GeoSci.xyz EOSC 350 (2017), 7 Oct 2016

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From YouTube: EOSC 350 Lecture 12: Seismic 4. Doug Oldenburg

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Seismic Reflection. October 7, 2016. Lectures slides are available at https://github.com/ubcgif/eosc350website/raw/master/assets/3_Seismology/Seismology.pdf

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Damn just a sec.

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Okay, so, first of all I wanna, thank everybody who came because at five minutes to there was just six of us here, we're deciding whether we should just stay long classical for six pack. Oh, how did I receive a merry-go-round.

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So what I'd like you to stay after spent almost all the time on a whole like different aspect, we're talking about seismic reflection, but right at the end, the last times we got just to fit rush and I just want to kind of briefly go over things. You can take a look in the gpg and get get a better idea of Facebook, just to kind of recap here when when the earth is sloping, then most doing it at a refracted ways.

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It comes down like this and comes along here, so always that refraction follows Snell's law with respect to the normal incidence of the boundary. So this is always the critical angle and then it propagates up yes and it can be recorded as a.

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Is something like that, so there's a direct arrival, and this is the you down.

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And then we introduced what was called a reverse shot. So basically, in this look in here we take so here's shot one and then here's the final receiver, oh and and now we take and we reverse it, and then we put a shot to here and then a receiver back over here. So you can see that the waves are travelling, exactly the same path and therefore must take exactly the same time. And that gives us a time point here, which is called the reciprocal time. And that's this guy up here.

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So you can get travel time, curves that go for the downslope travel time. Curves you go for the upslope and then you can get the velocity of the top layer. Just by looking at these slopes here they will be the same because they're just going to the top layer and then you've got two velocities that you've got here and unravel the information in those by looking at the intercepts.

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So there's an intercept time here and then there's another intersect time over here, so there's to intercept times that you get and you've got two velocities and you just kind of unravel those things to get the the two depths. One is this one beneath the dislocation and what is the depth between us? So that allows you to get some extra information.

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When you go out and if you're working for boulder or something like that, the jump trees are more complicated and so they'll use more complicated things to unravel what the how the boundary is shaped, but they're all kind of based on exactly the same principle. So you might hear things like plus/minus method or generalized reciprocal method, or something like that.

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But with the basics that you now have you, you kind of got more or less what's going on and everything else is just in sort of geeky, kids, so on next wasting when I try to finish this up, I just talked to the chief chief businesses that golder associates and he's been going around the province doing both seismic refraction seismic reflection and ma SW for various geotechnical purposes, LNG up in up in the north and / trooper, as well as some droves in British Columbia. So I'll show you some examples of refraction and reflection.

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Okay, I want to switch gears. I want to now due reflection, seismology from the point of view of what we study as up up to now. Nothing really changes we're going to have a shot and we're going to have receivers, but when we did refraction, we just looked at first arrivals. That was the only thing you cared about everything else after that was obscured, but now, in reflection, we're going to try to get out much more information, and this is why this is still powerful.

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So suppose that you had to think you had a geology that looked like this right. We got maybe some sand lenses here, we've got. You know some undulating bedrock, some loot liya, you know just complicated. Then the question is: how could you possibly get a picture of what is there? What we're going to do is to look at Wade information, seismic information that goes down and gets reflected back up. It will not come as a first arrival.

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It's going to be buried in the code of some place, but we're actually going to be able to work with this information and eventually get a picture that you just look at, and you can look at that picture and you're going to make some geological inferences right from the picture itself, so everything is going to be involved in how we take our seismic experiment. Take all of those traces process them give them around and make a picture with a particular kind of way. That goes all the way down and reflects all them back to us.

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So here's here's an example, as you said, a fairly large scale, so this is 5 kilometers. So this is no, maybe 30 kilometers. This way you've got lots of. This is sort of oceanographic. These are slumps that are coming down from ocean to ocean top yours or something you can see. There's lots of texture in here there's a few lines that have been put in so somebody has drawn somebody thinks they know it geologically.

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You know what's happening and they've looked at the underlying structure here and come in with you know where they think the oceanic crust is where they've got some to comments of whatever so a picture.

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Knowing something about the geology, you start to recognize, maybe events that are going in here and thinking. Okay, that's a particular interface. So maybe that's useful.

