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Unit Three - Geophysical
Methods Objectives - Students will be able to:
Explanation Density and Specific Gravity Density is simply mass per unit volume. The density of a substance is directly related to the atomic weight of the element composing it, ie., elements with higher atomic weight can be thought of as being “heavier”. Another way of stating this is that equal volumes of two different substances have different densities because each different substance has a different mass (ie, atomic weight) and crystal structure. There are many types of units which can be used to measure the mass of a substance (pounds, grams, etc.), just as there are many types of units which can be used to measure volume (cubic feet, cubic centimeters, etc.). The standard units of measure for density are grams/cubic centimeter. Different minerals or rocks can vary greatly in density, depending on their chemical make up. Density is such a characteristic property of a substance that it may be used to identify the substance. Geologists have found it useful to develop a system of comparing densities of different minerals or rocks by calculating a numerical value without units known as “specific gravity”. The specific gravity is the weight of a substance compared to an equal volume of water. Water is chosen as the substance for comparison because by definition it has a density of 1 gram per cubic centimeter (at 4o C), and also because it is such a common substance. For example, compare the density of a rock like granite to water: a cubic foot of water weighs 62.5 pounds and a cubic foot of granite weighs 168 pounds. The specific gravity of granite is: 168 / 62.5 = 2.7. Granite is 2.7 times heavier than water. Typical rock forming silicate minerals, like quartz and feldspar, have a specific gravity range of about 2.6 to 2.8. Specific gravity of sulfide minerals range from about 5 (pyrite) to 7.5 (galena). Native metals (gold, platinum, etc.) have very high specific gravities ranging from about 18 to 22. Differences in specific gravity between different rocks or minerals can be judged by simple hefting. It is relatively simple to calculate the specific gravity for a substance by comparing its weight in air to its weight in water. Table T7 provides a summary of specific gravity values for some common rock types and minerals.
Table T 7: Specific gravity values for selected common rocks and minerals. Earth’s Gravity FieldThe earth’s gravity field, like the earth’s magnetic field, is an invisible force field. In the late 1600’s Isaac Newton demonstrated the relationship between the density (or mass) of objects and gravitational attraction between them. Any two objects that have mass will have some gravitational force of attraction between them. The amount of attraction decreases as the distance between the objects increases. He theorized the gravitational pull between two objects is inversely proportional to the square of the distance between their masses. It
would seem a simple matter to calculate the earths gravitational force,
however, it is not quite that simple for a couple of reasons.
First, the Earth’s
gravitational field is not completely uniform because the earth is not
completely round. The field is proportional to the radius of the
Earth. The radius of the earth varies slightly from the poles to the
equator. We also know that the surface of the earth is not smooth,
but instead has many irregularities, such as mountains and oceans.
Second, the mass of the earth is not uniform. The mass of the
core is much greater than the mass of crustal material. The crustal
portion of the earth is made of a wide variety of different rock types,
each with a different density depending on its composition. For
example, basalt has a very high density compared to rhyolite.
Gravity Surveys
The acceleration due to gravity is measured with an instrument called a "gravimeter". A gravimeter uses a mass suspended from a sensitive spring and a very accurate measuring system to measure the extension of the spring as gravity increases or decreases. Gravity measurements vary by only a few percent. The gravity field has been measured at numerous locations on the earth, which has enabled detailed mapping of the gravity field (Figure F9). The standard unit of measure for acceleration due to gravity is the “gal” (named after Galileo), which is equal to 1 cm per second per second. Gravity surveys use the “milligal” or “mgal” (=0.0001 gal.) because of the extremely small differences in gravity which are measured.
Before the data can be interpreted, field gravity measurements must be corrected to account for several factors which effect the readings, also known as “reducing the data”. Geologists compare gravity values from adjacent locations by first changing each value to represent their equivalent values at an elevation of mean sea level. In other words, the corrections are made so the values represent a perfect homogenous sphere, called a “geoid”. If there are still differences in the readings after the corrections are made, then they may truly represent a gravity anomaly. The correction factors are made using sophisticated math formulas. Below are some of the more significant corrections which must be made: Free Air Correction: The height above sea level will have an obvious effect on a gravity value, because the higher the elevation (ie, the further from the earth’s center) the lower the gravity measurement will be. Therefore, measurements collected at higher elevations must be corrected by increasing the gravity values for higher elevations, and decreasing the gravity values for lower elevations. Bouguer Correction: This correction is also related to elevation. It accounts for the gravitational attraction of the material underneath the station where the data is collected. It assumes that an infinite slab of a specified density lies between the station and sea level, or in other words, the thickness of the slab is equal to the elevation of the station above sea level. The Bouguer Correction may have either a positive or negative effect, depending on the density which is assumed for the material comprising the slab. The formula for the Bouguer Correction is: BC = (0.01277) (SG) (h), where SG = specific gravity of the slab, and h = slab thickness Latitude Correction: As mentioned, the earth is not a perfect sphere. It has a larger radius at the equator than at the poles. Polar regions will have naturally higher gravity readings, and therefore must be corrected in a fashion which decreases the values. Terrain Correction: If a station reading is collected at the base of a hill, the mass of the portion of the hill which lies topographically above the station will have a gravitational ‘pull’ upward which counteracts the pull downward caused by the earth. A terrain correction for a station next to a hill will be needed to decrease the gravity value for the station. Likewise, a terrain correction for a station next to a valley will be needed to increase the gravity value.
Figure F9: Gravity map of west central Alaska.
Figure F10: Acceleration of gravity over object with greater density. Gravity
meter diagram and photo courtesy Tom Boyd, Colorado School of Mines
Field Methods | Geochemical Methods | Geophyscial Methods | Drilling Methods | Petroleum Exploration |
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Assumptions
can be made to "work around" these complications, and a value
for the gravitational force at a specific location can still be calculated
with a fair amount of precision. The standard method of measuring
this force is to measure the acceleration due to gravity. When an object
is dropped it increases its speed (velocity) as it falls. The
acceleration can be calculated by measuring the velocity at two different
times during the fall. The gravitational force can then be calculated
at any specific location on the earth. This value will be
a function of the mass (density) of the earth underneath the location,
which can be estimated by knowing the rock type at the location.
Gravimeters
are used in exploration geology by searching for irregularities in the
ideal gravity field which is predicted by the model. A gravimeter
can sense very small increases in gravitational acceleration over a
body of rock with a higher density (Figure F10). Gravimeters are
used in two different manners. One technique is to plot gravity
values along a survey line and create a “profile” (cross
sectional representation). Unexpected undulations (anomalies)
in the profile may represent buried mineral deposits containing dense
minerals. Another technique is to collect measurements of gravity
at grid locations or at numerous locations over a broad area.
These values can be contoured to create a gravity map. The map
can show where anomalies are located, and can be used to map the shape
of “basement rocks” in an area. This technique is
particularly useful in petroleum exploration where it is desirable to
search for 
