Sunday, May 30, 2021

Saunalahti hole


I was originally going to explore bigger caves in Nuuksio this weekend with Jarmo, but I forgot that I was dogsitting Olli's dog while they were moving. I couldn't take the dog to long hike and boulder fields, so ended up doing nice walks in the seafront during the weekend. One of those walks to go see the Saunalahti hole, a small crystallization hole in an otherwise solid ground rock.

I also recorded a 3D model of the cave, available here in GLB format and here in STL format. Or click this to rotate the model on your screen :-)

I've been here before as well, with Janne, some years ago.

Saunalahti is also a wonderful place to walk around. Just look at the forest and sea views, or the random sheep that I run into on the walk:

Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

Sunday, May 23, 2021

Herttoniemi boulder underside


While visiting (again) the secret Margarine factory tunnel, I  came across a small boulder cavelet in Herttoniemi.

Not big, maybe 2x2 meters, but counts as a cavelet, i.e., a mini cave. Or in other words, it counts as a cave for people who are starved of real caves, like we are in Finland.

Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2021 by Jari Arkko. All rights reserved. Thanks for Helene and Touko for accompanying me to the cave!

Airplane meal... on the ground!

I've been missing this... or have I?

More flying stories at Photos and stories (c) 2020 by Jari Arkko. All rights reserved.

Sunday, May 16, 2021

Avalanche transceivers as cave location devices?


A while ago, Bo Lenander asked a question in the Swedish Caver's Association if avalanche transceivers could be used as cave location beacons. As I have such transceivers, I started looking into this...


Radios have been used in caving and cave research for both communication and determining locations. Communication is needed for safety and other reasons. For instance, emergency situations, exploration progress and estimated return times to be communicated, etc.

Pinpointing locations is also a safety matter. But accurate measurement of locations is also useful for constructing cave maps. Underground map construction relies on sequences of measurements where small errors may accumulate over long distances, and being able to pinpoint a particular place in the cave allows parts of the map to be fixed with respect to their ground location and GPS.

Cave radios typically operate in low frequency areas, as higher frequencies are more easily absorbed by rock. For instance, the cave radios reported in [Jones] operate at 88 Khz and 10W, and System Nicola Mk2 radios [Naylor] at 86.95 Khz (also 455 Khz) and 3W. The devices fall in the category of short range communication [ETSI 300 330], but the radios can pass through even hundreds of meters of rock.

Avalanche transceivers are emergency devices operating at the 457 Khz frequency at 0.1W [ETSI 300 718] [Hereford]. The frequency corresponds to 656 meter wave length, and space 456.9-457.1 has been allocated for emergency location devices in [ERC 70-03]. A transmitting device sends a signal once per second, with the signal lasting at least 70ms and break between signals being at least 400ms.

Contrary to cave radios, avalanche transceivers are consumer devices, are mass manufactured, and come with protective enclosures needed to protect them for water and humidity. These devices appear to have a useful range in open air somewhere between 50 and 100 meters. Given that the devices operate with a much longer wavelength than their range and they use small antennas, their operation is largely in the near field and based on magnetic fields.

The basic avalanche rescue process involves a finding the strongest signal strength on the terrain plane, and then using metal probes for the final and most accurate pinpointing of a buried victim. Older devices simply provide an audio signal, but newer devices may provide some additional distance or direction assistance to help the transceiver search.

The question posed in this article is: Can avalanche transceivers be used for communication to and from caves, or localization functions? Yes or no? And if yes, at what limitations?

Other work

It has been difficult to find articles about the use of avalanche transceivers in caves, even though there are some articles.

Frank Reid tested unmodified and modified avalanche beacons in 1988 [Reid]. At this time, the avalanche beacons used an earlier frequency, 2275 Khz instead of the current 457 Khz. Reid found that free space range in those beacons was in practice around 30 meters, but with different antenna arrangements this could be made up to 100 meters. Tests in Buckner cave were able to find a transmitter 17 meters underground.

Lou Dawson reported that in the history of making the first avalanche beacons in the 1960s, some of them were actually used for sewer and cave passage location tracking by the city of Lockport [Dawson].

