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

Dry sand has a typical dielectric permittivity of 4–6, yielding a GPR velocity of approximately 0.13–0.15 m/ns, often rounded to 0.15 m/ns as a field starting estimate.

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

An air-filled void creates a strong impedance contrast at its roof (positive reflection) and floor (negative reflection due to reverse polarity when going from low to high permittivity material), producing a characteristic double-reflection signature.

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

Large cobbles and boulders in glacial till act as point scatterers, generating numerous overlapping hyperbolic diffractions that can obscure stratigraphic reflections and complicate 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

A fresh-water table in clean resistive sand or gravel provides a strong dielectric contrast at the capillary fringe and low background attenuation, producing a clear GPR reflection of the water table.

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

A brine-saturated or highly conductive layer absorbs nearly all incident electromagnetic energy, effectively blocking any GPR investigation of features below that horizon.

GPR - Ground Penetrating Radar Specialist Certification Practice Test

GPR - Ground Penetrating Radar Specialist Certification Practice Test

GPR Environmental & Geological Considerations 3