Seismic Reflection Interpretations: 1-5 Acquisition Basics Part 2

0:01 okay so now I'm going to talk about land acquisition for a few minutes so there's two common ways that we have been able to acquire land data so energy sources um vibrator which is more common nowadays and dynamite which is a little bit of an older let's say technology um so here we've got a comparison between records from a very early uh vibrator and dynamite shots and so both of these data sets it's kind of a fun study because both were required in the same area of Southeast Montana and so you could see on the left hand side that the Corel the vibrator records and then on the right hand side the Dynamite shot records and one thing you'll notice is that there's an improved signal to noise ratio in the vibrator records um and it's pretty

0:46 clear where you could see um let me turn on my pointer again um where you could see some slightly more coherent Reflections a b and c kind of down here and one thing to notice is that those are much harder to pick out in the dynamite acquired data of course if we're used to you're used to looking at 3D data or modern 2D data you know these would you know you kind of look at these and are like oh man that's going to be hard to interpret on uh there's a lot more uncertainty in these older records okay so going on to the next slide um every survey has its kind of distinct kind of quality zones that we often talk about that directly impact the interpretation so we often call this

1:33 what is The Sweet Spot that we're trying to image and so you may have your Target in that sweet spot um and then you have more of the The Fringe around the survey so when you shoot your seismic survey it's not saying okay well my reservoir is in these uh five square miles so that's where my seismic has to be shot but rather you have to build a bit of an area around that and so that allows you to have larger offset angles it also allows you to still maintain some of the the fold that you'll need for stacking the data to remove noise so always keep in mind that the seismic has to be a little bit larger than than your Reservoir itself so this is an example um of an

2:19 image that shows actual fold coverage from a land survey and so you could see how the fold builds up from the edges where it's much lighter so less fold um all the way into the center where it gets a little bit darker and you have a higher fold um areas of different fold will have different signal to noise ratios so of course that's something we have to keep in mind as we're interpreting our seismic data so those lower fold areas May mean that we have less statistical noise reduction in the stacking process and it can potentially make it harder to interpret the subtle features um these stripe patterns that you see in the The Fold acquisition could also cause what we we call acquisition artifacts in the seismic data itself and I'll show you examples

3:04 of that in later lectures um rather than geology and so these we often refer to like the striping pattern in 3D seismic as our acquisition footprint survey geometry also can vary a lot in land Acquisitions and this is depending on the terrain on the obstacles on the cost constraints even the equipment constraints and so there's a couple of different variations um one of these is cross spread shooting which I'm showing here which was first done back in the 1970s and then perimeter shooting is another method of shooting that involves the sources which are the squares in this image so you can see the the squares are the sources um and they are

3:50 kind of shot and placed around a regular or even an irregular area depending on your what you're trying to shoot um and typically the source and the receiver lines are coincident with each other and all the receivers on the perimeter are recorded for all of the shots and so this can make your 3D a little bit more flexible for undershooting obstacles like a house maybe or a road or you know anything that's that's built around that you have to shoot under um but you can't necessarily access that land itself and so um kind of the figure on the right hand side shows this idea and how it can work even for IR regular receiver areas and geometries where we're trying to shoot around something and so perimeter

4:36 shooting is one way to get more CMP image coverage uh with the least surface access and Source effort all right so now I'm going to talk about Marine acquisition for a few minutes and so this is a great image from liner's textbook I greatly recommend it if you want to learn more about these he covers it in multiple chapters rather than than five or 10 minutes like I am and so uh he has the this image is just an excellent overview of marine seismic acquisition and so you could see some of the key elements similar to the photographs I've been showing you of having a source vessel that's Towing air gun arrays and then also Towing long hydrophone streamers and so they can

5:23 cover the ocean um surface in many different ways uh one of the more common ways is to do it in kind of a racetrack pattern so going back and forth dragging those streamers and so the vessels will typically follow very carefully planned sail lines to ensure proper coverage of the target area and so these are just a few photos um of that seismic survey that I did in the Gulf of Mexico um from the ship that we used and you can see the streamers coming off of it so several of them are Towing streamers I think they were roughly six kilometers um with the geon's placed every 25 meters but it's also Towing the air gun and you could kind of see it um right here it's a little bit more of a wider