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This is slightly smaller scale. This is in meters, so this is like 400 meters from one side together, so yeah there was 30 kilometers, you do the seismic NH and you get. Something else looks like that. So you get just follow the dark lines and you can see some kind of continuity in here so something's coming up here and maybe there's even a bit of a break. That's coming up here again for a geologist who would look at this and know something about what the background is.

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You know he'd recognize like okay, maybe that's a particular interface. It actually does look like there's a fault. That's going in here, like they kind of think this event here should be the same as this guy. So maybe there's some kind of fault and you can see there's a little bit of a break a little bit of a break and interpreted those as smaller faults.

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So that's the idea. We somehow want to get pictures that look like this combine them with what we think we know about geology, that's especially even better if we happen to have a drill hole here. So now you have one drill hole, gives you some ground. Truth says ah this right here: I went from this rock to this rock. That coincides with that reflection event.

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That's true here, that's probably true up over here. So that's that's kind of way that you extend your information away from some kind of place where you've got ground truth, so the main uses for expiration the biggest one is actually hydrocarbon exploration. So by far the most of them, the greatest employment of TF physicists industry is through the oil industry and the main technique. There is reflection, seismic, but there's whole earth stuff.

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We can actually look at ways that go through the whole earth, maybe towards the end of the term I'm trying to get through all the materials so that we might have I don't miss a week or for electric at the end, where we could just not have any new material, but just take a look at all of the things that we learned and apply them to a whole bunch of different topics and what I'm that I really like to talk about is whole earth.

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Seismology and you know what happens when you have big earthquakes, something's kind of interesting and then we've got continental types of scales or we could have regional scale so that and then you can also do local scales where we got you get technical and environmental.

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The the basic thing here this is what most of this lecture is going to be on- is that what we want to do is to get an MH of reflection events, and so, let's suppose the finalists I'm a source right here. If I had a picture, ok of a reflection of that that occurred every time we get after your name is Riley, so, okay, so I it comes down.

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There's acoustic, impedance, contrast, read it Riley, so I get some kind of a reflection event that comes back to me and if I could plot that up, then I have trace and it would have a little bump right at that particular time. That coincides with the travel time for a wave to go from here tonight. Hey now, I come over here and I've got a slightly different situation, but my ideal size, the ground that I'm looking for, is something that comes comes down here.

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It's Robin in this case and reflects back to me and then maybe goes all the way back to kinda tak, a cup of tap go and relax back, so I have to now get two reflections right. So what's wrong and then I bought that up and the idea is that I could go along and get your all of these traces, the seismic traces like this, and then I put them up on a picture and I'd have now.

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You know something that yo kind of looks my true the sweeping sweeping around and it would actually have the structure.

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So all of those what I was describing was called kind of the ideal seismogram, and that is a seismic record or seismic trace that is associated purely with what is directly the knee the source.

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So we're going to work with that first, so we want to really kind of get the idea of what that ideal trade says and then we're going to contend with the fact that wait. That's not really what we get in practice, because I'm going to have waves coming from all over the place, and I need to do something but there's sort of two different metals.

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So I'm going to show you this slide first and then we're going to go to the app and kind of describe this a bit more in detail. So here's the overview just as I was describing before we've got an earth structure that might look like this, so I've got different layers of particular acoustic, impedance row and and B and I want to be thinking about waves that are kind of going straight up and down through these I'm, not going to worry about the fact that they might be scattering off at some angle, C.

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So the way in which we're going to think about that is just to kind of think about a plane, layered earth and what we call sort of normal incidence. So here's an earth. So this is depth and yeah. You can see we kind of try to hatch all these things, so these are different layers that are occurring in here. So each of these layers has got its own density and velocity and we can take that product and that's our acoustic piece.

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So if I thought so this is depth same meters and now, if I come down here, I know the velocity and the dance leg, so I can generate what the acoustic impedance of this layer is, and this and I can plot this down. Okay, so now I've got a plot of acoustic impedance with depth.

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You remember the very first lecture that we had.

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We had that the reflection coefficient was f2 minus said 1 /, zip, 2 plus 1, so if something was coming in in medium 1, this was medium too. If I had a wave with unit amplitude coming in then there'd be something that was reflected and that had an amplitude that was given by this. But it's so every time we have a change in the acoustic impedance.

