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.