Day 8: The return trip

The trip home went quite well.  It was fun to drive all the way across the island again, eat Jamaican Patties in the airport, and then see beautiful views of the Caribbean out the window. 

Now that we're back in Boston, our work turns to writing a report that incorporates the data we collected.  We will present our data in conjunction with the data from previous field trips and summarize our recommendations for future groundwater searches.  The report will also be sent to the Water Resources Management Agency (WMRA) in Saint Lucia as a reference for their future work.    

Yes, we were technically in Saint Lucia for a class project.  But the more important part of this experience is that we contributed to a much larger effort.  We became part of a team which will eventually find a solution for a real-world problem.  And our results, even though they don't point us to the absolute best place to drill in the Roseau Valley yet, will help with the long-term search for ground water in the area. 

Our whole team is shown below:

Group picture outside of the WRMA.  The people in the photo are from MIT, Harvard, and the Water Resources Management Agency of Saint Lucia. Photo credit: KMK.

Day 7: A chance to step back

On our last day of field work, we drove to a high point on the side of the Roseau Valley to look at the entire valley and get a larger perspective on the work we've been doing.  The north and south sides of the valley are flanked with rolling hills, while the valley floor is extremely wide and flat.  We are exploring the idea that the rolling hills, made of fractured volcanic rock, contain water within their fractures.  Furthermore, we are exploring the idea that the same structures that create the rolling hills might also exist below the valley floor, but covered by a thick floodplain of clay and silt.  Our resistivity measurements on the valley floor over the past week are helping us to find the volcanic structures below the valley and determine whether they have fractures or pores that contain water. 

Conducting a hilltop sounding, with the Roseau Valley in the background.

We took one short sounding on the top of the hill which produced some nice resistivity data.  We have inverted this data to get an estimate of the resistivity of hill material.  If similar resistivities exist below the valley floor, it may suggest the underground roots of the same hills that make up the valley walls. 

Resistivity data going down to 35m depth on the hilltop.

 

Sponsored Message: A Short Post About Coconuts

Brought to you by the friendly coconut farmers and water resource managers of Saint Lucia. 

While we were visiting the well this afternoon, one of our guides grabbed a bamboo stick and knocked down half a dozen coconuts from a coconut tree.  We collected the coconuts and brought them to the road, where the entire group took a break.  

If you cut a piece of the skin away with a cutlass, you can drink the coconut water from the center.  It is highly sought-after here in Saint Lucia-- in fact, you can find people selling bottles of coconut water on the street.  We drank it straight from the coconut.  

Mmmm coconut water.

Drinking more coconut water.  This kind of coconut actually has a smooth green skin when it's ripe. 

Then, our guides split the coconuts in half with a cutlass and showed us how to eat the coconut jelly.  This is the part that shaved coconut is made out of.  Actually, eating the miniscule layer of coconut jelly inside the fruit gave me a new appreciation for the shaved coconut that I usually see in supermarkets and take for granted at home.   And by the way, both the coconut water and the coconut jelly were delicious!

An open coconut filled with coconut jelly, which we ate with makeshift spoons made of slices of coconut skin.  

Day 6: Fixing bad data + making water quality measurements

Our data analysis from yesterday suggested that a few of the points we have measured in the past might have errors.  The resistivity values took an unusual turn at the deeper part of one particular sounding.  However, for resistivity measurements, an anomaly at depth may be representative of a surface feature somewhere along the line that just changes the possible paths for electrical current in the ground.  

So, in order to identify the errors, we went back to the field and surveyed that sounding for nearby disturbances, such as buried metal, culverts, or other obstructions that would change the conductivity of the ground.  We found that one of our electrodes was very close to a culvert during the final measurement, meaning that the measurement was suspect.  Tonight, we were able to remove that reading from the inversion and produce a new result.  

This afternoon, we  went on a trip to several local sources of water with our colleagues at the Water Resources Management Agency.  This included a well and a slowly-flowing stream which we tested for conductivity and pH.  Although this particular well was not in Roseau Valley, it is an example of a type of water resource that could be developed in Roseau Valley.  

Today's lunch spot: beside the largest river in the Roseau Valley. 

Sending a high-tech sampling device (a measuring tape and then a plastic cup on a string) down into the well to collect water.

Testing the pH of the small river at our site visit this afternoon.