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We actually get a reflection coefficient. So I could look down at this at this log here and there's. There's no change! No change! No change suddenly day that there's a change in the acoustic impedance and it goes down so z2 is going to be less than zed one because going from a high impedance to a bowl. So that means that this is going to be negative and then I go to plot a little bar there. That just tells me you know what the size of that reflection coefficient is.

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So it says it's negative and there should be scale on here. Maybe it's like point 25 or something, and then nothing happens again until you get down to this depth and then you've got now a positive reflection, negative positive, negative, positive right.

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So that's just a series of reflection coefficients in depth.

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When we measure our seismograms, our seismograms are always you know, the variable is always time. No, so nothing comes in and something comes in something else you know this is time. This is travel time. So we've got to convert these guys. This reflection coefficient log into something that instead of having an axis of depth as an axis of time, but we can easily do that because we know what the velocity is in each of these in each of these will go in the first book.

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The velocity was being one so there's going to be reflection at time, t is equal to whatever this thickness is so there'd be aged over the one.

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So that would be the time that I'd get my first reflection, except that when we deal with seismic waves, we're talking about stuff going down and have so that means is always there's an extra factor too, because I don't get sucked down there, but it all says back- and so this time here that you you see is usually an awful, sometimes they'll say that, but it's usually like to wait travel time. It's the time for the think. You'll.

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Actually get back you so stop okay, but the basic conversion or death to time is that for each unit up here where we've got a constant velocity and it's at a particular death, we can convert to time. So for these guys here, the extra time is, you know whatever, whatever that velocity was so we can take whatever the velocity was 44 here, compute that to a time increment, and that determines where this guy is as a function of time.

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Well, sorry, let's go here. Okay, so we've got now this weekend to call and we try to be consistent. We talk, we call this the reflectivity function, so this is now something that depends on time and you can see how these guys there they're matched up in terms of their polarity of the sign but they're their differences here and when they're actually occurring, because not all the velocities for each of the layers is same and so time gets crunched up.

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Okay, so we've got geologic log, gusta, campinas, reflectivity coefficients these are in depth, reflectivity function, which is now in time. So this would be the ideal seismogram if the seismic wavelet that was going down was like a pure delta function. It goes if it was a pure spike, but in fact it isn't the seismic wavelet that goes down that leaves leaves the instrument. You know might look something like this, so it's got a particular time, for it's got a particular width or you can think a lot of having a particular several frequency.

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If that makes much sense, but it's it's got some shape to it, and what that means is that when we take that reflectivity function, which was maybe something that looked like this so now, this is a function of contractor and maybe the reflectivity functions like yes.

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If every time we see the arrival here of some impulse, that means that this whole wavelet gets put on here. So it looks like so. What these impulses do is to tell you, okay, this is when the wavelet arrives. What's the way we're going to look like well, it looks like that. Sometimes the way, but might look like this- can have all kinds of kinds of shapes, but there's two things. One is the arrival time and then there's the shape.

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The proper mathematical terminology for this is that this final seismogram that I get here, sometimes it's called it trades, it's called size. Brown is really the convolution of this reflectivity function, with whatever the seismic wave lipids. So all that means is that every time I see one of these guys I've got to replace it by that particular impulse.

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Okay, so that now gives you what are ideal size, dependence, okay. So now, let's go through the app kind of do all these things again.

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So, uh let's just start from you, so here's.

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Aha, thank you. You can tell how precariously these lectures are planned. 11, small misfit, with the finger in your post, okay, uh here's what you'll see on the app we're going to define beans, reflection, coefficients transmission coefficients and the thing that I haven't ever talked very much about, but is important and perhaps kind of intuitively obvious. So we've got a way that comes down Scott you and amplitude, then part of it gets reflected and it is convenient and actually important to recognize that which way the reflection has happened and from which interfaces.

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So there we sometimes write these as two units like this is. This is an r1 2, which means it's a reflection coefficient for a wave that goes from unit one to you, too, that's important when we did this I.

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Wrote down this reflection coefficient, it is said to minus n 1 over Z 2 plus said what these numbers here don't have anything to do with the layer number. It could be a unit 30 or something like that. What these numbers have to do is with unit 1 is the unit in which the wave is coming in and unit 2 is the unit number that it's going into, so it's reflecting so r 12 is one that's reflecting at one and interface between unit one in unit 2, if I had something that was more complicated.

