GPR Environmental & Geological Considerations 3

Question 1

What approximate GPR signal velocity is commonly used as a starting estimate for dry sand?

Accepted Answer

0.15 m/ns

Answer

0.03 m/ns

Answer

0.10 m/ns

Answer

0.30 m/ns

Question 2

How do underground voids (air-filled cavities) appear in GPR reflection profiles?

Accepted Answer

As a strong positive reflection from the roof followed by a strong negative reflection from the floor of the cavity

Answer

As areas of signal absence because air does not reflect radar

Answer

As diffuse low-amplitude zones spread across many traces

Answer

As continuous horizontal reflections identical to soil layer contacts

Question 3

In heterogeneous glacial till with mixed boulders and fines, what challenge does the GPR operator face?

Accepted Answer

Boulders create diffractions that may obscure continuous layer reflections and complicate interpretation

Answer

Signal penetration is always maximized because glacial till is resistive

Answer

Glacial till has no measurable effect on GPR data quality

Answer

The till's uniform composition simplifies target identification

Question 4

What geological condition is most favorable for using GPR to map a shallow water table?

Accepted Answer

Fresh-water table in clean dry sand or gravel

Answer

Saline aquifer in clay-rich sediments

Answer

Brackish water in organic peat

Answer

Perched water in fractured granite with clay infill

Question 5

How does the presence of a shallow, highly conductive layer (e.g., a brine-saturated horizon) affect GPR survey depth of investigation?

Accepted Answer

It acts as a near-total electromagnetic barrier, sharply limiting penetration to targets below it

Answer

It focuses the signal downward, increasing the effective depth of investigation

Answer

It creates a reflection that can be used to calibrate depth estimates for deeper features

Answer

It has no practical impact on survey planning or data interpretation

Question 6

When surveying over a site with varying topography, why is terrain correction important for GPR data interpretation?

Accepted Answer

Without terrain correction, sloped surfaces introduce apparent dips in reflectors that do not reflect true geological structure

Answer

Slope changes affect only horizontal position, not reflection depths

Answer

Terrain correction is only needed for marine seismic surveys, not land GPR

Answer

Topographic variation has no effect on GPR two-way travel time measurements

Question 7

Which environmental condition most significantly reduces GPR antenna coupling efficiency and signal injection into the ground?

Accepted Answer

Dry loose gravel surface with large air gaps between the antenna and ground

Answer

Slightly moist topsoil with εr ≈ 10

Answer

Dense compacted sand with εr ≈ 5

Answer

Thin vegetation cover on uniform loam soil