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.