Volcan Baru Small Aperture Array

Updated January 5^th, 2003

The array will consist of eight vertical Marks Products L15B 4.5 Hz geophones which will be electronically extended to .5 Hz with a Roberts circuit. The array will be “L” shaped with 100 meters between stations. They will be connected with wires. At the apex of the “L” there will be a 3c 4.5 Hz sensor to help get the S – P times. Each station will communicate serially to a computer which will have GPS timing. That computer will in turn be connected to my present network. Each station will get its time from the central computer. The electronics will be mostly from Mauro Mariotti Seismostack www.infoeq.it. The data collection software will be Seislog and the analysis software will be Locas. I welcome any and all comments on the project. Write me at here. I presently have a network of six stations and I record events within 25 kilometers, and lots of activity within 100 kilometers.

This is a map of the general area. The summit of Volcan Baru can be seen in the upper left. This volcano last erupted about 600 years ago. The array site is just to the left of the town of Volcan and my home is just under the 14 on the map. More details in photos below.

In this map you can see the stile at the “Y” in the road leaving Volcan towards the airstrip. As you can see from the contour lines this is one of the flattest pastures in the whole area. There are cows in these fields. The soil conditions are not the best (deep topsoil that was once lake bottom). The soil is saturated during the rainy season, and even during the summer the water table is just a few feet down. But on the other hand it is close to home, it has power and internet, and I have permission from the owner. The airstrip to the left of the site is rarely used. As can be seen the roads enclosing the site really go nowhere, a private home and a coffee “beneficio”, a processing plant. There is no industry in the general area. There is a small river to the north. During the summer months the site is windy, but the stations will have a very low profile.

Here you can see great detail of the site. The house under the arrow will have power and a permanent internet connection with a public IP. The two roads carry very little traffic; mainly truck during the coffee harvest and lovers to the airstrip at night. The upper one is a dirt track.

All the stations will have to trigger at once to be able to declare an event, so I do not think that the vehicle traffic will affect it much.

Each of the eight senors will be in a watertight case (surplus USA army night goggle cases) which will be fixed to a 8x27x24 concrete blocks which I estimate to weigh about 450 pounds.

All the wires will enter the box through the only hole, which now has a PVC fitting.

Open and empty box.

Eight bases waiting to have boxes fitted. The loops are for carrying the bases around.

More photos as the project moves forward.

These are the geophones that will be used in the project. The were provided by Jim O'Donnell. Thanks Jim! I will show details of how the verticals will be mounted inside the cases. As mentioned above these 4.5 Hz devices will be electronically extended to .5 Hz.

The point under the level will be the location of the apex of the apex of the “L” of the array. I think that the points are within a few inches of where they should be. I spent two days checking these over. The house in the background is where I will get power and house the internet connection equipment (radios).

View of what I'm looking at in the first photo the little white vertical line is 100 meters away and where the first station of the “north” leg of the array.

This is an Excel (Open Office) list of the coordinates the array stations with a chart. The UTM grid gives me meters from the equator and prime so the x and y axis are really N/S and E/W. The stations will will be called (for now) AVB0-7 . The E/W stations are 0 and the odd numbers and the N/S are 0 and the even numbers. The meters are from a barometric altimeter and no the GPS altitude. I think the charted nicely.

Well now that the points have been marked the hole for the bases are dug. The little white piece of PVC tubing is the exact place for the geophone.

Here is the 4^th base being lifted on to the truck.

The bases being placed in position in the field. I have placed gallery of pictures at this URL as to not clutter this page too much.

The next phase is to dig the trenches and bury the tubes, pull the wires. At the same time I will bringing power to the main box, (not yet made but close) and making the wireless internet connections.

After that I will begin to install the electronics. I am still shooting to have some test done by the end of February. Still lots of stuff to work out.

March 9^th, 2003

Well it is close to the end of February but I'm nowhere near where I wanted to be. I had LOTS of trouble getting the cables in the tubes. In the end I had to dig up and cut the buried tube at each of the stations and pull the cable bundles one section at a time. It took five of us 3 full days work to do it. The next array will be easier! :)

Carla and Mariano at one of the lube stations, the wire bundle about to dive into the tube.

There is a link further on to a gallery of pictures.

What I have now is a central station with power, internet and GPS and eight remotes connected to the central station with wires. I have a computer running Seislog and accepting 8 serial ports. That same computer has all of the programs I need to collect and send the data to my home.

I have the geophones on their mounts ready to fit in the boxes, Including the AVB0 station which has three components.

These two gifs give “block” view of the array components.

This is the general layout. AVB0-7 connect to a central computer and each station gets its power and sense of time from the central station.

Here is more details of the relation of each remote to the central station.

View of what the inside of each station will look like. See photo further on with the seismo stack which will go in the box.

This is a view of the central station box. All the station cables are here along with the GPS and internet antenna cables can be seen. (one anyway). This box will contain the power supplies, computers, radios, and the RS422 transceivers you will see in the next picture.

