/sci/ - Science & Math

>>3974711
How do you measure the gravity? How accurate can your measuring tools get?

why would I want to study geophysics? what are some examples of interesting things about it?

wait

why does water bulge up over denser rock?

>>3974719

MINERALS, CHECK 'EM MARIE

>>3974724
damn, anon just made a good point

>>3974724

He's a geophysicist, not a scientist.

>>3974719
You can study anything from earthquakes to archaeology (we can see inside tombs and pyramids without opening them), not to mention forensic applications. Also, if you want to make a shit ton of money, you can go into oil or mineral exploration. We can pretty much tell you where there is something of value in the subsurface.

>>3974721
The pendulum method is the most ancient one. It's pretty ok if you have large contrasts in densities. The laser method is the most high tech I know (but there's always new developments). It is VERY accurate. It also depends on how much time you have to make the measurement. We can make gravity measurements on a plane that has some accelerometers and then correct our measurements using what the accelerometers recorded.

>>3974729
>>3974724
This one always gets people confused. Water will always even out to be have the same amount of potential energy, otherwise it would flow from the place of high potential energy to that of low. So now, imaging we have something like the OP picure (part a). As we move from left to right the water is going to need to bulge up over the excess mass to feel the same force as it did when it far from the mass. This is called an equipotential surface, or in gravity terms, the geoid.

>>3974750
How much will Exxon/generic mining company pay you to find stuff for them?

>>3974750
Does it have to be done from the surface of the planet? If we deployed a (chemically inert) balloon in Venus's upper atmosphere, is there any way we could detect large differences in the subsurface? (more than a couple hours on the surface is not an option)

>>3974755
Average starting salary at Exxon is about $80k but if you get a M.S. (highly recommended) you can start with $100k and have a lot more say in what you want to do with your career. If you don't have a M.S. you will probably hitting some sort of ceiling in your career. You also can't be a retarded asperger nerd, you have to know your science very well and be able to communicate and sell your ideas too. You also need to be social.

>>3974770
How much chemistry did you have to take in undergrad?

>>3974764
Yes we can. In fact, we have collected gravity and magnetic (which works almost identically to what I explained but with "density" substituted by "magnetic susceptibility") data over almost the entirety of Mars. The patterns are batshit crazy and are one of the main focuses of research in NASA's Houston center. We also have acquired, on Mars, a measurement called hyperspectral remote sensing which takes pictures of the surface in a huuuge band of electromagnetic frequencies; much more efficient than just visible light. This tells us about the mineralogy of the surface and even variations in ground temperature but has an almost negligible depth of penetration.

>>3974785
Just chem 1 and 2 is required but I took organic 1 and 2 as well (useful, but not necessary, if you want to work in the oil industry). Well, you also have to take a mineralogy class which, in a way, is like a solid state / crystal chemistry class. It's not that bad. Physics and math is the main focus though.

But what about stuff that has similar density? You wouldn't be able to tell them apart with these methods.

>>3974791
You are correct. If things had the same density, we would see absolutely nothing. And this is the case sometime. This is why we usually acquire magnetic data simultaneously with gravity data. If we have more independent data over the same object, we have a greater chance of figuring out what it is. Now, if the object has the same density AND magnetic susceptibility of it's surrounding, we simply won't see it.

In my next post, I will list all the geophysical methods we use in Exploration Geophysics (as opposed to Whole Earth geophysics, which deals with the very deep structures of the earth like: tectonic plates, the mantle, the core, etc)

>>3974711

I had to stand through close relatives saying that you guys "guesstimate" your prospecting,and that there might be more oil undiscovered. How great is the margin of error, and if negligible, thank you for giving me a usable explanation.

>>3974807
Excellent. I have never had any kind of remote interest in geophysics before, but this thread, and you, make it worthwhile. Thanks for a great thread.

>>3974750
>We can make gravity measurements on a plane that has some accelerometers and then correct our measurements using what the accelerometers recorded.
How do the accelerometers work? Do use some external reference point like GPS?

Hey Geophysicist man, weren't you here with a similar water-bulge post months ago? I replied in that thread a bit and since then have gotten some undergrad research project crunching numbers from the aftershock values of the recent VA-located earthquake.

How valuable do you think undergrad research is when you were/are looking for a job?

Geologist here.

Geophysics is a good field to get into. But what alot of the oil industry people want are people who are strongly cross-pollenated, so to say, between straight up geology and straight up geophysics. Having a strong geophysical and geological backround makes you a very valuable person.

This coming from someone who is very focused on geology, and only has the most simplistic understanding of geophysics.

