Geoscience Reference
In-Depth Information
5100
C
C
D
5050
C
C
A
B
5000
1000
1050
1100
Local X-Grid Coordinate (m)
fIGURe 8.4
Shaded-relief map showing drain line locations detected with magnetometry methods. A is an
iron water pipe, B is an unknown feature, C are associated with a known clay tile drainage system, and D is
associated with an unexpected drainage system. The crosses mark known clay tile drainage pipe.
Although the magnetic signal spatially associated with the drainage system is clearly seen in the
western portion of Figure 8.4, the signal fades away when traveling to the east of the survey region.
Examination of the soils identified a change in soil type between the western and eastern portions
of the site. A detailed examination of soil iron concentration extracted from trenches identified dif-
ferent levels of disturbed iron-rich soils related to drainage pipe installation. More iron-rich soils
were disturbed in the western portion of the site and may explain why the signal is stronger in this
region (Rogers et al., 2006). The magnetic surveys at the Oregon State University Research Dairy
demonstrate that agricultural drainage systems can be magnetically imaged, but success in doing so
appears strongly related to the amount of iron-rich soil disturbed during installation.
8.2
Self-potentIAl MethodS
8.2.1 s
e l f
-P
o t e n t i a l
M
e t h o d
i
n t R o d u c t i o n
Self-potential is a passive geophysical method—that is, it does not rely on the application of energy
from an artificial source. Self-potential, from an operational standpoint, is probably the simplest
geophysical method, essentially requiring only the measurement of a naturally occurring electric
potential difference between two locations on the ground surface. The electric potential difference
measured is associated with nonartificial electric current transmitted through the ground. These
naturally generated electric potential differences range in magnitude from less than a millivolt
(mV) to over one volt (Reynolds, 1997). The magnitude and sign (positive or negative) of a delin-
eated self-potential anomaly can provide indications as to the character of the subsurface feature
producing the anomaly. Naturally occurring electric potential gradients tend to be consistent and
unidirectional or fluctuate with time. Although somewhat of an oversimplification, the focus of
the self-potential method is usually to measure the consistent, unidirectional electric potential dif-
ferences, and fluctuating potential differences are typically considered to be “noise” or unwanted
signal. Caution is warranted; however, because some self-potential noise is relatively consistent and