Atanas Rusev recently reported "how he has used them for finding new entrances into large cave systems, and to identify dig locations for joining two caves", but unfortunately we have been unable to find a copy of the article [Rusev].

Absorption of electromagnetic transmissions in materials is of course in general well understood, see, e.g., [ITU] for a good summary of the relevant physics and equations. The equations are, of course, highly dependent on the nature of the materials, e.g., their permittivity and conductivity.

Unfortunately, while there are a lot of sources listing parameters related to absorption in the literature,  most of them do so only for construction materials [NIST] [Ofcom] [ITU] [Agilent] [RFCafe] [Achammer], not natural materials like different types of rock. Most literature also focuses on shorter wave lengths that are used for common communication (GSM, LTE, etc.) William Pullen's report from 1953 provides some information, such as, "attenuation measurements made with transmission lines antennas buried in soil indicated 7.5 db at 350 Khz to 11.7 db in 600 Khz" [Pullen], but it isn't clear if these numbers are useful for our case.

Tunnel experiment 1

A man-made tunnel in granite bedrock in Kauniainen was used for an initial test. In the very first test we used a 6 meters wide tunnel that goes in a circle, horizontally, around an area of rock. The rock area in the center of the circle is roughly 35 meters in diameter. A transceiver in transmitting mode was placed on the rock wall in one part of the circle, and then we attempted to measure the signal with a transceiver in reception mode as we passed through the circle alongside the center wall. See Figure 1 below.

Measurements with the devices are difficult, as no absolute reception level is indicated, exact accuracy of a given device being unknown, and the method at which the signal is presented in the software-driven user interface is also unknown. Nevertheless, the presence of a signal can be detected, and some indication of the strength and direction of the signal is given in the user interface. False positive signal reception is possible but in the author's experience relatively rare and random. 

The result from the first experiment in the tunnels is that strong signal was received in areas close to the transmitter (< 15m), moderate strength was seen beyond that but very weak/spurious signal was received behind the center rock. But it was still received. Unfortunately, not much can be determined from this experiment for any of the measured distances. This is because (a) false positive signals can not be entirely ruled out and most importantly, (b) wave guide propagation along the tunnels and around corners cannot be ruled out.

Tunnel experiment 2

The second experiment in the tunnels provided more information. This experiment was performed in the entrance tunnel, a tunnel that descends gradually while the ground above rises. One test was made 25 meters from the tunnel entrance, and another 55 meters from the tunnel entrance. See Figure 2 below. Tunnel height was 5 meters. Distances and gradients were measured using a DistoX laser and digital inclinometer device [DistoX].  Rock depths have been calculated based on this input and are approximate.

The tunnel exit might allow the transmissions to escape. However, no reception was determined at or above the exit, or on the way towards areas where reception was determined, so we believe that alternate signal path and reflections were unlikely.

The result is that a moderate/weak signal was clearly detected at the 25 meter location, above ground and with approximately 8.4 meters of rock in between to the transmitter. This signal was confined to about a 10-meter wide circle in the ground and was not present elsewhere. At the 55 meter location (12.4 meters of rock and altogether 17.4 meters away) a very weak signal was detected. Again, it cannot be shown that this was not a false positive signal, but while the signal was not continuously detectable, it was only detectable in a given area, so the current hypothesis is that it was an actual reception of the transmission from underground.

Cave experiment

We travelled to Finland's largest karst cave, Torhola in Lohja [Torhola]. This cave spans horizontally 30 meters and vertically 11 meters, but has over 100 meters of cave passage in a complex multi-level structure. The bottom parts of the cave are far away from the entrance, both in in vertical and horizontal direction. Yet they are relatively close to surface, as the cave begins from the top of a hill, and descends downhill towards a lake.

On a previous visit, we had looked at a small side passage at the bottom part of the cave. This passage might be at near the surface. On that visit, hitting rocks with rocks could be heard on the surface, but pinpointing the location where the sound was coming from was difficult.

On the experiment visit, we placed a transmitter beacon at the entrance of this side passage (the passage itself was flooded at the time). See picture below.