>>3974790
>be a philosofag
>take symbolic logic, upper-division non-classical logic, calc I-III, intro math stats+psych stats, and upperdivision probability theory
>take intro to physics, classical mechanics, astronomy, planetary astronomy, upper division planetary geology

I'm in a psych grad program but I kinda hate it. I've taken all the above in undergrad, what more would I need to get into a good M.S. program in geophysics? (GPA: 3.85 undergrad, 3.9 grad (so far). GRE: quant: 690, verbal: 740)

Do I need to get my quant above 700?

>>3974813
Well, the error is largely dependent on the methods we use. The more methods the better. Also, the error depends on the depth of investigation. If the target is veeeery deep, it needs to be very large to see it.

Think of your natural eyesight, if there is not much contrast between the object you're looking at and the surroundings, you can't see it very well. Also, the further it is, the more difficult to see. Now mix both those problems.

In exploration geophysics, we can see pretty well in the first 5km below ground. After that things get progressively noisy. But your relatives are right, it is all a guesstimate. That's how science works. But no one can deny that it is nevertheless very very useful.

>>3974849
Accelerometers are small capacitors whose plate spacing varies with a little spring. They measure the change in time of the potential difference across them. This tells us about acceleration, or movement of the platform where the accelerometer is located. They are also used to measure ground movement in seismology.

Yes we use (differential) GPS to locate each measurement sample. We also measure the elevation if the measurements are done on a plane.

>>3974901
P.S, I also have dyslexia and dyscalculia so getting my quantitative GRE above 700 is a major challenge.

>>3974910
>But your relatives are right, it is all a guesstimate. That's how science works. But no one can deny that it is nevertheless very very useful.

Isn't card counting in black jack "just" guesstimating? When it comes to games of chance, what matters most is that you're less wrong most of the time than the other guy.

>>3974910
>Accelerometers are small capacitors whose plate spacing varies with a little spring. They measure the change in time of the potential difference across them
Isn't that sensitive to the gravitational field as well as your acceleration? You couldn't use that to correct your gravity measurement for acceleration. By the equivalence principle, that correction can't be done unless you use an external reference. You could do it with the GPS, but not with the accelerometers as you describe them.

I suppose if they were very sensitive to changes in a-g, they might be useful, if the GPS isn't sensitive to high-frequency fluctuations in a. Is it something like that?

>>3974890
Yep, that's me. Just thought I'd come back with a little more time in my hands.

It's very valuable. It's much better than saying that you worked at pizza hut. However, it is EXTREMELY useful if you want to go into grad school (which you should). That's the first thing they look at. Make sure you get to present your research at some conference or, if possible, publish your results. Also, make a scientific poster. And put all that shit in your resume. Also get a good recommendation from your advisor.

>>3974899
Yep. Completely agree. The line between geology and geophysics is an arbitrary one drawn by universities because it' hard to learn it all in your undergrad. I double majored in geology and geophysics with a minor in math. But that's not typical. Now I'm doing a M.S. in geophysics. Oil and mineral companies really like the fact that I have a geology background.

>>3974901
The GRE depends on the university. I would recommend that you email some advisors in different universities and find out what you really need. Geophysics is a science that incorporates bits and pieces of other sciences so departments are usually pretty cool with diverse backgrounds. One of the best geophysicists that I met in the program was a philosophy undergrad. However, you will need to suck it up and take any remedial courses that they tell you to take. You probably need a few more math courses (diff eq, pde, complex analysis, linear algebra), some basic physics (electricity and magnetism, vibrations and waves) and some geology classes (mineralogy, stratigraphy, structural geology).

Paleo student here.
Shout out to all my geobros; thanks for the stratigraphy!

OP, maybe mention how denser chunks of continental crust sink lower while less dense chunks ride higher? (That is, mountains don't really show up on gravitational anomaly maps.)

>>3974911
Your GRE is fine. You can get into almost any school with a 1300. Also, no one really gives a shit about GRE scores. Get in contact with professors whose research you are interested in and ask them for advise. Be VERY respectful and pithy in your email. Also, no crying and bitching about what you don't know. They don't have time for your bullshit.

>>3974921
We "guesstimate" in all sorts of empirical research. That's how most experimental sciences work. We then have to devise tests of these hypotheses though.

>>3974959
We can decompose the accelerometer in signal into its frequency components and correct for any high frequency variations. We then correct the low frequency components with elevation changes. This gives us a reasonable measurement of local gravity.

>I'm going to put in a load of laundry, I'll be back in a few minutes. Ask away.

>>3974959
Sorry, this my previous post made it seem like we use the accelerometers to measure gravity. We don't. Accelerometers are intrinsic high-pass filters. They don't record the low frequency gravity changes.

