Development of Cost-Effective Soil Properties Sensor for Ubiquitous Wireless Networks
2006 Research Initiation Award Report
Investigators
Krishna Shenai (PI)—USTAR Professor, Dept. Electrical and Computer
Engineering
Scott Jones (Co-PI)—Assistant Professor, Dept. Plants, Soils, and
Biometeorology, Dept. Biological and Irrigation Engineering
Mac McKee (Co-PI)—Professor, Dept. Civil and Environmental Engineering,
Director - UWRL
Summary
Research is currently on-going at the Utah Water Research Laboratory (UWRL) on
the development and deployment of cost-effective technologies to accurately
forecast near real-time changes in soil moisture over large irrigated areas.
This technology employs a mix of hardware (including on-ground soil moisture
probes, satellite and meso-scale model data, and unmanned, autonomous aircraft
equipped with multi-spectral cameras) to gather evapotranspiration and other
relevant data over a large geographic scale, coupled with advanced software to
process/assimilate all data over diverse temporal and geographical scales to
provide inexpensive and easily accessible information products that will have
value to individual irrigators, canal operators,reservoir operators, and
irrigation system administrators.
Accurate and reliable assessment of soil properties at the field- or
watershed-scale is vital for reducing excess fertilizer application, developing
reclamation strategies and managing agricultural and hydrologic water
resources. Common measurements include soil volumetric water content (VWC),
electrical conductivity (EC), temperature and increasingly desired measurements
include fluxes of water, heat, and soil gasses. It is desirable to measure
these properties using numerous inexpensive and highly accurate probes spread
throughout themonitored area for a more accurate description of soil
conditions. Developing a distributed (networked) measurement capability will
require advancements in multi-function sensor design and measurement
capability, component miniaturization, signal analysis and data transmission
before real-time field-scale measurements become a reality.
Presently, EM measurements of VWC probes utilize a time-domain (broadband)
measurement or a single frequency, often in the low Megahertz range where
salinity and temperature effects are significant and must be accounted for.
Clayey-, organic- and saline-soils may be attenuating and exhibit
temperature-dependent dielectric dispersion or relaxation within the 1-1000 MHz
measurement frequency (MF) range in which most sensors operate. The result is
that the effective MF is reduced in the process and the ‘real’ permittivity
(e’) -VWC relationship is variable. These salinity and temperature effects are
especially prominent at lower MF making measurement frequencies above a few
hundred MHz more appealing for reducing measurement error (i.e., TDR, TDT).
Current soil sensor technologies have a large form factor, are expensive, and
use a significant amount of energy which reduces battery life or requires
alternate power sources. These undesirable characteristics prohibit their
widespread use in a network configuration. The probe tips of current sensors
are buried at the desired depth where they gather information which is relayed
to the surface for further processing by means of a physical link such as a
cable, antenna, or the embodiment of the probe itself protruding from the
ground. This physical link to the surface is easily damaged by agricultural
equipment and, due to the unsightly and obtrusive features of the necessary
surface link, makes use of these sensors in athletic fields or home lawns
undesirable. A large need exists for sensors which can wirelessly relay data to
the surface, allowing them to be fully buried and concealed.
This project will advance soil water content determination capability using
electromagnetic (EM) measurements of frequency-dependent dielectric
permittivity. We propose development of a multi-frequency EM sensor for
improved water content determination. Having permittivity at multiple
frequencies also provides information on soil texture (i.e., clay content)
associated with relaxation losses. We currently use a scalar network analyzer
and dielectric probe as a standard for characterizing the quality of other EM
probe measurements. As an option, adding temperature and electrical
conductivity measurements to the probe would further improve measurement
accuracy as well as provide information on solute concentration in the soil. We
are in the process of developing small-scale sensors for electrical
conductivity and water flux determinations (utilizing heat pulse measurements).
Prototype sensors have been installed in the TW Daniels Experimental Forest
Site to monitor rain and snowmelt infiltration. The circuitry for these sensors
is being miniaturized and would be readily incorporated into a novel EM sensor.
Providing a low cost and efficient sensor for agriculture, environment or
hydrology will address the growing need for spatially distributed measurements
of soil water content and other soil parameters used as inputs in modeling and
water management work.
A novel method for underground communication in a wireless sensor network using
magnetic induction will be developed. The wireless, fully-buried soil moisture
sensing node will be equipped with one or more ferromagnetic coils, which are
stimulated by a current oscillating at a certain carrier frequency. A
time-varying magnetic field will result, propagating from the buried node to
other buried or surface nodes. The sensor data is transmitted via the resulting
magnetic field by modulation of the carrier signal. The time-varying magnetic
flux density incident on a second coil onboard the receiving node induces a
modulated voltage, from which the data is recovered. Communication may be
configured for ad-hoc networking and use multi-hopping to pass data from node
to node and eventually to a local area surface node where data is analyzed and
transmitted through the air using conventional RF methods. The magnetic fields
used to transmit the sensor data will not be affected by soil type,
composition, compaction, or moisture content, unlike existing RF transmission
technologies. Magnetic induction transmission also requires less power than
short range RF transmitters due to simpler circuitry and lower operating
frequencies.
Contact Information
Krishna Shenai
krishna.shenai‹at›usu.edu
Scott B. Jones
scott.jones‹at›usu.edu
Mac Mckee
mac.mckee‹at›usu.edu