• Minerals. In particular, it is good for finding minerals hidden in complex rock formations. Precipitated minerals, metals that have already been smelted from the ore state by heat in the Earth’s crust, shine particularly bright on magnetic scans.

What it is not good for

• Amateur geologists. Magnetic maps can be extraordinarily complicated and require advanced training to properly interpret.