Questions
posed within the action maze
Selecting
surveys for calcine volume
The question was: Which of the surveys will
produce MORE data than necessary for this question if the whole
budget was spent on that survey alone?
Answer and remarks: Magnetics. If our
site is roughly 600 x 600m (see the sketch map in "survey
locations"), then a magnetics survey should cover 1200
x 1200m. 40,000 stations would permit 60 lines that are 20m apart,
and station spacing along the lines of 2m (38,000 stations).
There is never "too much" data for geophysical work
because the problem will always be underdetermined. We want to
learn about the three dimensional subsurface and we usually make
measurements only at the surface. However, increasing the resolution
will not improve our ability to learn about the volume of calcine,
so the 40,000 station magnetic survey is rather more than necessary.
Of course, using magnetics for this task assumes calcine has
a susceptibility different from the surrounding rocks, and that
bedrock features will not confuse the result.
Interpreting DC resistivity
for requirements
Seven questions were posed. Here they are with answers and remarks.
You may have other relevant comments.
1) Two sets of models were produced from the 3 lines of data.
Which parts of the models are the same for both sets?
The conductive parts, (warm colours) and
shallow portions of the models.
2) Which parts of the models are different for both sets?
The deeper portions of the model, and the
edges. This is due mainly to the fact that measurements have not
sampled the ground so well at greater depths nor at the ends of
the lines.
3) Is your confidence in information about the target (calcine
volume) higher as a consequence of producing these two sets of
models?
Yes. If inversion models are similar regardless
of the parameters used to obtain them, then it is more likely that
the data are the principle constraints on the mode.
4) How do you think the zone of calcine is depicted in these
models?
As the most conductive zone (reds and oranges).
5) For each of the 3 lines, what is the length of the zone
containing calcine?
L1:160m, L2:150m, L3: 280.
6) For each of the lines what is the thickness of the calcine
zone?
L1:20m, L2:20m, L3: 20.
7) Considering the location of these 3 lines, what is a rough
estimate of volume?
Lines 1 and 2 give a width of approximatly 155m, Line 3 gives
a length of approximately 280, and depth is roughly 20m. The volume
is therefore 868,000 cubuic metres. This is likely an overestimate
since the dump's sides are probably not perfectly linear.
Similar, complementary
or contradictory results?
Three questions were posed Here they are with some answers and
remarks. You may have other relevant comments.
1) How are the models from the two surveys similar (the same information
comes from both surveys)?
Their lengths (ie lateral boundaries) are quite similar, and their
depths are fairly similar
2) How are they complementary (information that is unique for
a survey, but which supports information from the other)?
Gravity produces models with boundaries, which would change
if density contrasts are adjusted. Resistivity models are "smooth".
You have to make a judgement as to the locations of boundaries. Also,
unlike gravity, the DC resistivitiy data include a cross line
(line 3).
3) Are there any contradictions (information from the two surveys
that does not agree with each other)?
The depths could be said to disagree somewhat, although given the
difference in physics involved with these two measurements, it
really should be said that there is remarkable agreement. |