Introduction to Earth Sciences

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Geophysics is concerned with the physical processes and physical properties of the earth and its surrounding space environment, and with the use of quantitative methods for their analysis. For more than seven decades, geophysicists have made significant contributions to the description of solid earth, based on its physical properties; on the exploration and production of its resources deep in the ground; and on an understanding and mitigation of the hazards associated with the earth’s dynamics, such as volcanic eruptions, earthquakes, tsunamis, landslides, hurricanes, droughts, etc. These types of events are so important that they directly affect where we live on the earth’s surface as well as the sources of our food, our energy resources, and our minerals—and such events can affect our very survival. Yet most universities still do not have a course focusing on an introduction to geophysics—the so-called 100-level geophysics course. …


We have developed this book with the vision of multidisciplinary education in mind, based on our many years of multidisciplinary research. Through the choice of these four topics—earthquakes, volcanoes, energy resources, and climates—we will also indicate to students that modern geophysics is a truly multidisciplinary field. From an academic point of view, this book can be used in 100-level geophysics and physics courses and in some introductory classes of the new multidisciplinary departments. We think that the book will also be very useful to the general public with an interest in problems related to basic energy, climate, earthquakes, and volcanoes.

Another major feature of this book is that it contains analytical problems as well as computational problems that include MathLab software and Mathematica software developed especially for the classes associated with this book. These problems can be used in lab classes or to help students improve their understanding of and intuition about the materials presented.


A View of Earth: The Earth’s Spheres


… A close look at the earth’s surface shows that 61 percent of  the northern hemisphere is covered by water,  and 81 percent of the southern hemisphere is covered by water.   However, on a planetary scale, the ocean is small. In fact, there is more water within the earth’s interior than in its oceans and atmosphere. Note that Earth’s surface is not flat; it has topography. Ignoring oceans,  Earth’s surface is dominated by two distinct elevations: most land is 0-2 km above sea level, and most of the sea floor is 3-5 km below sea level.






Seismic Waves and Evidences of Earth’s Interior Structure


A sudden deformation or sudden movement of a portion  of the medium produces two types of deformations: volumetric changes and changes in shape. P-waves  are caused by volumetric changes.  They are similar to sound waves, which are a series of contractions and relaxations. S-waves are the result of changes in shape.   S-wave motion is perpendicular to the direction of wave propagation.  It is important to become familiar with the differences between P-waves and S-waves because these two notions are central to geophysical studies, especially studies related to solid earth. Actually,  nowadays it is almost impossible to succeed in studies of solid Earth without a good understanding of P-waves and S-waves. Note that only P-waves are generated in the tank-of-water experiment because only volumetric changes occur nonviscous fluids, including water and gases.



Earthquakes and Volcanoes


… Figure 3.19 illustrates the first motions caused by a purely strike-slip earthquake.

From this picture, we can easily recognize the link between the polarities of seismic events and the focal mechanism. Seismograms exhibit small or zero first motions in the directions of nodal planes because the first motion changes from dilatational to compressional. So we can find the orientations of nodal planes from the first motions of P-waves; therefore the fault geometry is known. However, first motions alone cannot distinguish between the fault plane and the perpendicular auxiliary plane. Notice that in principle we can use any events in seismic data (e.g., S, PP, and PS, as described in Figure 2.30) to determine nodal planes, but only first P-wave arrivals have been used so far, as the later-arriving events are more difficult to identify because of interferences between events.





