Help!
Those who know something of my activities over the past 6 years or so may know that I’ve written two books devoted to different aspects of geophysics. Now I’m considering another one. This will be different in that it will be describing geophysics applied to another field of, ‘non-scientific’ study. I don’t want to mention that field, so will use the code word [poetics] and its practitioners as [poets]. Below is a draft of a chapter of that proposed work.
What do I need from my friends and acquaintances and anyone else interested in reading this? For those of you who are [poets], does this chapter give you an appreciation for physics, properties of matter and aspects of how they might be measured and data interpreted? For those who already know these things, am I misrepresenting anything, getting anything wrong? Please let me know what you think.
Note that I am taking a deliberate irreverent tone here because I believe that [poets] will be better able to understand and remember the concepts. It may annoy some of you but what the hell.
Also note that the chapter requires diagrams which I haven’t done yet.
Physical concepts for [poets]
This chapter is for non-geophysicists, non-physicists and non-engineers. It’s for those [poetically] inclined students who slept through their school physics classes. All others can safely skip this, or read for their own amusement, disparaging our feeble attempts at trying to simply explain a complex subject.
Soil, rocks, water – any material in fact – have physical properties. They can be solid, hard or soft or fluid, wet or vaporous. They have characteristics we can feel. We can measure the mass and volume of a material and calculate its density. We can, to a greater or lesser degree, pass electrical currents through them. We can hammer something solid, and it will transmit sound and vibrations and maybe even crack if we bang strongly enough. Some matter, depending on its constituent elements, emit characteristic radioactivity. All these properties – whether of specific objects or the substances they might be buried in – are physical. They are described by the physics of those materials.
Physics is only really done on simple systems, or more complex systems that have been greatly simplified. There’s an old physics joke. A dairy farmer decided that he wanted to increase the milk production of his herd. Thinking that science was the way to do it, he found a physics lab that was willing to look into the problem. After granting his life savings the farmer let the physicists alone for a year to solve the production issue. After that year, the excited physicists invited the farmer for a presentation on their results. After seating the farmer, the head of the lab went up to the blackboard (this is an old joke!) and after a few preliminary comments on the problem, drew a circle. “First,” the physicist proclaimed, “we assume a round cow.” Ba dum tss!
While silly, and perhaps not even very funny to a non-physicist, it does point out that, first, physics only works well on simple or simplified problems. Second, why it’s funny to physicists, is that these gross simplifications can work remarkably well. We are well informed that the original farmer *did* increase his production and make huge profits while the poor physicist had to write another grant application to try to keep his lab running. In that vein then, we will try to explain the relevant physics in as simple, and perhaps irreverent, manner as possible with no mathematics – apart from counting – to complicate the concepts.
We’ll look at the composition of matter itself and the properties of electricity, magnetism, electromagnetism, density and gravity, elastic-mechanical behavior and radioactivity and then look at some other important issues pertinent to the measurement of physical data. It should go without saying that the physics of materials underlies geophysics and its use by [poets].
Matter and electricity
There are two main concepts we need to start with and which we will combine here. There is a good reason for doing this, as we’ll explain. The first concept is that all matter – the elements of nature, their resulting molecules and chemicals and then ensuing minerals, plants, animals dead or alive – is ultimately made up of atoms. We’ll return to this in a short while.
The second concept is that the universe has something called electricity, composed of discrete electric charges. They come in only two flavors, vanilla and chocolate, or to be less flippant, positive and negative (+ or -) which are diametrically opposed to each other. These charges are associated with certain particles and have an electric field around them which exert a force on all other charged particles. The fields of ‘like’ charges (negative-negative or positive-positive) repel, that is push away from, each other. The fields of opposite charges (negative-positive) attract, that is pull themselves together.
Getting back to atoms, they consist of positively charged particles called protons, negatively charged particles called electrons and uncharged (neutral) neutrons. Protons and neutrons make up the nucleus – the core – of the atom. Since protons will repel each other, they need the ‘glue’ of the neutrons to keep together. The number of protons in the nucleus determines what element it is. Now think of this nucleus as the ‘sun’ of a solar system. Electrons orbit – they are the ‘planets’ in this analogy – this core. The circling electrons inhabit discrete shells around the nucleus. Each shell has a precise number of electrons which ‘complete’ it. If an atom needs more electrons after one shell is completed, they go into a newer shell further out[1]. To be electrically neutral, each atom potentially has the same number of electrons as protons. As much as it would like to be electrically neutral however, it is the need for complete shells that is more imperative.
