How to Record and Analyze Structural Measurements in a Mobile Stereonet

In structural geology, mixing up planar orientations with linear orientations causes severe interpretive distortion in equal-area projections. In RockGeo, every feature is explicitly typed:

• Planar Features (bedding S0, foliation S1, faults, joints, dikes): defined by three linked spatial attributes:
  • Strike: the bearing of the horizontal intersection line between the geologic plane and the horizontal reference plane (000° to 360° or quadrant format).
  • Dip: the maximum angle of inclination measured downward from the horizontal plane (0° to 90°).
  • Dip Direction: the azimuth of the horizontal projection of the true dip vector (perpendicular to strike, strike + 90° under the Right-Hand Rule).
• Linear Features (mineral lineations, fold axes, fault slickenlines L1): defined by two directional attributes:
  • Trend: the azimuth bearing of the horizontal projection of the line (000° to 360°).
  • Plunge: the vertical angle that the line makes with the horizontal reference plane (0° to 90°). Never confuse plunge of a line with dip of a plane.

RockGeo allows you to toggle data entry conventions at any time to match your field crew's workflow:

To add structural attitudes inside a field station:

1. Tap + New Measurement in the outcrop Structures tab.
2. Select the structure type: Foliation, Joint, Bedding, Fault, or Lineation.
3. Manual entry (recommended for critical accuracy): type the values read from your mechanical field compass (e.g., Clar 120/45).
4. Sensor entry (digital compass mode): place the edge or back of the smartphone flush against the rock plane and press Lock Reading. The app reads inertial sensors and the onboard magnetometer to compute instantaneous attitude.
5. Add qualitative field attributes: mineral filling (quartz, calcite), vein thickness, joint aperture, or roughness (JRC).

When you view your project stereonet, all readings are projected onto the lower hemisphere of a Schmidt equal-area net (Lambert projection):

• Poles to planes: each plane is projected as a discrete point indicating the normal vector perpendicular to the geologic surface.
• Great circles: circular arcs illustrating the plane intersection with the reference sphere.
• Statistical Kamb density contours: for datasets with more than 20 discontinuities, the Pro tier plots statistical contour lines (Kamb density isolines), allowing you to objectively identify joint sets (J1, J2, J3) without visual clustering bias.
• Fisher spherical mean vector: RockGeo calculates the spherical mean vector for each joint set along with its 95% confidence cone (α₉₅), essential for geotechnical design and geological reports.

For open pit mining, highway cuts, and civil engineering excavations, RockGeo includes preliminary kinematic screening tools based on Markland (1972) and Hoek & Bray criteria:

1. Input the slope face attitude (e.g., strike 045°, dip 70° SE) and the estimated basic friction angle of the discontinuities (φ, e.g., 30°).
2. The app overlays the daylight envelope, friction cone, and kinematic daylight windows:
   • Planar Sliding: poles to discontinuities that dip toward the daylight window with an inclination steeper than friction and gentler than the cut face.
   • Wedge Failure: intersection lines between two fracture sets that plunge daylighting out of the slope face.
   • Flexural or Direct Toppling: steeply dipping layers or joint sets dipping back into the slope rock mass.

While modern phone magnetometers and gyroscopes are fast for rapid orientation recon, field geology demands awareness of physical limitations:

• Local magnetic interference: proximity to banded iron formations (BIFs), magnetite-rich basalts, high-voltage lines, field vehicles, or even your steel rock hammer causes severe digital needle deviation.
• Figure-8 calibration: calibrate the phone magnetometer regularly by rotating the device in a smooth figure-8 motion before taking outcrop readings.
• Mandatory mechanical cross-checks: for dam foundations, tunnel portals, pit wall stability, and critical infrastructure, always verify representative structural orientations with a calibrated traditional mechanical compass.