6:09 feature um one of the issues we had to deal with is that we already this was a 4D survey that we were requiring so we had to work around the existing platform um and that meant we did two phases of seismic shooting we did the outside area and then we had to come in with some smaller vessels um and a separate vessel with a separate source and then our vessel with the receivers so that we could work around the platform so here's some actual equipment from that same survey um that I did that's used in Marine acquisition um so we've got the air guns up on the air gun deck so some of the extra ones they had on hand um the streamer Deck with all

6:55 the extra streamers um this was the Western goco tasmin that I was on and so at the time this was their data QC Center and I took this picture of the live data when I was on the ship because I was just kind of impressed with how noisy it looks and how this is what it originally looks like before it's turned into the amazing images that we're able to interpret so one of the things I mentioned that we have to be careful of are the ocean currents and Marine acquisition so they're a major Challenge and so shown in this this illustration those currents can cause streamer Feathering where the streamers drift off from their intended kind of straight line path that we would do in our perfect world um and so this affects our

7:41 subsurface coverage and is another thing that we have to account for during during processing and if we can't account for all of it during processing we have to account for it during our interpretation ocean bottom cables are great if you can afford them so they're a wonderful alternative and these are you have the cables that are laid directly on the seafloor and they contain both hydrophones and geophones um and so this provides a lot better low frequency response because um you don't have to deal with the signal coming back up from the subsurface through the ocean column um it also allows us to record both the pressure and the shear wave something we can't do on the streamers um although I mentioned before the

8:26 deployment can be more complex timec consuming which means more expensive and then ocean bottom nodes are another seafloor Recording Technology so this is where individual nodes can be placed in areas typically where the cables may be impractical like around subse infrastructure and so they provide you know very good data quality but are are even more expensive to deploy and then also to retrieve after the the acquisition is done and so this is a nice comparison just showing the difference between two uh Marine acquisition methods so we've got the traditional streamer acquisition on the left and then ocean bottom nodes on the right hand side and so you can

9:12 see how the different acquisition methods affect the Imaging capability in the final product so notice how the ocean bottom method seems to provide some better illumination of the more complex structure over here kind of circled in yellow to highlight it um then we see in the streamer data and so this is another comparison of uh subsalt images um kind of at the Meine level I think these are subsalt images at the Meine level um in the case of the toad streamer we're on the top part and then our OBS we have uh data on the bottom part and one of the things you'll notice is that we've got higher

9:58 resolution with the obs's um we also had wide azth node Imaging with them compared to the more narrow ath toad streamer image so again depending on what you're trying to uh detect to image the level of confidence that you need to have with your interpretation using a more advanced collection method could be ideal and so the last thing kind of the next to last thing I want to mention with acquisition is that GE ometry really matters so depending on if you shoot your seismic survey uh parallel to faults that you may know about or perpendicular to The Faults um that can totally change the appearance of them in seismic data and so this is an example

10:46 of two different geometries of a seismic acquisition uh from the heart textbook showing that you end up illuminating those features differently in the subsurface um you know on in this case I would be much more confident picking the faults on the left hand side than I would on the right hand side and so acquisition footprint is something that I mentioned a few minutes ago and so those are those artifacts in our data that result from um our acquisition geometry that we chose and they are for the most part quite unavoidable and you'll see them very very commonly and so these patterns if you're not aware of them they can sometimes be mistaken for geologic features and so one of the first things I do when I open up a seismic uh cube in

11:34 in my interpretation software is I scroll through it and I look for that seismic acquisition footprint to know how bad it is and how much I need to deal with it in my interpretation and so that's one of the main reasons and main things we want to be aware of when we're trying to understand how the acquisition parameters affect our data interpretation and so just a few Concepts I'll just kind of go over really quick so hopefully you've noticed that survey geometry really affects um your your seismic um it matters if you have different aamal coverages so you could have narrow ath or wider azth um other things are your data artifacts your acquisition artifacts May mimic geologic structures and if you have

12:19 better Asom mual coverage you'll get better Imaging of the more complex structures and you always want to check the acquisition um if you have access to that acquisition and processing reports you can look at maps of where the data was collected how it was collected and that might help you understand better any unusual features that you come across uh when you're interpreting thanks for listening