Back on the surface, a search was made to find the lowest display reading (i.e., nearest to the transmitter) at the xy-plane. This search was quite easy, as the indicated strength/distance numbers changed quickly and consistently from all directions towards the minimum. See the picture:

The best location for signal reception was roughly in the middle of this picture, in the ground few tens of centimetres from the bottom of the rock step:

The lowest readings, in the range of 11-17 correspond to being closer than 2 meters, when I've used the device for measuring in free space.


It clearly is not possible to use avalanche transceivers for long-range communication, given the limited power and used frequency. In fact, without modification these devices would not be usable for any communication, as they have no means to carry any user-settable signal (although this may be changing with supplementary vitals data link [W-Link]).

However, perhaps surprisingly they do appear to be usable as localisation device for caves at small depths (< 15m). The author has experimented with this in Finnish caves that are typically small and may never exceed that limit. 

Further experiments would be useful. We did not test the effect of antenna orientation, for instance. There seems to be also a big difference between different beacon products, and the ability of their user interface to provide information also differs.

Experiments in larger caves would also be very useful. For instance, the Swedish Lummelunda cave is relatively close to the surface, despite having several kilometers of tunnels. Lummelunda's depth (~20m) may exceed the performance of the avalanche beacons, however, even if the environment may be easier with less concrete, electrical wiring, and human settlements than in the tunnel in our experiment.

Further work would be useful to calculate the theoretical limits of 457 Khz communication through rock. Measurements relating to different rock types would also be useful.

Finally, it is still unclear if the results are relevant. Clearly, purpose-built cave radios and and localization equipment is far superior. However, it appears that in very limited cases readily available consumer devices may also be used. The author was able to pinpoint a location of a recently discovered tunnel part in the Torhola cave using the avalanche transceivers.


These experiments were inspired by Bo Lenander's questions [Lenander] -- thank you. Thanks to the city of Kauniainen and Nina and Markku for access to the tunnels for my measurements, for Ralf, Per, Tor, and Jarmo accompanying me on the first Torhola cave tour, for Helene and Touko for accompanying me on the second tour, and Jarmo and Tero for discussions in this problem space.


[Achammer] Achammer, T. and  Denoth, A., "Snow dielectric properties: from DC to microwave X-band". Ann. Glaciol. 19, 92–962, 1994.

[Agilent] Agilent, "Basics of Measuring the Dielectric Properties of Materials". Application Note.

[Dawson] Lou Dawson, "SKADI - First avalanche rescue transceiver beacon". August 2013.

[DistoX] Beat Heeb, "Paperless cave surveying".

[ERC 70-0] ERC, "Relating to the use of Short Range Devices (SRD)". ERC recommendation 70-03.

[ETSI 300 330] ETSI, "Short Range Devices (SRD); Radio equipment in the frequency range 9 kHz to 25 MHz and inductive loop systems in the frequency range 9 kHz to 30 MHz; Harmonised Standard covering the essential requirements of article 3.2 of the Directive 2014/53/EU".

[ETSI 300 718] ETSI, "Avalanche Beacons operating at 457 kHz; Transmitter-receiver systems; Part 1: Harmonised Standard for access to radio spectrum".

[Hereford] John Hereford and Bruce Edgerly, "457 KHz Electromagnetism and the future of avalanche transceivers".

[ITU] ITU, "Effects of building materials and structures on radiowave propagation above about 100 MHz". Recommendation ITU-R P.2040-1, July 2015.

[Jones] Herman Jones, "The cave radio".

[Lenander] Bo Lenander, "Enkel grottpejl??".

[Torhola], Suomen Luolaseura. "Torholan luola". The Torhola cave entry in the Finnish Caving Association's database.

[Naylor] Graham Naylor, "System Nicola Mk2".

[NIST] William C. Stone, "Electromagnetic Signal Attenuation in Construction Materials". NIST, October  1997.

[Pullen] M. William Pullen, "Geologic Aspects of Radiowave Transmission", Report of Investigations no. 162, State Geological Survey, pp 19-20, Illinois, 1953.

[Ofcom] Richard Rudd, Ken Craig, Martin Ganley, and Richard Hartless, "Building Materials and Propagation". Final report, Ofcom. September, 2014.

[Reid] Frank Reid, "The Easiest Cave Radio: Extending The Range of Avalanche Beacons". Volume 3 #2 June 1988.

