Depth test. see deeper

The deepest reach

The new GPR Roteg is constantly being upgraded. Now we bring longer maximal time window. But it’s tricky to document the depth range. A well-documented site with voids in known depths have to be found and measurements on the surface must be feasible. In previous campaigns, reflections from a depth of 90 m (The Amateur Cave, Czechia), resp. 200 m (Postojna Caves in Slovenia) have been matched to documented caves and cracks. Particular reflections on images from Hranice karst site (Czechia) had also been linked to recently discovered caverns adjacent to Hranická Abyss.

Several geophysicists question reflection from geological interfaces in depths around 500 m and claimed 25 MHz antenna system’s reach up to 40 m at maximum. Surface waves from far metallic objects or filtered-in artifacts were supposed to cause the deep reflection from scientists’ point of view. Nevertheless, such metallic object were not found in corresponding distances! The issue went so far that an article about deep measurements was not published.

To defence our results, we designed a survey in old limestone quarry of Malá Dohoda close to Ostrov u Macochy, Moravian Karst region. Almost vertical walls lines the quarry floor, located 30 m bellow the surrounding terrain, eliminating surface reflections even on long distancies. The wave propagates straight-forward and no reflections from far-located metallic conduits could be sensed.

We set the time window up to 16 000 ns, which should cover up to 1 000 m in limestones. Towing the antenna on quarry surface provides optimal measuring conditions. A 190 m long profile spans whole quarry length, a 28 m long perpendicular profile covers the width. Measurement details were as follows: 25 MHz antenna, 20 kV pulse transmitter, 10 cm trigger interval, 57 260 samples, stacking up to 10 to suppress noise from 4.8 km far Kojál radio transmitter (emitting an overall wattage of 600 kW on frequency interval 87-778 MHz). An one km distant geological section, constructed from deep-drilling cores HV101 and HV102 (on the right), provides comparison to measured data.

The deepest reach

False, air waves from surface objects could theorically arise at times around 200 ns, but not such a feature is seen there. If some were nevertheless reflected, they must have travelled into the area outside of the quarry and couldn’t reach back into the receiver antenna. A power line of high voltage, located 320 m aside our profile, would create a continuous line reflection accross the whole radargram at times around 2 130 ns. Neither this feature was recorded. The image is, then, not affected by the air waves from distant objects. So what is it, drawn in the radargrams?

Strong reflections down to 300 m depth are predominantly generated on carst voids  and cavities. Nearly horizontal interface in the depth of 475 m incides with limestone rock type alterations. The deeper strata is characterised by horizontal layering up to depths of 780 m, below where the combination of sandstone, conglomerate and graywacke should appears. Further down, a granite plume should be seated, possibly forming the interface at depths around 860 m.

Detected interfaces on the radagram complies well with both the stratigraphy from the nearby geological section and the characteristic reflection shapes for the particular rock types and undulated interface below 750 m depth. For time-depth conversion a characteristic value of 0.12 m/ns was chosen as it was calibrated on several known undergroung caves nearby (Amateur and Holstein cave, e.g.). Then, the difference in depths given by radargram comparing to the geological section is about 20 % – in sense GPR brings deeper location of interfaces. The section, however, is located one km from the site and was constructed from, beside others, seismic measurements, which demand velocity estimations, too.

A perpendicular profile is shown on the left and a detail of the voids up to 300 m depth from the previous radargram is on the right.

Perpendicular profileDetail of the voids

Beside measurements in ice, far different to the surveys in rocks, you probably won’t find any radargrams spanning down to a 200 m depth, either on the internet or elsewhere.

We expect this survey results will lead to antagonistic reactions. The raw data can be provided so anyone can assure himself that nothing was artificially added to the data. Only gain, trace averaging and frequency filtering (to eliminate the signal from Kojál radio station) were applied. We don’t mind measuring this or any other, geologically documented profile again to show the GPR Roteg capabilities (any questions, ideas or contact welcomed!).