As an example, a single proton orbited by one electron is a hydrogen atom, the simplest element possible. Two protons, with two neutrons to keep them together, will have two electrons orbiting. This is the helium atom. Those two electrons inhabit the first shell, completing the helium atom. Since helium is electrically neutral with a complete first shell, it is an inert, or noble – an element that deigns to involve itself in any interactions with other atoms. It is sufficient unto itself.
Each increase in the number of nuclear protons – and the necessary neutron glue – is a different element. More protons mean more negative electrons attracted to orbit it by the positive nuclear charge. But, because the atom really wants to have complete electron shells, they may lose or gain electrons, creating atoms with net charge called ions. Again, because they really – really – want to have complete electron shells, they can share electrons with the atoms of other elements. Or, positive and negative ions of different atoms may cling together – opposites attract, remember. Chemistry and molecules ensue[2].
Let’s look at water, H2O. Oxygen has two electrons in its first shell, nice and complete and contented. It has six more in the next shell, but it really wants to have eight in that second, outer shell. Hydrogen has only one electron in the first shell. So, two hydrogen atoms will share their single electron with an oxygen atom. Hydrogen thus has another electron to complete its first shell and oxygen has two electrons to complete its second shell. This is called a covalent bond. Covalent bonds are the strongest chemical bonds, the shared electrons providing a tough lock.
Let’s look at salt, NaCl, sodium chloride. Sodium, Na, has two content electrons in its first shell, eight equally content electrons in the next shell but only one in its third, outer shell. Chlorine also has satisfied electrons in its first two shells and seven more in its outer shell. But it wants eight in that shell to be full. Sodium feels better by shedding its lone outer shell electron, leaving it with a net positive charge – a cation – Na+ in the vernacular. Chlorine is happy to pick up any free, lonely electron leaving it with a net negative charge – an anion – Cl– in the jargon. Positive and negative attract, so the sodium cation and chlorine anion will stick together in a relatively weak ionic bond. When you mix salt with water, the heady mix of electricity, strong covalent water bonds, weak ionic salt bonds, means that water has a sea of anions and cations floating around in it. This is more than just an example of applied physics – ahem, I mean chemistry – but important, as we’ll see, in the geophysical methods that involve electric currents and electromagnetic waves (see more below).
One more aspect of matter that will be important to us is that those elements, atoms and molecules that make up everything come in two different types on the macro scale (that is, the scale at which we humans experience them). These are fluids and solids. Solids are generally, well, solid. They’re structured and often stiff like rocks and minerals though they can be broken up into small pieces, like dust and cookie crumbs. Fluids, which can be either liquid or gas, are more amorphous. They flow and ooze like water and oil or diffuse like steam and air. And, of course, as we just alluded to by mentioning water and steam, one can become the other and we can even add ice to this picture – different aspects of the same molecule, gas, liquid and solid.
Now, back to electric charges. Think of two oppositely charged (+ and -, positive and negative) particles constrained from falling into each other. For the sake of argument, you can think of holding one in your right hand and one in your left – do not try this at home. This is called a ‘dipole’ (two opposite poles). As we’ve mentioned, there is an electric field emanating from each charge. Any other charged particle, entering into the dipole field, will be forced to its oppositely charged pole. With a supply of charged particles into this field, the resulting stream is called a current. In this case it’s a direct current (DC). The force field of the dipole is called the electric potential, more colloquially the voltage. If we, again continuing the argument, juggle the two poles between our hands, changing the configuration, the stream of charges goes one way, then the other, as the poles change. This is an alternating current (AC).