Day 5: Data analysis begins, with some photos

Much of today and yesterday were spent on coding for data analysis.  If we can analyze the data correctly, we now have enough information to make some preliminary conclusions about the resistivity of the material below the Roseau Valley.  So we split up the work of performing inversions and analysis, and are now contributing pictures and text files and spreadsheets to a shared group folder at a frightening rate. We work in the morning before going to the field, at night after dinner, and at whatever other times we can manage in between.  Hopefully we'll have an interesting result.  

In the afternoon, we had enough time to run two more soundings in the field.  They're going faster now that we have established a team routine.  Guided by our preliminary results from last week, we chose two new locations to probe and conducted soundings there.  The activities in the field are much the same as in previous days, so today I decided to show a few more pictures of our group at work.  

Taking a break. We often have more people than we need during a sounding, so we rotate taking breaks to make sure each person gets enough rest from working in the sun. 

Pounding an electrode into the ground before a measurement. 

Recording a measurement back at the current source, our home base during soundings. 

Day 4: Data collection

Today we conducted three more lines of resistivity measurements, which took pretty much the entire day.  Each line had electrodes spaced at maximum 150 meters in each direction from the starting position.  Using the three lines, we were able to cover 900 meters of the valley.  The three vertical soundings that we measured will be combined to generate a 2-D image of the layers of rock below the valley.  

Diligently placing electrodes 150 meters away from the central point.

Day 4: Flora and Fauna of Saint Lucia

While doing fieldwork in and around the Roseau Valley, we have seen many interesting kinds of plants and animals.  These include:

• Mongooses (or mongeese?)
• Banana trees
• Soursop fruit trees
• Yellow/orange coconuts

And:

An African snail climbing a banana tree

A horse which usually has a cattle egret sitting on its back

A dwarf coconut tree that's only about 10 feet tall

And a bunch of MIT and Harvard students.

Day 3: Mapping on Saint Lucia

Yesterday I tested out our handheld GPS device while we were driving to the field site.  Here's our path:

Thank you, Google Earth!

To me, a map like this reminds me how small this island actually is!  Note the twisty-turny roads getting into the valley- they greet us every morning with some beautiful views. 

Day 3: First Measurements

At last, some hard-earned data.

Measurements today!  We headed back to the Roseau Valley to try our equipment one more time after making some changes from the day before.  This time we had more success. 

To recap, we're using surface measurements of electrical resistivity to infer the resistivity of deeper rock layers, hoping to find the tell-tale signs of a freshwater aquifer.  To learn about layers deep below the surface, we must separate our positive and negative electrodes by a large distance; typically, we can only learn about layers that are above a depth of one-half the separation distance.  Today, we spaced the electrodes up to 300 meters apart, meaning that we could "see" features up to 150 meters deep.  We also took many measurements at closer spacing during the process.  

Electrodes measuring voltage and current in the ground.

 The picture above is a typical set-up for an electrode.  In the photo, we have:

• a metal stake (the actual electrode)
• a measuring tape
• wire, to connect back to home base (up to 150 meters away)
• a hammer, to put the electrode in the ground
• a banana leaf (decorative, of course). 

In addition to the resistivity measurements, we also collected self-potential (SP) measurements by recording the small background voltage drop, usually a few millivolts, between two distant electrodes in the ground.  We subsequently measured the self potential at points 60 meters apart for 1200 meters along the road.  The SP measurements will hopefully help us determine where subsurface water is flowing.  

The SP measurements.  Note- we marked every 60 meters with a banana. 

Taking SP measurements.  The bucket is for washing and recalibrating the electrodes. 

 All in all, the measurements today appeared to be successful.  We will analyze the data later through an inversion to identify any aquifer layers.

Day 2: An important lesson of field work

One important lesson about field work is that sometimes, things don't go according to plan.  

The Roseau Valley, the site of our fieldwork

This morning, we drove to the Roseau Valley, in the heart of the banana-producing region of Saint Lucia.  Along with our colleagues at the Water Resources Management Agency, we began to set up an array of electrodes to take measurements of ground conductivity.  But before too long, we realized that one piece of equipment wasn't working.  Unfortunately, it wasn't the same piece of equipment that was malfunctioning yesterday- that piece was just fine.  We would inject current into the ground and receive negative voltages on the meter when we should see positives.  Sometimes we would see positives, too; the meter was more erratic than anything else. 

Hammering an electrode into the ground in Roseau Valley.

Debugging the equipment in the field.

