Terraloc Data Troubleshooting
These pages provide examples of common data problems encountered during seismic survey, and how to resolve them.
If the problem is that no data shows on screen, you can jump to the “No Data” section HERE
Missing Trace
A missing trace will look like this (Trace 1):
– Check the Trace menu (shortcut x) to ensure it is enabled.
– Check the geophone(s) connection.
– Check pins/receivers in connectors.
Bad Traces
Examples of bad traces are shown below
– Check the geophone on that take-out is connected correctly.
– Check the geophone is upright.
– Check the geophone is firmly planted in the ground.
– Check the function of the geophone on that take-out by swapping it with another.
Poorly Coupled Geophone
A poorly coupled geophone will generally result in a “bad trace” (see 10, 12, 14, 15 & 17 in the example).
– Pull the geophone out and push back into the ground close to its original location.
– Ensure it is pushed in vertically – pushing it in at an angle, and then straightening, loosens the coupling with the surrounding material.
– If the ground is hard, it might be necessary to manually make a hole for it, but do not make the hole too big, the geophone should be contacting the ground on all sides.
– If the ground is hard consider using geophone tripods instead of spikes.
Incorrect Geophone Polarity
– Check the polarity flag in the Receiver Spread menu (press 5 on front panel).
– Check the connection is correct on the cable (it should be very hard to connect the wrong way due to the asymmetric design of the take-outs and clips).
Reversed Cable
A reversed cable will produce a strange looking pattern of first arrivals across the traces. With a 2-cable layout it will normally produce a distinct “step”.
With single cable, first arrivals might appear to be getting earlier, further away from the shot.
– Physically flip the cable.
– Alternatively remap the take-outs to the correct traces in the Receiver Spread menu (press 5 on front panel).
Not Enough / Too Much Delay
First arrivals will either appear too-far down the trace, or first arrivals will be lost somewhere ‘above’ the start of the display.
– Check the pre-trig/delay in measurement parameters menu (press 2 on front panel).
– To move the first arrivals down the trace, reduce the delay or add a pre-trig.
– To move the first arrivals up the trace, remove the pre-trig or add a delay.
Noisy Data
Noisy data can manifest in many ways, but typically there will be lots of activity in the traces before the first arrivals.
– Try more stacks, to average out the noise (increases signal-to-noise ratio).
– Try to make more consistent shots so stacking is more effective.
– Try using a more powerful source.
– Be aware of surrounding sources of noise: stop people walking along the line when shooting; try to time shots when the noise monitor shows lowest levels of ambient vibrations; avoid shooting if trains or vehicles are passing; in extreme cases collect data at night when there are fewer sources of noise.
– If absolutely necessary, a mild filter can be applied (press 4 on front panel): but be aware this can have a negative impact on the result if applied incorrectly.
Insufficient Shot Power
Not having enough power will appear as either noisy data (see previous) because the signal struggles to get above the background noise, or in quiet environments, a simple lack of signal in the more distant traces.
– Try a different display setting to ensure it is lack of power and not a lack of gain (amplification).
– Try increasing stacks.
– Try using a more powerful source a heavier hammer, a “betsy”/”buffalo” gun, an accelerated weight-drop, a vibration source or, if appropriate, explosive charges.
No Data or Just Noise Showing On Screen
There are a number of reasons why no data appears on the instrument screen during acquisition:
– geophones not connected
– spread cable(s) not connected
– incorrect trigger settings
– trigger failure
– measurement boards not detected (check the “Instrument” tab).
If problems persist, contact [email protected] for additional assistance.
Dispersion Curves
Below are examples of good (top image) and bad (bottom image) dispersion curves from the Dispersion Analysis tool. Below these are some suggestions for adapting the analysis settings.
Good data: shows a clear, continuous surface wave train across most or all traces, with signal amplitudes noticeably higher than background noise. In the Dispersion Analysis, this produces a strong, continuous dispersion curve with few interruptions or competing high-energy features.
Bad data: dominated by noise, with weak or absent surface wave energy. Signs include noisy traces before and after arrivals, poor continuity of the surface-wave train, and little or no clear dispersion trend in the Dispersion Analysis. This reduces confidence in the calculated subsurface properties and may indicate poor source coupling, excessive attenuation, or unsuitable ground conditions. The first step is to try increasing the shot power, which should overcome the first two causes.
A default view (based on settings of 5 – 100 Hz, and 50 – 1500 m/s) will work well for most sites. But:
- If the site is very soft and the dispersion is unclear: try 4–50 Hz, 1–500 m/s because the signal may occur at lower frequencies and velocities.
- If the site is very hard and the dispersion is unclear: try 5–100 Hz, 200–3000 m/s because the signal may occur at higher velocities.
- If you still cannot see the fundamental mode: widen the range of values further, until the curve becomes visible, then narrow the values again to tighten the dispersion plot around the curve.
- If no clear dispersion is visible at any setting: check the seismogram, considering the comments above on the “poor” example. Some sites may not produce a visible dispersion curve.