Switching back to atomic matters, think of a material that is either amorphous – atoms and ions floating around in or as a liquid – or structured like mineral crystals or even semi-solid like dirt. These materials can have very promiscuous electrons. They move easily from one atom to another. Or, they can be very chaste electrons and are constrained from moving. The first type of slutty electrons move randomly, from one atom to another, in any direction. But, if you think of putting an electrode, a positive electric pole (anode) in one part of the material and a negative electrode (cathode) in another part, the resulting electric potential field will force the electrons into a more coherent stream – a current. Sex club materials are conductors. Church going matter is an insulatos. None of this is perfect as all conductors are not the same, nor or all insulators – we’ve all heard of semi-conductors. The more conductive a material, the less electrical resistance it has. Less conductive materials have higher resistance. Resistivity and conductivity are inverses of each other – high conductivity is low resistivity and high resistivity is low conductivity.
Magnetism
We all know what magnets are – pieces of metal that stick to some other metals. We know the earth has a magnetic field and that a compass, a free-floating magnetized needle, aligns itself – maintains its orientation – with the earth’s field. We discussed electricity above and it turns out that it and magnetism are two sides of the same coin. When you have an electric current there is – automatically and inescapably – an induced magnetic field. A magnet, of any type, has a north and south pole – a dipole. Unlike electricity, there is no such thing as a magnetic monopole. The north-south poles – akin to positive and negative charges – cannot be separated.
We remember that electrons orbit – move around – a nucleus. A moving charge is a current and hence a magnetic dipole field is created. An electron spins and while this is somewhat difficult to visualize, that is also a current with an associated magnetic dipole. Protons also have magnetic fields – but much smaller than what is produced by electrons. Most atoms have their electrons orbiting and spinning in different directions, so their magnetic fields cancel. But sometimes they don’t. In some metals, most importantly iron, the tiny magnetic fields can align to produce a large magnetic field. An iron bar, for instance, in an external magnetic field, has its tiny iron magnets forcibily lined up, creating a stronger field around it. If the external field is strong enough, it can force the tiny iron magnets to permanently stay aligned – fridge magnets!
What constitutes an external magnetic field in geophysics? Due to the complicated flow of ionized plasma (a fourth state of matter in very hot conditions) in the fluid core, we have large electrical currents in the Earth producing a huge bar magnet with north and south magnetic poles. In addition, the solar ‘wind’ and radiation produces magnetic fields in the very upper atmosphere (as exemplified by the Aurorae – northern and southern lights). These are the external magnetic fields that can induce secondary magnetic fields in near surface minerals, like irons, that can be measured by geophysical means.
One of the most important magnetic minerals is magnetite. Minute amounts of magnetite – and other magnetic minerals – can be found in lots of natural materials like certain minerals. Things like pottery or soil have less or more of these minerals, causing small but measurable magnetic fields – their tiny quantities of magnetic minerals lining up in the external magnetic fields. You also have the case that a hot material, like a clay pot being fired in a kiln has no magnetic field because the heat makes all the particles move around randomly and more energetically than the external field can align them. But once the pot – or a hearth – cools below a certain temperature, the external field becomes more dominant and lines everything up and the magnetic particles freeze in place, becoming very weak, but still very measurable, permanent magnetics.
An important characteristic of materials is their magnetic susceptibility. It is a measure of how much a material will be magnetized – how much of an induced magnetic field will be produced – in the presence of an external field. Maybe think of this as a material acting as a magnetic amplifier – how powerful is its amplification. It is controlled by the amount of magnetic minerals, like magnetite, in a substance.
Electromagnetism
We’re not through with electricity and magnetism yet, alas. We’ve already discussed that electricity and magnetism are different faces of the same coin. The whole coin is electromagnetism. To stretch the metaphor even closer to breaking, it’s a coin that’s flipping through the air, glinting in the sun.
We know that charges have electric fields associated with them and magnetic dipoles have magnetic fields. But together, electric currents and moving magnetic dipoles have an aura – sorry for a new metaphor – around them. This aura is an electromagnetic field which manifests itself as an intertwined wave of electric and magnetic fields. Electric fields, magnetic fields and electromagnetic fields and waves are all manifestations of a single phenomenon.