In this picture are the electronics for the array. From left to right. The first seven identical stacks are for stations with one geophone and from bottom to top in these stacks are the amp, period extension, digitizer, power supply and RS232 to RS422 converter.

The next (eigth) stack is for the 3c station and is just like the first seven but has three amp and period extension boards. And the last last stack is the central station stack with 8 RS422 / RS232 converters and on top is a power card and on top of that will be the GPS interface card and GPS. From this stack the 8 serial cords goto to an 8 port device and into the computer with Seislog which will be connected to the internet via an 802.11 radio.

Here is a link with more photos of the work.

May 7^th, 2003

Well the array is up and running!!! Still some testing and lots of details to attend to but it is 98% working. Here are some more photos of the last few detail of the work. I hope to be locating soon with the array.

It has not all been without problems. The biggest is that while the area is homogeneous in it's nature. It is a very wet and soft area. I would call it “S” deaf, not entirely but close. I also made some mistakes when installing the buried wires. I very carefully put soldered all the connections and places heat shrink over each of the inside strands and heat shrink over the whole cable. Well in two instances when shrinking the outside tubing I melted the inside insulation into a glob and all the wire shorted out. Luckily both in places that were not to hard to repair. The other mistake , fixed now is that the buried tube goes under a dirt road between stations AVB0 and AVB2. When cars went over the road, the road when down and the tube got shorter and “pulled” on the station. Very clear on the wave forms. I have since decoupled the tube and AVB2 and the problems has disappeared.

Here are some pictures:

What you see here is the array all set up in my work room

This the results of the polarity test with all the units in the rack. Lines 2 an 3 are the horizontals. I tried lot of thing to do the polarity test, from dropping things to a plate and hammer. In the end the best results was from tilting the entire rack. Plain old tilt test.

This is results of the phase test. The input was a square wave to the vertical channels. The time for all the cards is syncronized by a DCF77 simulation from the GPS 1PPS and is sent to all the cards every second. The square wave looks funny because of the filtering.

Well out of the lab and out to the field.

This is what the the individual remote stations look like in the field. I plan to paint them with some warning of “High Voltage” and death. There are 8 of these.

Here is a picture that shows the outer rain and sun protection box, 4 layers of ½ stryofoam covered with mesh and plastered over with concrete. Keeps the bulk of the rain which is nearly 400 inches a year off the water tight inside box just in case. Keeps the sun off a bit also.

Here is the inside of the inside box with the business of the remotes, This is AVB0 which has the 3c system.

You can see the plate with two horizontal geophones the vertical geophone is hanging under the plate. I can level and rotate this plate eaisly. The 3c seismostack is in the front of the box. The top card is the rs422 card which takes the RS232 signal and converts it and send it to its mating card and then is converted again to RS232 and then into the serial ports at the computer.

This is a view of the central box with all the common stuff. I manage the entire array with VNC.

So there you have it. That is the location, installation and hardware. Here are some of the first results.

Sorry about the fuzzy pictures. If any reader wants the actual files just get in touch. What you see here is the first event that I recorded with the array. There are 8 stations and 10 channels. AVB0 - 7 and AVB0 has 3 components.

Here I have selected all the vertical channels and expanded around the P. I have added a line to help see that you CAN see the time difference. The I use a program call FK from the Seisan suite of programs to compute the azimuth and apparent velocity.

This is the output of the FK program. You can see what it shows. For a good explaination of this please refer to page 216 of the Seisan manual. It is available in PDF format.

I then pick an S from one of the horizontal channels and a P from one of the vertical channels and then I can compute the location.

Well here are the locations of the first ten events. I still have to compare them with the results of the University of Costa Rica. I will do that in the next few days. I also still have quite a bit of tweaking to do and I really do have to learn a bit more about the FK analysis.

Now I have to make the page pretty!! And learn how use this thing. :)

Once again thank to everyone who has helped and contributed. I could not have done this by myself.

Jens Havskov, Seisan and encouragement, doesn't let anything slip by.

Oyvind Natvik, Who wrote the Windows Seilog and puts up with all my questions.

Arie Verveer, Wrote the original Windows drivers for Seislog.

Mauro Mariotti, Electronics and support.

Jim Odonnell, provided the geophones.

Ramon Ortiz, Locas software and oversight, really got me going

Chris Chapman, constant great comments and observations. And some impossible for me to get resistors.

The Janson Family, The land the array is on.

Max Jaramillo, Trucks, loaders, sand and gravel, you name it

Carla, my wife who puts up with the hours I spend at this and helps me in the field.

PSN list.

Special thanks to Mauro and his father who in many ways have worked just as hard to make this array happen.

Special thanks to Jens Havskov and all his colleges of the University of Bergen

Once again any and all comments welcome.

bru2 (at) volcanbaru (dot) com