Remarks & Solutions
Talings Project: Calcine volume

This page contains solutions and remarks for the activities which are not self-testing. They are
1. spreadsheet, 2. Survey model types, 3. Using raw DC data. Activities 4. and 5. do not need solutions.
Questions posed within the action maze are also answered.


1. Spreadsheet 1

(a) Single surveys, from the "Selecting surveys for calcine volume" page:
Survey Lines, stns, etc. comments
2D DC / IP 4 lines, totalling $6,700. Include: field work, processing, and 2 days mob/demob for 3 people. Use the contractor's survey list for costs.
Gravity 7 lines, totalling $7,650. Include: field work, processing, mob-demob for crew of 2.
Seismic tomography 3 lines, totalling $7,200. as above, but crew of 3 is needed and processing is double that of DC / IP or gravity.
Magnetics 10 days (40,000 stns), totalling $7,500. Only crew member is needed.

(b) Two surveys, from the "Resulting information" page:
Here is coverage you can expect from the two chosen surveys.

Note that the budget should never be the only consideration. The geological or geotechnical project must have adequate information. The final choice will be a compromise between optimal information and optimal cost.


2. Survey model types

These are the model types expected for the five surveys listed in the exercise. They are not necessarily displayed in the order shown in the matching exercise activity. This issue should be clearer if you have studied at least a little geophysics. The introductory Learning Objects in the AGLO collection will be enough for this.


DC sounding survey

Gravity survey

Seismic survey

VLF survey

DC resistivity survey

3. Using raw resistivity data

  1. Bad data points: These are areas of the pseudosection plots that have no dots on them.
  2. The diagonal stripe of data points with rather different values from their neighbours starts at x=50m and decends to the left.
  3. The vertical axis of a pseudosection is in "n" value which represents the separation between transmitter and receiver electrode pairs. It does not represent true depth, but apparent resistivities measured at larger separations (larger values of "n") do include information about resistivities at greater depths than measurements taken with small values of "n".
  4. Recall that values are apparent resistivities, not true physical properties of the ground.
  5. There does appear to be structure which will be interpretable if we can determine where in the ground higher and lower true resistivities exist which caused the measurements to vary as seen in the raw data.
 


 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.