Electromagnetic (EM) waves fill our entire space. Light is an EM wave. Radio and TV, mobile telephone, WiFi and Bluetooth are electromagnetic waves, as are radar and microwaves in your oven. Radiative heating – infrared – are EM waves. The entire range of EM goes from gamma rays (harmful ‘radiation’) with the highest frequencies onto the lower frequencies of ultraviolet, visible light, infrared and a very long range of low frequency radio waves.
Because EM waves are changing electric and magnetic fields, they will cause some materials to resonate, producing their own electric and magnetic fields – induced EM waves. Depending on the electric and magnetic properties – resistivity and conductivity, magnetic susceptibility and some others – these materials act, again drawing on metaphor, as EM amplifiers.
As something you really may not need to know, but you will see reference to if you learn more about such phenomena, are something called Maxwell’s Equations. These four laws, rules, or equations describe all the electric, magnetic and electromagnetic behavior we’ve just described. Along with Newton’s laws and the laws of Thermodynamics, Maxwell’s equations describe the entirety of classical (before Relativity and Quantum Mechanics) Physics. Those who understand these laws in their full mathematical glory control the universe[3].
Density
Perhaps the most fundamental characteristic of matter, though of less importance to us here as poetic geophysicists, is density. Density is simply the amount of matter – solid, liquid or gas – in a given amount of space. That is, so many kilograms per cubic meter, or whatever other units you want to use. Interestingly, the same material can have different densities. If you take a block of … something, whatever … and measure and calculate its density, you’ll get one value. Then if you drill big holes in that block and measure it again it’ll be less dense. Or if you take pieces of two different materials mixed together, you’ll get different densities of the composite depending on what ratios of each you add. This is the concept of bulk density – the density of the whole thing, warts, holes and all, which can be different from the inherent density of an undisturbed, whole material.
Matter gives rise to gravity. Gravity holds the universe together. It holds the earth together. It makes planets revolve around the sun and moons and satellites orbit around the planets. It keeps our feet on the ground and makes apples fall from their trees. The denser an object is, the more gravitational pull it has. The closer one is to a particular mass, the more gravitational force it has. While permeating the universe and keeping everything together gravity is actually one of the weakest forces in nature. You will not feel the miniscule gravitational pull of a bowling ball at your feet, but it turns out that we’re smart enough to construct instruments that can measure such weak forces.
Mechanical properties
Think of a rubber ball. You can squeeze it, squash it, bang it against a wall and it cheerfully bounces back to its original shape. Its ability to keep its shape after abuse is due to its elastic properties. So, how to think of elasticity? We can think of springs – you compress a spring, or pull it, and it ‘springs’ back to its default length when you let go, bouncing back and forth until it gets tired and stops. Going a bit further, we can think of matter as being made of a collection of tiny, tiny springs. Bang it with a hammer and we see the signal travel through the material as the minuscule springs compress and stretch as waves. Press on it for a very long time and the material deforms permanently – the springs weaken -anelastically. Take a sledgehammer to it and you break apart the spring connections and encounter brittle behavior. All of these modes of behavour are mechanical properties.
Anelasticity and brittle behavior are more in the realm of structural geology and geodynamics and doesn’t have a lot of relevance for [poetical] studies. In terms of geophysics overall, elasticity is very important since it provides the basic underpinnings of seismology. But seismology is a very small part of the geophysics used by [poets] as we’ll see. It’s important to be aware of it but we won’t dwell on it much further.
Isotopes and Radioactivity
Getting back to the nature of matter and the nuclei of atoms, we’ve described how each chemical element has nucleus with a distinct number of protons and an equal or greater quantity of neutrons that keep the nucleus stable and together – the glue as we’ve called it. In some cases, the same element can have more neutrons than others. They are isotopes of that element – chemically the same, but with more or less different nuclear attributes. These isotopes are identifiable, statistically distinctive and proportionally distributed.
As a nucleus gets heavier and heavier with more protons and neutrons, it becomes unstable – some isotopes being more unstable than their sisters. There are too many more protons than the neutrons can keep glued together. Sometimes more neutrons can even add to the nuclear instability. An unstable – a radioactive – nucleus can then spontaneously eject particles – two protons and two neutrons for instance – making itself lighter, happier and incidentally remaking itself – transmuting – into a new element. The two protons and neutrons are a helium nucleus and are ejected with some force so that they can penetrate matter to some extent. These nuclear bowling balls are called alpha radiation. Since alpha particles are big and heavy, they don’t travel far and can be stopped by relatively light materials.