Earthquakes and Volcanoes 


 … Ecuador and its capital, Quito, for example, are under the threat of both volcanic eruptions and earthquakes. The whole of Central America is under the twin threat of earthquakes and volcanic eruptions, as we can see in Figure 4.3a.  An earthquake in Guatemala in 1976 caused 22,000 deaths, an earthquake in Nicaragua in 1972 led to 5,000 deaths, and an earthquake in El Salvador in 1986 caused 10,000 deaths. Volcanic eruptions occurred in Irazu in 1964 and Arenal in 1970, both in Costa Rica. On the west coast of South America (Figure 4.3a), an earthquake in Chile in 1960 caused 5,700 deaths, and one in Peru in 1970 left 66,000 dead. The volcanic eruption of Nevado del Ruiz, Colombia, in 1986 left 23,000 dead. In the Pacific Rim (i.e., cities located around the edge of the Pacific Ocean), the situation is quite similar (Figure 4.3b). Volcanic eruptions and earthquakes have occurred in New Hebrides (Vanuatu), New Zealand,  Papua New Guinea, and the Philippines.  Japan is well known for its volcanic eruptions and earthquakes, as are Indonesia and the less-densely-populated areas of Kamchatka (a 1,250-km-long peninsula in the Russian Far East, between the Pacific Ocean to the east and the Sea of Okhotsk to the west, with a 10,500-meter-deep trench along its Pacific coast) and Alaska. These are clear examples of the correlation between earthquake occurrences and volcanic eruptions.



Climate and Heat Radiation


About 50 million years ago, the earth was free of ice, and giant trees grew on islands near the North Pole, where the annual mean temperature was about 60 degrees Fahrenheit, far warmer than today’s mean of about 30. Remember what the North Pole is today is now located in the middle of the Arctic Ocean amid waters that are almost permanently covered with constantly shifting sea ice. So there has been a very large climate change in the last 50 million years.

There is also evidence that the earth was almost entirely covered with ice at various times around 500 million years ago; in between, the planet was exceptionally hot. These very large swings in temperature characterize climate change.






Electromagnetic Radiation


(Figure 6.1), (Figure 6.2), (Figure 6.3)


Electromagnetic (EM) radiation is all around us and in the entire universe. Yet EM radiations are hard to get a handle on. Although we see with light, which is an EM radiation, we can appreciate the beauty of a Fiji coral reef in blue waters (see Figure 6.1), the warmth of sunshine and the sting of sunburn. These observations are brought to us by electromagnetic waves. In other words, electromagnetic fields cannot be seen, but nonetheless, they are real and can work (by applying forces) on objects and exchange energy. It took scientists until the 1890s, to accept this fact.

We are now very familiar with the fact that X-rays can reveal a broken bone (see Figure 6.2), and with microwave popcorn (the so-called “microwave generation” tends to overuse microwave ovens to prepare their food), with the natural electrical phenomenon of lightning (see Figure 6.3), and with the less-powerful magnetic forces which can cause lodestones to point north and can have profound impact on humanity. The effects of lightning include lightning flashes, acoustic pulses, heat stroke, and EM pulses. Lightning can also destroy electronic and electric networks. These facts and phenomena are also other natural manifestations of EM waves.



Energy-Balance Equations and the Greenhouse Effect 


… Let us now modify our climate models by asking what effect the atmosphere has on the earth’s temperature. To answer this question, we consider the one-layer model depicted in Figure 7.4. In this model, the incoming shortwave radiation (after removing the reflected component) is transmitted by the atmosphere and is completely absorbed by the ground. The ground emits as a blackbody.  The atmosphere absorbs all of this energy and re-emits energy as a blackbody from both surfaces: i.e., into space and back to the surface. In this one-dimensional model, the emission is shown in Figure 7.4 as only up and down relative to the earth’s surface. Here we have to estimate the average temperature of the ground and that of the atmosphere. We can do so by using the following energy-budget equations in the atmosphere and in the ground:







Climate Forcings 





The decrease in Earth’s glacial area can be seen clearly in the satellite images of Mount Kilimanjaro and Lake Chad (Figures 8.16 and 8.17), which illustrate climate-change impacts. They show a continued decline in lake-surface area from 23,000 ${\rm km}^2$ in 1963 to only 300 ${\rm km}^2$ in 2001. Hurricane Katrina, which took place during the 2005 Atlantic hurricane season, was the costliest natural disaster and one of the five deadliest hurricanes in the history of the United States. At least 1,800 people lost their lives during the hurricane or the subsequent floods.