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So this is unit one two, three four so now I've got let's say b4 here v3, if I'm looking at the reflection coefficient from here yo now when I use this formula, I think ok, the the incident unit, one that's really this guy here and then here is the one that's traveling into so I would use your really be impedance from said from unifor to be here and unit three, so the one we're generically writing this stuff here.

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These numbers here have to do with you, know the incident and the other one to which is information traveling. Not particularly you, ok. So that's a little bit more generalized in here where we say our. I J plus 1 earth.

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Okay, see whether it's an eye or a jay, a quick.

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Anyway, I plus 1 minus said: I tells you what that inner? What that reflection coefficient is at that interface and then here's also the transmission coefficient. So if we now go down.

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Something like that, if you're actually trying to figure out what the amplitude is of the wave coming down, so we've got a weight coming down, but now it's transmitted so there's a transmission coefficient between the goes across here. So it loses a bit of altitude. So maybe the transmission coefficient only like 49 and then so. If that was point 9 and then the reflection coefficient here was point 5 and then there's another transmission coefficient here of maybe point 9.

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You can see that the total amplitude here is being reduced, not because of the reflection coefficients, but also because of the transmission.

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Okay, so now we've got this situation here: layer, one layer, 200. What I'd like to do is to take this situation, we're going to alter the layer, thicknesses, alter the layer, velocities and densities and get our reflection coefficients and reflectivity function.

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So this is this is what your dashboard looks like so d. 2 is the.

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So we go 1, u 2 g-3, and so, if d 2, if this is equal to 50 meters ah right now, d 3 is a hundred meters. So this is another 50 meters fit here and then I've got three velocities right now. These are all essentially the same host of 1500 and the densities are all the same: 2300 children, ER, and so when I look at this. This is the density as a function of depth and that's pretty constant and then the velocity as a function of depth is essentially constant.

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So my impedance is a constant, so I don't have any reflection coefficient over if I took. Let's say this second layer so I take this guy here and alter his velocity.

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So now, let's, let's make a big.

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So now what happens at 15 years, the velocity increases to thirty seven hundred meters per second squared or per second, and my velocity structure looks like this. The product of the density and velocity looks like the fist of my reflection coefficients. Look like this. So I kept positive and a negative okay, so you can take that and you can play around with it.

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You can do all kinds of eight minutes, it's the product of density and velocity, so you can make the density go up and the velocity go down and not end up with any reflection. So could do that.

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I'll still have a positive increase in velocity, but I I could decrease the density, don't have very much option but I'll try.

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Well, I decrease the density as much as I could, and you can see I've made that reflection coefficient really pretty small. If we altered numbers moving, bigger I could easily still find combinations of role and be that give me the same number so I can even have velocity is changing, but still not necessarily have a reflection coefficient generally. That doesn't happen. Usually of these things are more heavily just, but the what's happening with velocity.

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Ok. So now we've got.

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Something that looks like that now we want to do is to convert this system here, which was in def going to convert that to time. So that means for each of these layers. We've got to know what the velocity is, and then we know what the layer thickness is. So we could compute what the travel time was to any point inside the earth.

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So- and you can see so- here's our reflection coefficients in depth- here's our travel time, so this is now how to convert to a travel time to to death. So every particular depth we can actually calculate with Tula travel time is, and then that allows us to convert from depth to time. So we can see the first positive reflection. Coefficient is actually going to be about point O, six and then the negative reflection coefficient is point one.

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Ok, so we're almost there. The last part is then to take that and we're going to put a wave lid on it. So in this case here the wavelet looks like this and like a Gaussian wavelet and we've got a couple of things that you can you can play with. One is the frequency of the wavelet, the dominant, so let's suppose I've got I've got a wavelet that they didn't look something like that.

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Ok, so if you were going to look at this thing- and you see uh well there's no- this has got some width to it in time. So it's got some subtotal time. Duration.

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If I take the reciprocal of that.

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So 1 over time is going to be frequency, so this is going to give me kind of like a a dominant frequency of this, this waiver. So if this wavelet was point 01 seconds so 10 milliseconds, ah then that would say someone over zero point of one is about a hundred first.