Another way neutrons can relieve themselves is by beta radiation. Beta particles are electrons that are fired out of the nucleus when a neutron turns into a proton (plus and minus is zero, remember). As opposed to the alpha bowling balls, beta particles are high-velocity bullets, travelling faster and farther and able to penetrate through thicker materials.
When a nucleus transmutes by alpha or beta decay – the new nucleus has an excess of energy – it’s in an excited state. To calm itself it can radiate some of this energy as a very high frequency electromagnetic wave/particle (again, reference quantum mechanics) dubbed gamma radiation. Gamma rays are very energetic and very penetrative – laser death beams as opposed to bowling balls and high velocity bullets.
Radioactive elements decay in unique and statistically predictable ways. As an example half of any given amount of the isotope Uranium-238, the most common and abundant type of uranium, will decay to Thorium-234 in about 4.5 billion years – its half-life. This leads to a long chain of radioactive transmutations into daughter elements that ultimately lead to stable Lead-206. The relative abundance of isotopes in a composite material can tell us how old it is. This is the concept behind Carbon-14 dating. Carbon-14 is an unstable, radioactive isotope of carbon with a half-life of 5730 years. The relative amount of Carbon-14 in a biological sample can tell us how long since that material died and stopped ingesting C-14.
We’ve described nuclear isotopes and radioactivity in some detail for the sake of completeness, but also because it is very important for many [poetical] studies. It is, however, more physics (and even, sigh, chemistry) than it is geophysics. But even so, there can be geophysical methods that measure the ambient radioactivity in an area that can help us focus field investigations.
Waves
Waves are very important in geophysics. Just about everything we can measure, the signals of physical characteristics we can record, can be looked at as a wave. Generic features of waves are used to describe any wave motion, be it on a rope, in water, sound, seismic, radio, or the way gravity and magnetism changes in an area and even the wavy nature of topography.
The simplest wave is a sine wave that you may remember from school maths. At some time or place, the wave starts at an equilibrium point – zero if you will. It smoothly increases in size to a maximum, ramps down back to zero then down to a minimum value before rising back to zero again. The length, or duration, of this one complete cycle is the wavelength or period. How many full wavelengths (or periods) within a set length (or time) is its frequency. A high frequency sound – a soprano voice – has its acoustic wave changing very quickly in time, so that it has a short period. A baritone – low frequency with a longer period (wavelength) has fewer full cycles in the same amount of time. The maximum/minimum value of the peak of the wave is its amplitude – its strength. A high amplitude sonic wave is louder than a low amplitude one. All more complex waves can be considered as mixtures of simpler sine waves that makes analysis tractable.
There can be some confusion here with wavelength versus period. In concept they are the same but differ in nuance. Wavelength is usually used for waves that can be measured with a ruler or tape measure (water waves or even topographic highs and lows). Period is usually used for waves that change in time (sound and radar waves). Even so, they can be related to each other – a sound wave of a particular period can have a calculated, equivalent wavelength. But in popular speech, they often mean the same thing.
Signals and sampling
If it hasn’t been understood yet, by osmosis, waves are a synonym of signal and in turn of data. We have a wave, a signal, coming at us and that we can measure as data. We might do this by means of a strip chart recorder – a measured signal is detected somehow and traced out by a pen on moving piece of paper that we can later use as data and analyze it. Here we have a continuous measurement of data and its wave shape as an analog signal. Such data isn’t necessarily easy or efficient or amenable for further analysis.
In our digital age we can try another approach, by simply take discrete measurements every so often and record them (write them down in a notebook or into a computer file). These are very amenable to further analysis and interpretation by slide-rule, calculator or more reasonably by computer. It turns out we don’t need to see the minute details of the signal, but just representative parts of it. The question then becomes, how often do we do this in order – how often do we sample that signal – to accurately define the wave?
