Pervasive post-Eocene faulting and folding in unconsolidated sediments of the Mississippi River, Central U.S. as imaged by high-resolution CHIRP seismic data
Xenia Fave*, M. Beatrice Magnani**, Brian Waldron***, Kirk McIntosh****, Guo Lei**, Steffen Saustrup****
*Florida Institute of Technology, Melbourne, FL
**Center for Earthquake Research and Information, University of Memphis, Memphis, TN
***Ground Water Institute, University of Memphis, Memphis, TN
**** University of Texas at Austin Institute for Geophysics, Austin, TX
Despite being located in the stable continental interior of the North American plate, in 1811-1812 the New Madrid Seismic Zone (NMSZ) experienced among the largest magnitude historical earthquakes that ever occurred in the U.S. Paleoseismological evidence shows that large earthquakes have been occurring every 500 yr in the region for the past few thousand years, and historical and instrumental seismicity demonstrate that the NMSZ fault system is actively deforming today. By contrast, motion rates emerging from almost twenty years of geodetic observations substantiate a very slow rate of deformation across the NMSZ faults, suggesting that present velocities are not representative of the long-term deformation rate of the NMSZ fault system, and that deformation has likely been accommodated along structures additional to the NMSZ.
In the summer of 2010, a high-resolution marine seismic reflection survey was carried out along the Mississippi River as part of a multi-year cooperative effort to investigate the spatial and temporal distribution of deformation in the Mississippi Embayment. Coincident to the seismic reflection profile, the survey also acquired ~300 km of CHIRP (Edgetech SB-512i) data from Cape Girardeau, MO to Caruthersville, MO. The CHIRP used a 0.7-1.2 kHz source pulse and recorded to a depth of 5-50 m sub-bottom. Here we present the preliminary interpretation of part of the CHIRP profile along the Mississippi River north of Hickman, KY, where the survey imaged a highly reflective sedimentary package down to a depth of ~50 m. The sedimentary sequence is about 20 m thick and appears to be bounded at the top and at the bottom by angular unconformities. The package is mildly folded and pervasively faulted, in some cases by extensional faults that exhibit up to 2 m of displacement and that reach the riverbed.
Based on exposure of Eocene deposits 7 km to the east of the study area, and on the correlation of electric and gamma logs of nearby oil, gas and water wells, projected from 12 km to the west and which penetrated the Eocene units at a depth of 67 m, we determined that the reflective package corresponds to one of the elements of the Jackson Fm, (i.e. the top of the Eocene and of the Tertiary sequences), sealed at the top by the basal unconformity of the Mississippi River Quaternary alluvium and at the bottom by the Claiborne Group deposits.
Beatrice and I were talking during my goodbye dinner and she said she thought the project description she wrote was maybe overly ambitious. I personally feel like I could have accomplished a lot more with an extra two days or so of training picking horizons and faults before Beatrice left on vacation. That would have made my work during the two weeks she was gone more effective. This would have meant that upon her return we wouldn't have had to spend so much time repicking and after some preliminary corrections and looking at the surfaces could have moved on to coorelating faults. However, I don't think there's any way we could have loaded the data any faster than we did, especially because of the different technical difficulties we encountered. I do wish I had gotten further but realistically I think we did things as fast as we could and handled the problems that arose in as timely a manner as possible.
I think what I did achieve was a result of being honestly interested in the results. I was excited to pick horizons because I really wanted to see how the faults were orientated and I was looking forward to beginning to coorelate those faults and then map the surfaces. When we did finally map the first surface it was really exciting to see what I had been picking in two dimensions become this three dimensional structure. I'm looking forward to finishing up the other four layers and drawing conclusions. I think part of my fascination was the variety in what I was learning. On the boat I learned about the equipment, the process of data acquisition, observed how serious and minor problems were solved, and at the same time overcame a lot of my shyness and felt like I really bonded with the group. Then when we returned to Memphis, I learned so much about faults and stratigraphy from Beatrice as we got excited about different formations in the data and tried to figure out what was going on. Learning about faults in an immediate application with true-life examples dirctly in front of me strongly affected my grasp of the possibilities. I also of course learned about importing data, became very familiar with ProMAX and SeisWork 2D. I had fun experimenting with the different settings and testing out some of the functions. Then working with Brian and Beatrice to determine what we were looking at gave me a chance to learn about well logs, electrical well logs, and how that type of characterization is done.
