I found this excellent 114-page report posted at the West Virginia Mine Safety Organization website (Required reading in my opinion):

Natural Resources Canada: CANMET Mining and Mineral Sciences Laboratories Summary Study of Underground Communications Technologies
Final Project Report
by Pierre Laliberte, Eng.
Experimental-mine
Project: 603478-00-0
Report CANMET-MMSL 09-004(TR)
Version: May 2009
URL: http://www.wvminesafety.org/pdfs/und...

The report concludes that Wi-Fi signal frequencies at 2.4 GHz (ActiveMine's frequency) outperform the lower frequencies when it comes to the following:

a) Range as a function of roof height and line-of-sight frequency
b) Range as a function of roof height and frequency
c) Turning around or passing through pillars, concrete walls and other obstacles.

The report concludes that there doesn't appear to be one communications system that meets all the requirements of the United States' Miner Act of 2006. Most of the systems that use optical fibre, coaxial cable, Leaky Feeder cable or any other cable to carry communications will not function if they break unless there is a redundant system and an alternative route that allows communications to reach the surface.

The report concludes that wireless mesh networks require a communications link between nodes. In surface applications, each node can be connected to many other nodes by radio link. In underground galleries, nodes are usually arranged in linear configuration, and the signal could be impeded or cut off completely if any galleries were to collapse. Again, there has to be an alternate route so that the mesh network can get communications to other nodes and ultimately to the surface. That may not be possible in long dead-end galleries that are not connected to other levels or the surface. In addition, there has to be a separate back-up power system for each node in case the electrical power cable is cut.

**A survey of 13 mining companies was carried out in the summer of 2008. The results of that survey are presented at the end of the report. The findings suggest that the level of knowledge of new underground communication technologies varies considerably. It also appears that the mining industry is still reluctant to use these new technologies even knowing they are reliable. Finally, 8 of 11 respondents reported that their current underground communications needs are being met by manufacturers.
Note: In my opinion, the sample size is too small to be statistically significant.

Key survey results:
a) < 10% of the 13 mining companies used MESH networks for their underground mine communications. > 80% of the 13 mining companies used Leaky Feeder technology for their underground mine communications. 60% of the 13 mining companies were using optical fibre and LAN Network technologies. 70% of the 13 mining companies were controlling their vehicles and equipment remotely.
b) Most of the 13 mining companies had little knowledge about MESH Networks.
c) Those mining companies currently using MESH Network technology are using this technology primarily for alarms and monitoring.
d) Most of the 13 surveyed mining companies are using Leaky Feeder systems for communications.
e) Most of the 13 surveyed mining companies don't use any technologies for location and tracking.
f) The reliability of Leaky Feeder systems was rated good. The reliability of MESH Networks was rated between average and good.
g) The purchasing cost of Leaky Feeder and MESH Networks was rated between average and good.
h) The ease of installation for Leaky Feeder systems was rated good. The ease of installation for MESH Networks was rated between average and good.
i) The maintenance cost for Leaky Feeder systems was rated good. The maintenance cost for MESH Networks was rated average.
j) The ease of maintenance for Leaky Feeder systems was rated good. The ease of maintenance for MESH Networks was rated between average and good.
k) The ease of operation for Leaky Feeder systems was rated good. The ease of operation for MESH Networks was rated between average and good.
l) The capacity of Leaky Feeder systems and MESH Networks was rated between average and good.

Sections of particular interest:

Page 7 > Summary

Pages 44-53 > Wi-Fi Communications (Active Control Technology discussed on Page 52)

** Pages 54-68 > Mesh Networks (MeshDynamics and Active Control Technology discussed on Pages 56-61)

** Page 68 > Table 9 - Summary - Meshed Networks

** Pages 74-89 > Equipment and Personnel Tracking Systems (Active Control Technology discussed on Page 87)

** Pages 88-89 > Table 10 - Summary - Tracking and Location Systems

** Pages 100-106

Pages 107-109 > Conclusion