2007-01-09_REPORT - C1980007

• Permit No: C1980007
• IBM Index Class Name: REPORT
• Doc Date: 1/9/2007
• Doc Name: 3rd Quarter 2006 Seismic Monitoring Report
• From: Mountain Coal Company
• To: DRMS
• Permit Index Doc Type: Stability Report
• Email Name: TAK
• Media Type: D
• Archive: No

Velocity Model
The largest contributor to reliable location of seismic events is the velocity model utilized in the
hypocenter determinations. The model is typically defined for compressional wave arrivals and
may take several different forms depending upon the location algorithm utilized. In this report, we
use a layered-earth model as input to the location program. Using a layered-earth model,
velocities are constant over certain vertical intervals and change at discrete horizontal boundaries.
The travel path for this model is a series of line segments with abrupt emergence angle changes at
velocity boundaries.
Table 3 Preliminary Velocity Model
Layer Top
0.00
0.07
0.27
0.37
1.50 _
2.50
6.00
Velocity in km/sec
2.21
2.73
3.01
3.18
3.96
4.40
6.00
Velocity in ft/sec
7470
9227
10174
10748
13385
14872
20280
The velocity model given in Table 3 was generated for use in a very detailed study at West Elk in
2005 (Swanson and Koontz, 2006). Note that for that study the stations were directly above a
mining panel and all rays were assumed to be direct. At larger distances refraction of rays from
deeper layers will occur and the deeper layers will be more important. As additional work is
completed the velocity model will be refined. Additionally, the large elevation differences between
stations (Table 1) will be accounted for by station corrections.
Peak Ground Acceleration
The sensors used in this array are three-component accelerometers. The sensors report the
acceleration of the ground at their installation point. The complete characterization of the ground
motion (what it was doing at every point in time) is called a time history of the event at that station.
Figure 2 is an example of a time history for a typical event recorded by this network. Time
histories are often used by structural engineers to quantify the performance of a structure if the
ground at or under the structure should experience that time history (intensity).
In order to summarize a time history a single measurement of ground motion is often used. While
many measures (summaries) of a time history are possible (peak particle velocity, duration of
motion, etc.) an often used 'summary number' is peak ground acceleration. To calculate the peak
ground acceleration from Figure 2, one would form the square root of the sum of the squares of
the three orthogonal accelerations for each time point (resolving the amplitude of the vector of the
ground motion) and pick out the largest. That value, usually reported in percent of the vertical
acceleration due to gravity, is the peak ground acceleration.
None of the summary designations are completely satisfactory (for instance, the frequency of the
ground motions is important) but peak ground acceleration has the advantage that most persons
can relate to it we all contend with 100% vertical acceleration at all times. Thus we can