I think being on my own so much in an entirely unknown and “slightly” dangerous city also really shaped my summer in an entirely different way.
All in all I had an excellent time this summer. I think my experience was very reflective of what grad school will be like what with the group of students, the long work days, and the independence in decision making. I am still planning on going to grad school for sure, but although I had an excellent and nothing short of fascinating time working with reflection seismology I am still considering some other geophysics disciplines. One of the things that was very eye opening this summer was the several discussin that were had about proposal writing and finding grants. This was something I hadn't really heard about in detail at any point so it was great seeing some proposal research and writing in action.
I wrote th following post while sitting in Memphis airport Friday afternoon, but finally got my internet up and running last night in my house...Reflections Post still to come!
Been a little longer than usual between posts. The past three weeks I've been picking horizons and faults along my stretch of the Mississippi. My days were spent trying to find a reasonable interpretation of the data. It was tough going without feedback for two weeks, and I definitely worried that I was making bad choices on some of the lines and that a lot of my work was going to have to be corrected before we could move forward when Beatrice returned. At the same time, I met with Brian (one of the other P.I.'s on the boat) a couple times where we tried to determine whether I really was looking at the Eocene.
The first time we met we plotted my lines in GIS, so we could search for nearby wells. There were a lot of shallow, coal mining wells right next to my site. We went and looked at the paper well logs and did a rough estimate of where my formation was in comparison to mean sea level . This allowed us to get an idea for what type of sediment was present at each site at the elevation that we expected to find my formation. The data wasn't entirely conclusive but we were near or in clay for each case so at this point we decided I was indeed looking at Eocene. Then Brian got electrical well logs that were deeper. In these he traced a marker bed of the Memphis sand which is a very thick layer of about 400 feet through all three logs. Above this he could see the Cook Mountain and Cockfield which make up the Eocene/Claiborne. The problem was that both of these layers were very clearly present at a much deeper elevation then where I was seeing my formation. As a result we thought that I was actually looking at Quaternary! Which would have been extremely exciting because that would mean all the faulting I have observed is A LOT more recent than we initially thought.
Beatrice, Brian, and I met and took another look at some of the well logs. Looking at them indepth and using our basic trigonometry skills we ended up determining that I was actually still looking at Eocene, so the excitement there died down but this interpretation makes a lot more sense.
Then Beatrice and I met and we looked at some of my lines and began correcting them. I think I've learned more about picking horizons in the past two days then I did in the two weeks that I was working on it by myself. I managed to get all of the top Eoene done during our second day working together. Then we used that horizon to interpolate to a 3D surface of the area. Which unfortunately I don't have a screen shot of but it looked really cool! After that I finished picking the second layer and started picking the unconformity that marks the base of the Alluvial which sits on top of the Eocene. We believe the Eocene was eroded away in many places so what I am looking at is actually the very bottom of what a long long time ago was a thick layer. When I finish picking the unconformity we can map it in 3D and compare it to what we see for the Eocene. If there are some trends present in both or even better yet if a fault is present in both then that would mean these faults were active in the past 170,000 years!
Unfortunately, I actually left CERI today... but Beatrice and I have discussed it and I am going to try and keep working on the horizons from home. Hopefully it won't be too slow to run landmark on my computer. I'm keeping my fingers crossed on that account. Then in October I am almost definitely going to head back up to Memphis and finish everything up!
In the meantime I've also got to start writing my abstract which we're going to collaborate on remotely. Additionally, I downloaded ArcGIS Explorer, onto my laptop. This will allow me to look at and help interpret any new data Brian can find on my slightly mysterious geological time layer.
I'm really glad I've got the chance to come back because there is a lot more I would like to do with this project before calling it a wrap. Its been an absolutely fantastic summer but I'll put those types of reflections in my next post.
So last Thursday everything was looking pretty dismal. Seisworks has two display views. One is mapview which shows all the lines you upload as if you were looking at them from above. In my case this makes a pretty rough looking grid. The other is Seismic View which displays your data and allows you to pick your horizons. My lines were all showing up in mapview but were coming up as big white rectangles in Seismic View. A bit of a problem. I came in and had a stroke of genius or maybe just a sudden urge to try anything new to fix the problem. When we upload the data we have to select a value for the trace number and for the shot number I was using traceno and source respectively. Switching these two fixed everything. It doesn't make any sense but it was a hallejulah moment! I don't know why it plays a role especially since in my case these two values are identical but I was so happy it was working I didnt question it.