We tried a number of controlled experiments to diagnose the problem, and even the simple tests never quite worked out.  In the end, I would say that field work today was somewhat frustrating.  We accomplished little due to faulty equipment.  Tonight we will work on the equipment again, and we will regroup or redefine our objectives in the morning. 

Day 1: Alligator clips, anyone?

Today, we performed the all-important duty of testing the field equipment.  This turned out to be non-trivial.  

We are using a DC/DC converter to transform the 12.4V from a car battery into the range that we need for making measurements, anywhere from 100V to 800V of DC voltage.  According to our colleagues here, the converter worked last Thursday, but today... we had no such luck, at least not in the morning. 

The suspect converter, pre-debugging. 

When there was a problem, our first instinct was to consult the manual or the manufacturer.  But of course, the manufacturing company of the converter:

• is based in France, where it was already after 5PM at the time of our problem,
• is exclusively French-speaking, and
• does not put manuals online. 

So when the converter beeped constantly and wasn't able to produce the proper voltage, we decided to take it apart and trace the signals.  

The inside of the converter.

Protecting the debugging activities from a sudden rain shower.

After trying many things and seeing many strange behaviors, we decided to bring the converter indoors and take a break.  Our theories that the device only worked by magic when it was upside-down or when our professor was in the room were... not satisfying.  But after a while in the AC, the converter stopped beeping and started behaving correctly instead.  So even though we never conclusively found the source of the error, we suspect that moisture in the instrument may have caused it to behave badly. 

Performing surgery on the electronics equipment.

In the course of our debugging, I was reminded that electronics equipment is harder to come by in Saint Lucia.  Alligator and banana clips, for example, might not be available at the nearby hardware store, and might only be available at a few places on the island if they are available at all.  Shipping things from Amazon isn't an option either.  In the case of the voltage converter, we simply do not have a replacement if this one breaks.  We're hoping that it will perform well during the next week. 

In the end, we left the electronics out on the table in the air-conditioned room overnight, hoping that we'll be able to use them more consistently in the morning! 

Day 1: Defining Our Scientific Objectives

This week, we will be looking for underground rocks which contain groundwater that could be used as a source of fresh water for the people of the Roseau Valley in Saint Lucia.  In order to detect the presence of water, we will conduct a series of surface conductivity measurements that will reveal the locations of any electrically-conductive, and potentially water-containing, rock layers at depth.  

Placing a stake in the ground for a test measurement of electrical conductivity.

For each measurement, we will set up two distant stakes in the ground as electrodes and apply a large voltage to induce a current between them.  In between these two electrodes, we will set up two more stakes with variable position and spacing, and we will measure the voltage drop between the two inner electrodes. By performing an inversion of the data, we can use the measured resistivity or conductivity values at the surface to give us information about the resistivity, and thus the water content, of the deeper layers of rock. 

Based on what we find, we will probably conduct a number of one-dimensional soundings and two-dimensional horizontal and vertical conductivity profiles in the Roseau valley.  

Stay tuned to find out more! 

Day 0: Arrival

Today, a group of MIT and Harvard students arrived in St. Lucia to begin our field geophysics project.  After landing at the airport, we first drove to our hotel.  The whole drive, which brought us from the extreme southeast corner of the island to the extreme northwest corner, took less than two hours. 

St. Lucia is home to a relatively well-preserved tropical ecosystem.  While driving on narrow, sharply-turning roads (and driving on the left side), we saw the palm trees, banana trees, ferns, and pine trees that live in St. Lucia's tropical rainforest.  (Yes, there are pine trees in the rainforest here!)

A rainforest in St. Lucia, as seen from the car.

St. Lucia is a developing country plagued in certain places by weak infrastructure and poor sanitation.  On the drive to our hotel, for example, we saw several examples of bridges that had been destroyed by recent floods.  We saw other buildings in need of repair.  Along the lines of development, the geophysical study we will conduct over the next two weeks is actually aimed at improving public access to clean water in a nearby part of the island.  

A few vehicles and buildings in St. Lucia.

But the most striking thing about St. Lucia is its exceptional natural beauty.  It's no wonder the island attracts hundreds of thousands of tourists every year. 

The Caribbean Sea near Dennery, St. Lucia.

Now that we are settled in, we are very excited to begin working here.  Tomorrow, we will begin by meeting our St. Lucian colleagues and discussing our strategy for the next week.