It turns out that for a particular wavelength/period, we need to sample the wave at least twice. That allows us to uniquely define the waveshape of the signal. If it’s less, we miss things and will misinterpret it. If we sample more, there is no problem other than perhaps getting more data than we need or can handle. This is in the realm of something called digital sampling theory.
One other important aspect of data and of signals – whether analog or digital – is something called noise. Noise is sometimes incoherent and random, sometimes not. One person’s signal can be another person’s noise – Beethoven to one versus Lady Gaga to another. Noise is omnipresent and can sometimes overwhelm, despite everything we can do in the measurement or post-processing realm.
More generally though, talking about more concrete things like finding your keys dropped in a field after a dust storm, how often do we need to dig a hole in the hopes of finding them? This is also sampling theory, but since we not looking for sinusoidal signals, it’s a bit different. Just to make things a bit more reasonable, lets talk about finding your car buried after a huge snow storm. Here, for a car about 2×3 meters in size, we can probably find it if we dig a hole every two meters. If we do it every four meters, there is a good chance we’ll miss it. Digging every meter will guarantee we find it, but at a back-breaking expense. What if we’re looking for the highway buried by the snow? In that case, you can probably dig a trench and be sure of finding it if you ‘sample’ – place your trench – at right angles to the known orientation of the road. It becomes obvious that any sampling procedure – for concrete items or physical signals – needs to be carefully planned based on your needs and expectations.
Anomalies and Interpretation
After we measure physical fields we need to be able to make sense of them. Generally speaking, physical measurements, depending on the way we record and visualize them, can look like pimples and blobs on an otherwise blotchy surface. Pimples and blobs that are conspicuous can be termed anomalies. The word anomaly simply means something strange, out of the ordinary, out of place.
In geophysics an anomaly means that you see a signal that sticks out from all the other measurements. It is intriguing but not necessarily indicative of something interesting or a signal from something we hope to be there. It could easily be just noise. It is, alas, a characteristic of most – if not all – geophysical measurements that they do not show us explicitly what is there. Do you remember one of the first scenes from the movie “Jurassic Park”? The ‘scientists’ use some single, sharp sound source (in actuality a seismic ‘shotgun’) that results in a highly resolved ‘radar’ image of a buried, complete dinosaur skeleton. We will not bore you with debunking the multiple mistakes, errors, misnomers and plain bullshit displayed – ‘shoot the radar into the ground’ indeed!
In any case, strange blobs indicative of interest might or might not be that different from other strange blobs from uninteresting things. It is possible, to some degree, to characterize the sub-surface in more detail than that and pull out real physical parameters that allow us to identify specific materials. But that is difficult and more often than not, especially with less experienced and knowledgeable practitioners, we descend to ‘anomaly hunting’ and then can make the mistake of identifying them with our targets. Let’s not be too strict here, seeing a few geophysical anomalies, especially if they conform to the known characteristics of what we’re looking for, is much better than digging holes and trenches, no matter how well-planned. Again, the information we get from a field measurement will depend not only on the methods used, but the skill and knowledge of the one analysing the data.
Analysing signals, interpreting data, is an important skill that can be more akin to artistry than science. Yes, there are scientific methods of analysis that can to some degree quantify the physical parameters that we’ve discussed above. But in the end, even the most sophisticated analysis needs interpretation. It requires the knowledgeable and experienced mind of an ‘interpreter’, someone who can take the data, the images, the analytical results and spin a meaningful and realistic story of what is buried.
Further, having said all sorts of disparaging things about blobs, they can also cohere and align into patterns that are recognized by the knowledgeable practioner. One only needs to think of a radiologist who can interpret the various shadows on an x-ray that mean nothing to most of us.
Summing up
To sum everything up, geophysical measurements of physical fields are powerful methodologies to characterize the subsurface and to find specific articles buried in an area. But geophysical exploration requires several things. It’s important to understand the physical properties of matter and how they react and create physical fields, the intricacies of measurements the earth and how these impact the process of interpreting them to understand things under our feet.
[1] The why of this is in the realm of quantum mechanics, but hasn’t much to do with Schrodinger’s cat.
[2] We promised physics and are now getting into Chemistry. Physicists are loathe to say so in mixed company, but they consider chemistry to be just applied physics.
[3] Sort of.