The same day Beatrice and I realized that I had a problem with my mapview. All of my data was being displayed as staircases instead of smooth lines. This creates a problem when you're trying to pick a horizon because it lets a line cross another line twice which means the program thinks that place is in two places at once which doesnt make any sense. We originally thought it might be because we had decimated the data, taking only every 10th ping. So Monday I removed all the decimations but still had staircases. I began wondering if it was a problem with precision and so I increased the significant digits in the latitude and longitude files. Then spent another couple hours of help editing the GMT script that converts the latitude and longitude to UTM coordinates until it also displayed another digit. Sure enough much smoother, and for lack of a better word, prettier lines.
Since then I've been going through each line and picking horizons and making sure they match up at every intersection. Its what Beatrice called "an exercise in patience". I couldn't agree more. Thank goodness for pandora, Itunes, grooveshark, and lunch breaks.
Now my biggest problems are ensuring all my horizons line up. As seen in the pictue below this is not always as easy as it sounds...
Its a little bit hard to pick out the yellow. But the left screen shows one East-West line and the right screen shows an intersecting North-South line. On each the yellow horizontal line segment is where i placed the top of the eocene before comparing. then on the left screen seisworks plots my northsouth pick as a vertical line so i can see if it matches up with the eastwest pick. and vice versa on the right screen. clearly there are some huge differences and its hard to assume which one is more correct. Headbanging and snack breaks result.
Everything works! I fixed Seisworks this morning and everythings coming together. More details to come I just didn't want you all to worry over the weekend.
After managing to upload all of my profiles to Landmark and getting ready to start interpreting them with seisworks...Seisworks decided to have issues and not display most of our profiles. The exception being the first east west line and the very first chirp line we ever did. But neither guo lei's MCS data, Onur's MCS data from the Caribbean, or the other chirp lines would display. So working with what we've got I am now looking at the displays on Promax again, writing down where the Eocene begins and ends and then print out those sections of profile. Where I will write on paper where I see faults and pick my layers. Additionally I will print out a map with GMT of the grid and label on it the eocene and exceptional faulting.
Then hopefully in the meantime the Seisworks problem will get fixed and if not, there's a chance I'll be coming back up to Memphis sometime this upcoming semester to get the computer analyzing going.Which would probably be the most work intensive couple of days of my life so far, but isn't that what science is all about? Computer bugs, looming deadlines, and insane, manic nights that lead to the most amazing results.
I am going to make this work.
Having completed my preliminary look through the data, I proceeded to make navigation files for each line. I used a workflow in Promax that read in the SEGY data, output it in a Landmark Format, Selected every 50th Ping in each file from which to create a database, that we then converted to a spreadsheet. So our final result was a spreadsheet with the shot number, trace number, Easting, and Northing. When I finished creating the spreadsheet, Friday afternoon, I wanted to try and map the grid lines we had done with GIS so we could get a feel for where in the river this had taken place, etc. I didn't have any issues getting the data into GIS but everytime I projected it, GIS placed it very south of America let alone the Mississippi. I tried several projections but with the same result and figured there had to be something wrong with our coordinates. I compared my nav files with those of Guo Leis and found a huge difference. Beatrice and I talked about it Monday morning and discovered that what we thought were Easting and Northing were actually arcseconds of Latitude and Longitude.
Fortuitiously, she had a scrip on hand that would convert from Lat/Long to UTM coordinates and so with a few minor modifications that problem was fixed. I am now uploading these new navigation files and pairing them with their corresponding segy files in Openworks in order to get one step closer to interpretation. Every time I upload a line it plots it on a map that already has the lines from their voyage two years ago so you can get a feel for everywhere we've been. Additionally this allows you to compare CHIRP data for one area with the local MCS data or data from the previous trip.
In other news, my Mom's coming this weekend and we're headed to Graceland! Elvis' 75th birthday is this year so its a great time to be going. We're also headed to the Memphis Zoo! And of course a Riverboat Cruise! Then there's the hodgepodge of restaurants, stores, and exploring downtown to look forward to with her.
