old base model for VES data interpretation was horizontally-layered medium.
We consider as the new base model (fig.1) a combination of
horizontally-layered structure (HLS), deep inhomogeneities (DI)
(3-4 on fig.1) and nearsurface inhomogeneities - NSI (1-2
TES field technology
TES field technology includes the next rules:
VES locations on profile are regular with equal distances.
At each reference point two pole-dipole sounding (AMN & MNB) are fulfilled.
Step in current electrodes distance growth is constant (linear) and equal
to sounding step. In this case places for current electrodes grounding
will be the same for all VES points on profile.
Step on profile should be equal to (0.1-0.05) AOmax. In turn
AOmax depends on depth of investigation hmax.
Field measurements may be carried out also with AM (pole-pole) array and
expander (dipole-dipole) array.
In each case the TES field technology aims to investigate a depth interval
from hmin up to hmax. The horizontal size of any
object to be visible should be approximately equal or more of its depth
(3-4 on fig.1). The step on profile for detailed investigation of such
object should be 2-5 times smaller than its size. The object laying deeper
than hmax (5 on fig.1) is not visible due to restricted penetration
depth. All objects at the depth smaller than hmin (1-2 on fig.1)
- may influence and the more noticeably, the smaller their depth is, because
they are closer to the current and potential electrodes position. But step
on profile and minimal AO distance are not enough for their study. These
objects are considered as geological noise.
between NSI and deep objects depends on our selection. Some bodies in definite
depth interval we consider as useful objects and adjust field technology
for their tracing, while some others on smaller depth we consider as noise.
Distortions of the electric field (or VES curves), caused by such NSI objects
may be divided into two principal types: caused by object near dipole element
of array and caused by object near single electrode. These effects depends
also on the fact: is this dipole group or single electrode moveable or
unmoveable. These effects are shown on fig.2. Model is represented with
single NSI of half-spherical form over two-layered medium (fig.2,a). Pole-dipole
array can be applied in two ways: single electrode is moveable while dipole
group is stable or dipole group is moveable while single electrode is stable.
Stable electrode or dipole group can be placed in NSI limits (cases 1 and
2) or out of it (cases 3 and 4). In the last case moving electrode or dipole
group will cross the NSI.
Our standard technology is two-sided pole-dipole sounding with stable
MN group and moveable current electrode. For this case we use more local
terms to classify distortions: P and
C - effects.
119899, Russia, Moscow, Moscow State University, Geological Faculty, Department
V.A. Shevnin,I.N. Modin
Tel. & fax: (7095) 939 49 63