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So if you had a sinusoid of a hundred Hertz right, so if you just had a sinusoid that was going along here- and it was, you know, it was a hundred Hertz. That means you get a hundred cycles per second. So that means that each cycle is about one one-hundredth of a second, which is final one. Second, okay, so that's the relationship here, frequency kind of related to the time width of this wavelet and then that's going to go on to so now. Let us.

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So now I take these reflection.

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Reflectivity function could ball it with this wavelet and now I'm going to get a seismic trace. That looks like this.

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So that's not great. So this is the information. Then that's telling me I've got two layers and the time difference between these guys I know what the velocity is is going to tell me what the thickness of the layers.

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So, if you look at that you're gonna, say wow, you know that's not going to help me that much especially if you thought about something that was like had a lot more layers and a lot more complicated I mean you know he just gets squiggles all over the place, and it's going to be. You could imagine this it's going to be hard to interpret, but there was one thing- and this is you know sometimes simple things- make a huge difference. At four years.

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People would plot these seismic records, as just these they just plot these things is, you know, here's my squiggle trace right and then you get a whole bunch of them right and he get squiggles that look like that and you're. Looking at this stuff and can't make any sense, somebody came along in a frittata Caillou's to notice me. It was in the early 50s and he said you know you know what we should do. That would really help a lot is if we should shade the top hat everything that's positive. We should shave lat.

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So now, if you did shave good flag for Shane, it is black, which is what you're seeing here and the thing that that does and you've already seen a little bit of that is it. It suddenly now draws your eye and you can see patterns in these. What used to be just a bunch of squiggles to this day it, despite the fact that they do now 3d data acquisition and vibration, a huge amounts of processing, probably the greatest batsman in understanding, sighs, great wavelets was Hector, sighs, citizen well, I!

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Go why we shake the tough parts in right, uh yeah.

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So anyway, what is going to happen here is is not that we're going to get information from a single trace the mind can't do that, but the mind is very good at capturing images and making sense of images, and that is kind of where all this is going.

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So we've done the ideal, so good, okay, so we're going to hang. Hang our hats up there, just just just for a moment, so we've now got the ideal seismogram. The next thing is to to try to get this, and maybe I should just escape this for a second and.

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Maybe let's look at this guy.

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Okay, what you're looking at here is called a shot gather. These are the signals that you would have observed on all of these geophones due to a single shot.

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The sean is here and you're going to see this again in the in your team based learning in your case history. This is this is actually a really interesting example about the different kinds of ways: p waves s ways, it's an example where you're looking for the water table you're looking for stuff, that's at the base of the table, just just lots of interesting that's going on, but here is a shot gather, and this is what your data are going to.

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Look like, cuz everytime you acquire data, it's always going to be like okay, I've got a shot and here's my receivers and I'm grinding them along and then I'm going to plot each of these traces down. It looks like this, so this is time and I've got in this case about 90 different to geophones with praises.

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That's your information now what we were doing before in our roof? Well, let's just hope. First of all, let's just look at this thing because there's a lot of stuff, that's that that's going on here! First of all, there's if you're sitting out here your first of all notice that there's direct arrivals. So if you're sitting up here, there's a straight line, it comes down through here. So that's when the arrival comes first, so that is a direct arrival and that's the guy, that's very, very long! Sarongs and you see its other line.

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That's coming in here! That's the first arrival, not a straight line, so that is going to be.

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30 refraction, thank you. So that's a refraction event, so so far in what we were doing before, that's the only things that we looked at right, but look at all this other stuff, that's happening.

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There's information there, some of its noise, some of its noise, sometimes and signal other times yeah we've got first of all this there's a straight line. There's an event! That's coming in here. You can see this. If you look at this line here and you look at the velocity is associative. First of all, is this velocity going to be higher or lower than either those velocities.

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It's lower right, so it's got a steeper slope, but the slope is 1 over V, so I slow and if you actually figured that out. If you looked at that slope- and you know measured the time just since you find like oh that's 340 meters per second, which would tell you immediately that it is.

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Nathan where's me there yeah um yeah retro. That way, which is the direct arrival from where this would all right. What sorry, but.

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Either way: good, yes, 340 meters per second, it's the speed of sound in air. So when this explosion goes off, okay, there's actually also a wave that travels in the air as a pressure wave gets, the geophones makes the vibrate, but it doesn't come in as first Robin comes in later.