So since arriving back in Memphis I have been working on two things.
The Cruise Report: This is basically done. Guo Lei and I split up the writing and he did the Cruise Summary, description of the MCS acquisition, and the description of the Chirp acquisition. I wrote up all the intro stuff (cover page, table of contents, etc), the Research Vessels section, the Navigation and Track lines section, and then edited and formatted both our halves to make them read and look uniform as well as reselecting pictures because even though we have about 1000 to choose from we chose four of the same pictures in our different sections. We still need to do a couple figures with GMT or GIS and then I'll go through and renumber all the figures and make any changes and edits that Beatrice suggests.
Chirp Data: Beatrice uploaded the Chirp data on Tuesday so since then I have been using Promax to go through each line (there's 68 in total) and diligently scanning and searching for any faulting, folding, or other regions of interest we may have missed. Some of the lines come up great and you can really see a beautiful fault and others keep you guessing.
This is what my workstation looks like:
We completed our river acquisition on July 1st. Then on Friday we finished packing up everything we'd put on the ship and putting it back in the van for Kirk and Steffen to return to the University of Texas. I really enjoyed the time I spent on the boat. I learned a ton about acquiring and processing marine data. As well as handling emergencies with equipment. But I am also very excited to begin the interpretation and analysis of the data we collected.
Beatrice and I met today to discuss my project for the summer. My first two weeks are going to be spent bringing in all the seg-y data we have from the Chirp into Landmark. While I'm bringing it all in I am going to be looking through all of the Chirp data for interesting features. This includes the region I previously mentioned where the Claiborne was covered in faults. I will be determining the strike and dip of this region and possibly even the stresses that would be necessary to produce the numerous faults we saw. There were a couple other places we noticed as we moonwalked down the Mississippi where the Claiborne also was very visible so I'll certainly be taking a look at those. Then there were sections of the river when no one was watching the Chirp so we have no idea if anything was there or not so it will be my responsibility to play the whole trip back and perform and in-depth search for anything and everything of consequence.
Once all the data has been brought into Landmark and I've scanned through it, I am going to begin creating a 3D profile of the Claiborne section as well as interpreting other areas of note. Meanwhile Guolei will be bringing in and interpreting the streamer data we collected. So while I will be looking intensely at shallower formations he will be looking at every layer including the cretaceous, paleozoic, etc. Interpretation should take another two weeks and then we're going to take a real look at everything we've got and begin the analysis and I'll hopefully start writing my abstract and planning my poster for AGU with the direction we're choosing to take.
Alongside all of this, Guolei and I will be working together to write the official trip report for this project. This will be a daily log of the events that occured along with a detailed explanation of exactly how we did everything. We already began on the boat by taking pictures of the setup we had for the streamer, airgun, and chirp as well as measuring exactly where everything was in relation to the barge and the Strong. Guolei made detailed diagrams with the dimensions and objects which will be included in the report.
So we were riding along on our merry way down the Mississippi when fate handed us a bad card. The streamer got caught on something underwater. We got the boat to stop in time so that we didn't run over it but still created enough stress on it that the streamer snapped. Then when the Tiger Shark started reeling it in from the buoy end of things...the rope also snapped. This left out poor streamer caught underwater. We tried searching for it with a boat hook and then a grappling hook that the chief engineer quickly constructed but to no avail. The next day they sent another small boat with sonar mapping to go look for it and the good news is that its still caught at the same place and hasn't started drifting down to Louisiana, the bad news is they still weren't able to get it up. Luckily we had a spare streamer which is a bit older on board so we're still collecting data.
Interestingly enough when we compared the last couple hundred meters of data we collected with the first streamer to the first few hundred we collected with the second streamer, the second streamer gave us much better results! The streamers are identical in theory and the area we covered was the same so I would like to believe that it was my amazing processing skills that made the difference since I only did the second line, but its rather unlikely.
Today I also had the opportunity to take Brian (our hydrologist and my co-buoy-watcher) through all the steps that we do to process the data beginning with creating a .nav file from our gps and geode information all the way through an fk migration. It was great because by teaching it I discovered a lot of small details I had never questioned before and Brian asked great questions so Steffen came over to help us really understand a lot of the different steps. As a result even though I thought I had a great grasp of what was going on, I now have an even better understanding of why we do so many steps. We could even stop it between each new module so that we could see the difference between the before and after.