[RFCafe] RFCafe, "Dieletric Constant, Strength & Loss Tangent".

[Rusev] Atanas Rusev, "Radiolocation, Using Avalanche Beacons in Cave Exploration". September 2019.

[Tsuchiya] Toshio Tsuchiya, "Absorption (A) and Transmission Loss (TL) in Seawater".

[W-Link] Wikipedia, "W-Link".

This article has also been published at TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogsspot or TGR. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

Kauniainen is melting!

Kauniainen ski hill is melting! Just a few days ago it was almost completely skiable, but now... just two plots of snow left.

Another local skier, Jussi Vuokko, skied the hill on May 10, and made a comparison to 2020, the situation was much better. In 2020, there was just a two meter plot of snow left. What a remarkable difference!

The good news about this year though, is that one the plots of snow left now is a big mound... it will surely last until June... but what about July?

This article has also been published at TGR. Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos, videos, and text (c) 2021 by Jari Arkko.

Saturday, May 15, 2021

Volvo is back! (But how long?)

Volvo is back! Refueled, washed, with summer tires and in sunshine it feels like a decent car now 🙂 

And only 250 or so for the repairs, including cannibalising parts from the shop owners own Volvo, rest of it went to the junk yard 🙂 But my lights work now, without having to pull fuses after every stop.

Read more car stories in the Planetskier series at Blogspot. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

The long walk


Long walk... enjoying the spring weather while on calls... also picking up my car from the repair shop :-)

Read more car stories in the Planetskier series at Blogspot. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

And now the loaner car broke down!


Now the loaner car is broken and needs to be towed to the shop that already is repairing my Volvo. (Should I ask for another loaner? Should they give me one?)

Read more car stories in the Planetskier series at Blogspot. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

New driver's license!

Youngest son gets a driver's license and a shiny new ... key to the blue rust bucket 🙂 

I will be viewing things now on from the back seat when my personal driver takes care of transport. The same Volvo btw has now been used by three generations from the ex father-in-law to me and now the son. Although I do wonder how long the car is going to last now, all pieces ready to fall apart already. Any bets?

Read more car stories in the Planetskier series at Blogspot. Photos and text (c) 2021 by Jari Arkko. All rights reserved.


Loaner, because they haven’t made the part that my Volvo needs in 10 years

And the car is broken as you saw here...

Read more car stories in the Planetskier series at Blogspot. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

Vihti. And then the car breaks down.


Clutch failure. Stranded in Vihti... waiting for the call from towing company ...

I had been skiing in Vihti. Felt good... then as I was leaving, disaster struck. The clutch pedal lost all pressure and car wouldn't go any longer.

To add to the injury, the towing company claimed that they need an extra 100 € for towing from Vihti to Espoo, as the insurance wouldn't cover it. Well... I had no choice. These old cars can't be repaired at all shops. Plus I needed to get back home.

Magnificent skiing views:

This article has also been published at TGR. Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos, videos, and text (c) 2021 by Jari Arkko.

The front... mirror fell off. Fixing it.


Look at my professional repair operation!

The mirror that had fallen off... and glued with different kinds of glue... now glued with silicone. Not really meant for this job, but it was the only gluey stuff that I had home that I had not yet tried.

But I am particularly proud of the rubber band and tissue -approach to holding all this in place while the glue sets!

Read more car stories in the Planetskier series at Blogspot. Photos and text (c) 2021 by Jari Arkko. All rights reserved.

Monday, May 10, 2021

3D Model of the Torhola Cave

Digitalisation of caves is proceeding! :-) I now have the first model from a bit larger cave, the Torhola cave in Lohja, southern Finland. This cave is Finland's largest karst cave, with around 100 meters of complex tunnels on 3-4 levels.

I’ve been using the new iPhone LiDAR sensors to build detailed 3D models of caves. This is the first bigger project in this series. I have hit the practical limits of the iPhone software for modelling complex structures, so the model has been recorded in four parts, which were then joined together in post-processing in a computer. On the iPhone I used the native sensors and cameras, and the Polycam software. On the computer I used Blender.

The picture above is a static side view of the model as if you were able to observe the cave from outside, but there’s an actual full 3D model behind, including textures so one can also peek inside and see what the tunnels look like etc. 

