Geoscience Reference
In-Depth Information
fIGURe 8.3
A magnetometry data collection effort in the field is often broken up into separate survey units
that together cover a study area. The data from the individual survey units are later merged together to produce
a magnetic field map for the entire study area. Edge-matching is the process of matching the magnetic baseline
at the end of one survey unit to the beginning of the next. Bidirectional surveys can produce striping, with
every other transect having a slightly higher or lower magnetic baseline when compared to its neighbor.
8.1.5 M
a g n e t o M e t R y
a
g R i c u l t u R a l
a
P P l i c at i o n
e
x a M P l e
The Oregon State University Research Dairy in Corvallis, Oregon, sits on 73 ha of working land
2.4 km west of the main campus. The dairy contains 40 registered Jersey cows and 130 registered
Holstein cows and houses 20 to 140 various aged heifers. Near-surface geophysical surveys were
conducted on a 1 ha portion of a pasture that is clay-tile drained with the most recent installation
approximately forty years ago. This field was selected for study because excessive levels of
Escheri-
chia coli
were found in adjacent Oak Creek that seemed related to the method of spraying liquid
effluent as part of the nutrient and manure management strategy. Successful mapping of agricultural
drainage systems was essential in understanding the relationship between effluent spraying and
creek contamination.
Drainage pipe locations could not be imaged using a 500 MHz ground-penetrating radar antenna
due to extreme attenuation of the signal caused by soils with high clay content (Rogers, 2003).
Drainage pipe locations were successfully mapped using a Geometrics G-868 optically pumped
gradiometer. A 100-meter-square study area was divided into twenty-five 20-meter-square subunits
using nonmagnetic polyvinyl chloride (PVC) stakes to mark the subunit corners. Nonmagnetic
(fiberglass) survey tapes marked the subunit perimeter, and nineteen blaze-orange, 0.95-gauge plas-
tic weed trimmer line were used to mark north-south running transect lines spaced every meter.
The magnetic survey was bidirectional with the sensors separated by 0.5 m in a vertical gradi-
ent mode. Under ideal conditions, establishing the survey grid and conducting the aforementioned
magnetic survey could be accomplished by an experienced team of three in 4 to 5 days. Golden
Software's Surfer 7 (Golden Software Inc., 1999) was used to create a shaded-relief of the plot with
no postacquisition processing (Figure 8.4). An iron water pipe, the clay tile drainage system, and
other magnetic signals spatially associated with subsurface objects are evident in the shaded-relief
plot (Rogers et al., 2005).