We did have a small hiccup today with the airgun when it stopped shooting properly. We brought it on deck and indoors and Steffen began taking it completely apart while all of us watched. He replaced a couple o-rings, cleaned, and lubricated everything and we were set to go. It started working just fine after that. So although this gave us a small time delay (about an hour) I was glad that I got to see what the inside of the airgun looks like and how it all works together.
So we found a new fault! More like a set of faults really. We were drifting backwards down the Mississippi when it showed up on our CHIRP data. Its unbelievably clear. Also it appears in the Claiborne layers which is where the aquifer is that provides water to Memphis and the rest of the region. So this is really a big find. The CHIRP is a single-channel device with both the sender and receiver directly on it. It emits a noise that sounds sort of like a chirp which is of course where it receives its name. It gets very high resolution data on the first 10-30m but becomes very noisy after wards. Because of how interesting and important this find was we decided to spend the next day recrossing the area with just the CHIRP. So on Friday we made five North-South and five East-West passes over these faults. From the data we can tell it is dipping towards the North and East. Beatrice and I have discussed it and she thinks it would be a great region for me to focus on this summer. So when we return to Memphis I am going to have to figure out how to put CHIRP data into the landmark software and produce a 3D map of the surface. More details forthcoming. In other news I finally moved a lot of my pictures from my phone to the computer and have begun posting them on facebook along with a few good ones here.
The Tiger Shark:
The Barge with the streamer and airgun being pulled behind it:
So It's been almost a week on the river. Basically my days begin at 530 when i wake up to get ready for breakfast at 6 to leave the hotel at 630. Then we drive out to a port, board the Tiger Shark (a small boat that helps us out all day, pictures to follow), and enjoy the windy ride to the Strong. Upon arrival the Strong will drive to wherever we're beginning acquisition and I either show off my huge muscles by helping put the streamer and airgun in the water or go help in the lab turning on and getting ready the five computers we use to collect data. After everything is set up I either spend most of the day on the back of the boat keeping an eye on our streamer and airgun to make sure they maintain a proper distance from each other, that no logs get stuck on them or drag them down, etc. Its hot back there but we've got a fan and pass a lot of time playing cards, reading, talking, and of course everyone's favorite napping. If I'm in the lab I keep an eye on the chirp data coming in as well as the different profiles. Basically just making sure that everything's coming in alright. Today Steffen explained all the programs to me as well as taking me through a detailed explanation of how we process data. I got to do some picking but must say its even trickier then it was back in Socorro. Looking at the results we could tell there had been some pretty large errors but I intend to get better. Everyone says its a matter of experience and I've got plenty more days to get some of that. At one point one of the crew members makes lunch and being in the South it tends to be something fried which my stomach is growing accustomed to. When we finish the number of miles we want to get done (generally 12, which doesn't sound like a lot but since we're going at 2 mph its at least 6 hours and generally more since we often hit some technical snag and have to stop or backtrack). Then when we're done i put on another batch of sunscreen and help pull everything in or help convert the chirp data from .jsf to .spy and turn of the computers. We pack up our stuff, get back on the tiger shark, make for land, get in the van, drive to the hotel and then take a refreshing jump in the pool. Then we head out to some restaurant before collapsing in my bed to begin again the next day.
This has led to my getting between 5-6 hours of sleep a night for the past week which is not exactly conducive to patience during picking but I intend to get in bed earlier tonight. Granted I've been saying that every night....
We've been having some different issues. Everything from the streamer buoy getting stuck in an eddy to our latest issues with the Compressor. It gives the high pressure air to the airgun (excuse me, pneumatic energy source) and hence is rather important. When they did the pilot study for this experiment in 2008 their compressor broke down and they fully replaced it. The new one is now having some of the same issues so everyone is getting worried. .. One of the hazards of the field. Also every computer has special rules about how it has to be turned on (what can and can not be plugged in at the time).
So today Brian (one of the P.I's), Guolei (phd student of Dr. B), and I took the tiger shark to go visit some interesting bluffs enxt to the river. While discussing the sediments present Guolei and I ran into a pretty nasty patch of Poison Ivy. We scrubbed our legs and hands when we got back so hopefully no damage will be done but we'll find out tomorrrow...