Just how different was your uni courses from a straight up physicist?
Did you still do the same amount of maths, relativity and all the other "poster child" subjects?
Is fluid mechanics a good background to have for geophysics?
Can someone with a physics B.Sc get on a geophysics M.Sc?

Sorry about all the questions, i'm trying to decide between regular physics and geophysics.

>>3975045
A typical geophysics undergrad substitutes a few of the advanced physics courses (relativity, qm, etc) for some basic geology courses. This is why people expect us to do a M.S. to get into serious stuff.

You will be perfectly fine going into geophysics for a M.S.

The typical (completely anecdotal) undergrad background of geophysics graduate students is:

- Geophysics: 35%
- Geology: 20%
- Physics: 15%
- Electrical Engineering: 15% (because of signal processing)
- Other Engineering: 10%
- Other in General: 5%

You'll be fine. Take "Physical Geology" if it's offered in your university. This will give you an intro to geology that you will need and this will also show you whether you like the subject or not.

>>3974711
>>3974711
EEE undergrad. speaking,

do you use spectroscopic methods of analysis in upper layers of soil measurement when using ground penetrating radar? say in the region of MHz to GHz?

Is ground penetrating (low frequency) sonar of any use? Can it be used for spectroscopic measurements for properties of materials?

>>3975198
To clarify by shallow layer I denote several meters like 2-10 meters deep.

>>3975198
I don't know if they use spectroscopy in GPR. It sounds like a cool idea though. I don't know how much the characteristics of soils change in only a few meters though. They might, but I don't know. By the way, GPR is extensively used in archaeology and forensic science.

I don't know about spectral analysis of sonar but I am currently researching the spectral characteristics of different rock formations based on seismic frequencies. You can tell a lot but it's not straight forward, you need some well-control to confirm your findings.

Methods we use in Exploration Geophysics:

- Gravity: as explained before

- Magnetic: Similar to gravity, but this is dipolar field. We can make mathematical corrections for this. Also, the driving rock property is magnetic susceptibility and not density. Different minerals have different magnetic susceptibilities.

- Electric: Basically we put two electrodes in the ground at some spacing and at some electrical potential difference. A current will move from one electrode to the other and we measure this current using another pair of electrodes in the middle. Then we calculate the resistivity of the rocks using Ohm's law. Another method is to use a large coil to induce a current in some deep rock formation which will then induce a current in our receiver coil. We can tell the conductivity (basically, resistivity) of the material that caused this secondary current. It's like a huge metal detector.

- Electromagnetic: We fire a low frequency electromagnetic wave into the earth which will partially reflect from any electrical impedance (high resistivity) surface and we record the back-propagating wave at the surface. It's almost like an echo-location technique. We can tell the location of things and their resistivity with this.

>continued

- Seismic: Basically the same as EM but with elastic waves reflection from acoustic impedance (stiff rocks) surfaces and we record the returning wave at the surface. This is almost exactly echo-location. With this we can tell the elastic properties of rocks and where they are located in the subsurface.

- Remote sensing: We take EM pictures of the ground with a plane. Quite simple. However we might use many many frequencies in our pictures, not just the visible EM frequencies. This allows us to characterize different minerals by their "color" (spectral signature), which we have a huge database of. So we can tell the chemistry of the ground simply based on a picture. We can also tell you the chemistry of vegetation and tell you if a portion of a forest is suffering from some disease. Vegetation can also absorb specific minerals from the ground their characteristics are imposed on the spectral signature allowing us to describe what materials the plant's roots are touching based on the plants overall "color". This is also useful in archaeology.

- Borehole geophysics: When you drill a well, we can lower many tools to measure various rock properties including: density, resistivity, magnetic susceptibility, sonic velocity, fluid properties, pressure, temperature, gravity, etc.

A good geophysicist understands all these methods and has a good background in geology as well.

>Questions?

>>3975337
I see. I wanted to detect organic compounds such as plastics in soil so I was looking for whether there is a way of detecting them using GPR or not. What do you think is it applicable or not?

>>3975382
I don't think you'll get anywhere near the resolution of spectroscopy, but there may be some value in this. It really depends on how much material are you talking about. Is it mixed with the rock grains or in the porosity (with the fluids)? What are the electrical properties of the plastic and do they differ significantly from the properties of the surrounding rocks (or soils)? What are the bulk properties of the mixture of soil and plastic?

If there is vegetation in the area you are studying, you could also do spectroscopy on the leaves to see if they show any alteration from the plastic intake. This could add to your interpretation.

This sounds like an interesting project to me, but you may want to contact a professor or researcher in this specific field for advise.

>>3975423
Do you know/can advise me any database of soil-ground properties please? Or source of multiple databases?