Focus: Dowsing with Magnetic Resonance
Dowsers claim they can locate underground water using a tool that senses the water’s presence, but physicists have recently marketed their own dowsing tool. A distant cousin to medical MRI (magnetic resonance imaging), the instrument uses magnetic fields to map the density and depth of subsurface water. But the conditions outdoors are harder to control than those in a modern hospital, so deriving the equations that convert raw data into images is not so easy. In the 17 May PRL a team derives the complete set of equations and shows that incorrect assumptions built into a commercial machine’s software lead to extremely inaccurate results for water deeper than about 40 m.
In place of the powerful magnet that dominates hospital MRI’s, surface nuclear magnetic resonance (NMR) imaging uses only the Earth’s feeble magnetic field, which is 10,000 times weaker. According to NMR theory, if a collection of nuclei is placed in a static magnetic field, and a second oscillatory field is applied, the nuclei will respond by generating their own oscillating magnetic field, which can be detected with a suitable wire loop. By applying a series of pulses of varying lengths of time, the imaging system receives responses from the water protons at different depths below ground. (Only nuclei in a liquid can respond under these conditions.) The field strength of the pulses decays with depth, and protons exposed to weaker fields respond best to longer pulses.
The commercial system–from Iris Instruments in France, based on a prototype developed in Russia–uses a 100 m diameter wire loop laid out on the area of ground to be investigated. The loop serves as both transmitter and detector of the pulsed fields and is wired to the electronics and a computer. The problem with the system, according to Peter Weichman of Blackhawk Geometrics in Golden, CO, is that the data processing software in the Iris machine does not correctly account for the electrical conductivity of the ground. This conductivity leads to significant time delays as the magnetic pulses travel down and back up through the ground, but these delays are ignored by the software.
Weichman and his colleagues derive the complete set of equations,accounting for the delays, and show that their solution works well down to at least 80 m. (Deeper water is almost undetectable because the pulsed field becomes too weak so far from the transmitter.) They show that the incorrect assumptions made by the Iris system lead to an impossible negative water density for water buried deep enough to cause significant time delays–about 40 m.
The team plans to further develop the technology and use it to aid in cleaning up toxic spills. With it they can detect not only the location of toxic liquids that seep below a clean-up site, but also the distribution of pore sizes within the rock–which strongly influences the spill’s tendency to spread. The instrument is already being used to search for groundwater in deserts, and Weichman says the new results appear to explain anomalies observed by a researcher using it in Israel.
Pierre Valla, President of IRIS Instruments, acknowledges the inaccuracies in his company’s product. “What we have been using up to now is some sort of first order approximation,” he says, but the company is already working on improvements.