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We're actually going to see this thing you're in more detail when we come to do the ground-penetrating radar, because then there's also going to be an air waves that come through, that it's going to be real dominance can be poor, okay, so this guy's noise, somehow one way or another. We want to get rid of him and there's techniques, processing techniques, especially that allow you to get rid of energy.

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That's coming in at a particular angle, so it's called FK filtering for everybody, but certain like 48 frequency domain stuff- and you could put this in and you could probably get rid of get rid of that.

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Then we got this stuff in here. Is his listeners ground roll from the point of view of seismic reflection, this stuff you just want to get rid of it is. It is not doing you any good, because it's not a reflective event, it's not something! That's bouncing off you and come or bouncing off something and coming back, it's rolling along the surface here.

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Well, the seismologist who's doing reflection, says: okay, I got if you're rid of that. So maybe you could use the same kind of techniques because it kind of linear right, you sort of see house right there.

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You could maybe get rid of those and then you're left with all of this other stuff right I mean you, take a look at this stuff and it'd be pretty hard to make much sense, though another thing, but we're going to do that, we're going to take all these other things, but we're going to need a whole bunch of these shots, we're all going to have one, but we're going to have a whole ton of these guys and then we're going to extract particular records from each of these shot gathers put them together, do something with them, and lo and behold, you're going to get some nice reflection of its and that's going to be what's useful.

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The other thing is this: guy I just said was really you know it's noise, it's it's contaminating stuff for the seismic reflection, but when we do nasw here's our signal, this is the guy that we're going to try to work with we're going to try to look at that information and we're going to find that, as that goes around, as we go from, you know these near near offsets to farther offsets that the waves in your change just a bit and they're going to change in concert with the shear wave velocity which we can are going to use for rigidity right.

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That's engineers are interested in it hey, so that's 0 where we're going.

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Uh-Hmm, okay, so I think I'll, just I'll just conclude one more thing out I. Maybe you could read the gpg uh just to do the following thing: yes, here's what I want to plan.

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The ideas as follows: if I have a shot and if I have receivers so now remember the reflection event is going to come. You know midway between each shot and receiver, so I'm going to have particular points that just look down in here right.

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I'm going to have particular points in the subsurface that are reflections. If I then take different combinations of sources and geophones. You can imagine the situation like this, where I've got a shot and a receiver and I've got another shot and a receiver and I've got another shot and receiver. They could all be kind of reflecting from that same midpoint.

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So what we're actually going to do is to try to take these plots here, which are common, shot gathers and we're going to extract particular traces from each of them, so that we have a whole sequence of seismic traces that there are common point gathers so that they correspond to a location, that's halfway between a shot and a receiver, and then we're going to have a whole bunch of traces that have reflections from essentially the same point.

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And then we can go ahead and work with them, stack them up and then pretty offer pictured your kind of get rid of the most. So that's what we're going to do, but if you could read the gpg because oh monday is a holiday, so then I only have wednesday to try to finish finish this stuff up.

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So I want to try to do the reflection, seismic and a little bit of a mess w and then.

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Yeah, in order for us to go- and I forgot to mention the just sec, our itinerary, which is so we revamp things a bit or schedule.

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Yeah, so we were supposed to finish that reflection day. I didn't quite do it so Monday's thanksgiving wednesday, yeah, we'll finish up reflection, seismic as well as try to do a little bit of mas w enough to kind of get you going and then we'll have on friday, that'll be the seismic team based learning and then good news for you guys. I will not be here the following week, so we're going to have you're going to get somebody who's really good at GPR. It's going to be talking to you, as so.

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He'll have two lectures on GPR and then we're going to have a seismic Wiz and.

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The labs so so, there's no monday lab, obviously for thanksgiving the wednesday people will keep their schedule. So you'll still do your wins left the Monday people who didn't you know we're eating turkey, yet the next Monday to do their lab and there'll be no lab for the Wednesday group. So there's we're missing a little bit. The reason that I want to do that is I want a front and load. The labs are really important part of learning the course.

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So everything is integrated here and so the earlier we can get you to do the labs it integrates with the course you know it just makes it easier. So we're trying to do that as quickly as possible and then the add that the GPR is going to take us through till the end of October will have a day for a midterm review and then the midterm will schedule for November. Second good, all right have a good weekend.