The European heat wave in the summer of 2003 (Figure 8.18) caused massive loss of life; the deaths of at least 70,000 people have been attributed to the heat. The heat wave of 2010 (Figure 8.19) was centered in western Russia and lasted from late July until the second week of August 2010, surpassing the 2003 heat wave. In Moscow, the daytime temperatures reached 38.2 degrees Celsius (101 degrees Fahrenheit); in Kiev, nights reached 25 degrees Celsius (77 degrees Fahrenheit), crops were destroyed, fires swept across western Russia, and estimates put the Russian death toll at 55,000.

The Pakistani floods began in July 2010 following heavy monsoon rains in the Khyber, Pakhtunkhwa, Sindh, Punjab and Balochistan regions. Estimates are that more than 2,000 people died and more than a million homes were destroyed.


Energy production and consumption


World oil demand in 2011 was 89.11 million barrels per day, and world oil production was about 88.54 million barrels per day. So about 0.6 million barrels per day of spare capacity was needed to keep up with the demand. Oil prices go up at some point.

Spare capacity refers to the world oil market’s ability to respond to shortfalls in production. The total spare capacity in 2014 was about 0.5 million barrels per day, which provided very little cushion for fluctuations in demand. The world demand in 2014 was 92.4 million barrels per day, and world oil production was about 93.1 million barrels per day, so there were about 0.7 million barrels per day of overcapacity, and the oil prices decreased at some point.

Figure 9.14 captures the differences between consumption and production for the past 35 years. It also shows the differences relative to oil price fluctuations. One of the first observations is how small the differences between consumption and production are relative to overall production or to the consumption. Yet these small differences have a huge effect on oil prices, at least in the short term.  In other words, if for some reason the margins between consumption and production were to drastically change overnight, oil prices could collapse or skyrocket. The next question is, what are the chances of such large changes in the difference between production and consumption happening? To answer this question, let us review the leading producers and consumers that can cause such changes. Table 9.2 shows the leading producers, and Table 9.3 shows the  leading producers. Besides the United States and to a lesser extent Brazil and Canada,  we can see that oil production by leading producers has been quite stable, with a small decline by Iran and a small increase by Iraq, both for geopolitical reasons.


Biomass, Solar, and Wind Energy Resources



Fundamentals and Technology. The following conclusions can be drawn …

(1) If the wind speed doubles, the power in the wind increases by a factor of 8. Therefore it is important to select areas with a high wind speed and/or higher-altitude areas to increase the wind speed

(2) If the weather is cold (higher density), more power exists in the wind available for the turbine. In other words, a wind turbine at a given wind speed produces more power in the winter than in the summer because the weather is colder. The same conclusion can be extended from day to night if the temperature difference is significant, as it is in deserts.

(3) A turbine that is two times larger in a cross-sectional area has twice as much wind power available to it. This conclusion explains why the size of wind turbines has grown dramatically over the last three decades (see Figure 10.21).


Source rocks and seismology


One of the major requirements in 4C-OBS experiments is that geophones be well coupled to the sea floor in order to record high-quality P-waves and S-waves. Because shear waves do not travel through water, the geophones must be in direct contact with the seabed in order to capture the motion of the seabed and not the change of pressure in the seawater (see Figure 11.17). The process of ensuring direct contact between the geophones and the  seabed or any other solid material is called coupling. The 4C-OBS experiment requires the precise coupling of geophones with the seabed.


Exploration, Production, and Monitoring


The subsalt E\&P challenges are even more daunting when we focus on the specific case of ultradeep waters because, in addition to the thick salt bodies overlying these reservoirs, we have to account for the extreme temperatures at and below the sea floor  ($150^{\rm o}$ C or more) and the extreme pressure at the sea floor and below (about 30,000 psi or more).

Figure 12.4 shows four ultradeep water fields in the Gulf of Mexico. Moreover, the ultradeep water reservoirs are generally very heterogeneous and have low permeability, and oil in these reservoirs is highly viscous. We need to develop new acquisition technology which can operate at these temperatures and pressure so that ocean-bottom and borehole measurements can be added to current sea-surface measurements in order to improve the descriptions of these reservoirs. We also need to increase the resolution of seismic imaging through improvements in acquisition and processing so that time-lapse seismics, which we will describe later, can be used to optimize drilling through these complex reservoirs.