Air inside of AIr
You all may recall that at the orientation when Beatrice was describing the method they use to image the Mississippi they use an airgun that creates an air bubble to generate the waves. To get rid of some noise they use a special airgun that puts an air bubble inside of another air bubble. Then I can't remember who I was talking to but we realized we really didn't understand how you put air inside of air and kept it different. The answer: Pressure gradients! So yes there is really just air inside of air so if you looked at it you wouldn't see the second bubble but! Because of how they're released at separate times with different volumes they do actually remain distinct. CRAZY!
Asian leaping carp
Today while visiting the bluff we scared up some giant fish! These are the much heralded Asian leaping carp! They're at least three feet long and jump at least one foot out of the water. We saw three during our short little voyage. Apparently during the pilot voyage they had one jump up on the tiger shark! Still waiting for that to happen one morning on our way to the Strong. Its one way to wake up!
Want to follow the official blog for this voyage?
Switzerland For The Win!!!!
I'm Swiss by birth and of course we're watching quite a bit of the World Cup on the boat since Dr. B is Italian, so it was really exciting to see them win their first game, and then that they beat Spain...SPAIN! SO GREAT!
The Mississippi! So today we got on the boat and started collecting data. Before we could do that though we had to put the airgun, the chirp, and the streamer in the water. We got everything set up in a pretty timely manner. However the river is full of logs, branches, and the occasional full tree and the buoy setup we had for the streamer got hit by a tree within minutes of being in the water. It got hit so hard the ziptie tore and it started floating away down river. But we have a small boat with a fantastic crew that just goes around and tries to push/ pull them out of the way before they can become a problem which chased it down for us. Everyone is being fantastic and I'm having a lot of fun with the whole group of scientists and crew. Only downside for right now is when we played a couple friendly matches of volleyball post-river my team lost twice in a row. We're plotting our revenge for tomorrow night. haha
I'm in Memphis! Right now the number one thing I'm looking forward to is sleeping in a room that isn't freezing, but Monday the real fun begins!
I'm starting off this internship by sailing down the Mississippi for several weeks. During the trip we'll be imaging the layers underneath the New Madrid Seismic Zone(NMZS) using an airgun as our source. We are also going to be taking CHIRPP data. During the voyage I'll be helping with the data acquisition, doing some quality control testing, and taking a preliminary look at the data as it comes in.
When we return to Memphis after our 300 km trip I'll be responsible for taking a look at a short length of the river. This is going to include doing different tests on it to find the best parameters for analysis as well as using ProMAX extensively. By the end of the internship I should know this profile inside out.
So goals for the summer:
First three weeks:
Learn all about acquiring data out on the boat. Dr. Magnani's talk about her research has given me a pretty good picture of what its going to be like out there but I am still very curious about every step and how its going to go each day. I saw the Mississippi when we flew it and that is a seriously big river! I can not wait to be out there on it!
Start talking to the grad students about their projects and what grad school is like.
Second three weeks:
Become really comfortable with Promax and any other softwares we're going to use to analyze the data.
Be able to look at a profile and recognize where there is noise and/or multiples and what is the actual signal.
Third three weeks:
Continue the analysis, I'm not entirely sure what I'm going to be looking for in my length of the river but based on what we did in the Promax class I'm guessing there will be lots of filtering and who knows maybe I'll be the lucky duck to find another fault.
Write an abstract for AGU before going back. Best to do these things while everything is still in my mind plus the first couple of weeks of school tend to be busy enough on their own.
More generally I'm really looking forward to this summer as an opportunity to get a good idea of what grad school is like, what being a geophysicist is like, which I like better: field work and the gathering of data or the analysis that takes place in the lab, and hopefully a more focused image of what I want to do after earning my physics degree.
I was in a canyon today for the first time! We did a two mile hike through some mountains, stopping occasionally along the way to discuss the geological formations around us. We saw evidence of many different types of faults including a normal strike slip and later saw some folding. The orientation week started off really amazing and has not slowed down since. We are kept busy running from installations of broadband seismographs to lectures about graduate school while at the same time having just enough type for random games of balderdash (geological version of course!)
A lot of fun activities are still on the way including another hike, and later tonight learning how to "predict" earthquakes.