This model is a composite of four parts, turned out to be difficult to scan all in one go. This should be useful for understanding the cave, drawing cross cuts and maps, or peeking inside. But this is still very experimental, none of the softwares were really meant for this, it is difficult to get good texture video in very tight passages, there’s overlap from the different parts, and the model is also too large (600k pieces, lots of detail, but chokes my browser if I try to rotate it).

The overall process for creating a 3D model of a cave is as follows:

  1. Use the iPhone's capabilities and the Polycam application to record a 3D model of the cave you are in. Start by creating a new model, then "record" the scene, and when finished, perform the Polycam processing as a "space".
  2. If necessary, continue the model collection by using either the "extend" capability in Polycam or create additional models.
  3. Export the resulting model(s) by using Polycam's export capability, e.g., to GLB format. Note that you will need use the non-free version of Polycam to do this. Take care of export the kind of data you need in post processing and the final model; GLB for instance includes both the model and the texture data recorded from the videos.
  4. Import the model into suitable post-processing software. Perform any corrections, merge different parts together, etc. that are needed.
  5. Use the model and the post-processing software to build any final results that you may wish generate, e.g., cross-cuts, maps, measurements, simplifications (decimation), and so on.
  6. Publish your results.
But this is still difficult. Some of the problems that I encountered:
  • Collecting high-quality video and LiDAR data is difficult in cramped cave tunnels. The sensors work best when there's a reasonable and consistent distance to the photograped walls, but I found out that I'm often too close, just centimetres from the surface.
  • Due to the nature of the complex cave, there will be a lot of situations where a "hole" is left in the model, e.g., because a cavity extends further out than the sensor can reach, and it will not be possible to crawl into the cavity. These holes cannot be distinguished algorithmically from cave entrances, and they make the processing difficult. In particular because the holes appear like fractals in small and large scale.
  • In the course of recording the Torhola cave, I experienced one failure to use Polycam's capability to extend a model; possibly due to leaving the continuation point to a tight spot where it was difficult for the software to recognise an earlier recorded part. But I've also seen this issue in some other situations, and I sometimes had to return to the starting spot of a recording. This is of course impractical in caves.
  • I also experienced one case of a firmware or sensor problem, where the iPhone indicated that I do not have rights to read the sensors. This happened in the middle of recording a part of the cave.
  • It is incredibly difficult to align manually different recordings together. I left on purpose some forms in overlapping parts that would ease alignment, but it is often difficult get the alignment done well along all axis.
  • The overlap itself is difficult to remove; my current models contain overlapping segments because of this.
  • The models are big, on the iPhone the raw data for Torhola was 4GB and over a million faces. Post-processed with Polycam, it was only 65 MB but that's still large (600k faces).
  • The large models can't easily viewed on the web interfaces using software such as model-viewer, even if they can be rotated and modified on dedicated software such as Blender.
  • Operations needed for cross sections and other useful things in Blender are designed primarily for objects. They seem to operate rather poorly for the spaces that caves represent (more than objects). I have not determined the exact causes for this, but I suspect it is due to the algorithms in Blender's Bool tool that determine what's "inside" and "outside" of an object. 
But, the results are still tremendously interesting and illuminating. And this is a fun thing to experiment with. I’ll keep exploring if this can be made to work more fully.

The raw model for Torhola is available also, both in GLB and Blender formats. Do note that these models are being processed and fine-tuned; as noted they also have several issues. On my list of things to do next is various cross-sections (see here for an example from another cave).

I have also started to maintain a directory of all 3D models I have from Finnish caves. The directory can be found here. There's also a similar directory of all cave maps, here. Both models, maps, and all other information from Finnish caves is also directly integrated to the Psgeo mapping service that the Finnish Caving Association uses. The most recent version of the software (with support for 3D models) can be accessed here. For an example of how the models are maps are visible in the map, click on this.

This article has also been published at TGR. Read more urban exploration stories from, and other underground stories from Read the full Planetskier series at, or all blog articles from Blogspot or TGR. Photos and text (c) 2021 by Jari Arkko. All rights reserved. Thanks for Helene and Touko for accompanying me to the cave!