Continuous recording cableless systems have many benefits but potential drawbacks arise when there is inflexibility in data collection.
Out of all recorders on the market, only Sigma offers wireless and cable telemetry-based real-time recording (RTR) options, doing away with any need for data harvesting. But when RTR is not being used, Sigma also offers the most choices in data retrieval. This is essential because flexibility in harvesting is the key to high productivity and data security.
If the recorder is a shoot blind type and data can only be harvested by loading ground boxes in a download rack, the problems can be significant. Up to one third additional channels may be needed to cover when boxes are in the staging location and not on the line acquiring data. The first time the observer sees data may only be after collection of ground units. Delays in collecting up boxes also adds to the risk of hardware damage or theft, and data loss. In some cases, the observer may know nothing about this for many days.

The situation can be even worse for ground units which rely on internal batteries. It may only be possible to recharge such batteries when they are within certain temperature limits, further reducing use of equipment for data acquisition or affecting harvesting operations.
Sigma Acquisition Units (SAU) use external batteries doing away with any need to collect up boxes simply for battery recharging, and no SAU needs to be taken off the line just so that data can be harvested. The result is a system which can be left deployed and active for as long as necessary and then moved direct to the next receiver location, with data downloaded using a variety of methods while recording continues. No need for extra equipment, no need to stop production.
The simplest harvesting method requires only a data cable connected between the SAU's Ethernet port and a standard laptop PC with Sigma harvesting software. This feature is included in the basic Sigma set up at no extra cost.
The next two methods use optional omni-directional WiFi, installed either inside the SAU or as an external plug-in option. The advantage of the former is that there are not external parts; the advantage of the latter is that the operating range is extended. Harvesting then is just a matter of passing-by the SAU (by foot, quad, truck, boat, helicopter etc) with a Wi-Fi enabled harvesting PC. To save power, Sigma's Mesh radio Network is used to switch on the Wi-Fi just when it's needed.

Another method uses ruggedized USB memory attached to the auxiliary port of the SAU. On connection, the external memory takes a copy of the data as selected by an ASI file. Data is then copied from USB to Sigma processing computer. Lastly, Sigma also offers rack-based download as an option - though we are pleased to note that few bother to take us up on it. Any mixture of harvesting techniques can be used as software keeps track of what has been retrieved. Transfer speed is very fast, only being limited by the read-speed of the internal SAU memory.
To make the process even more efficient, all downloading uses iSeis's "Smart Harvesting" TM capability. Unlike systems which may have to copy gigabytes of continuously recorded data to the harvester before searching for actual seismic files, Smart Harvesting takes just the relevant data from SAUs. For some operations this can accelerate download times by factor of ten or twenty.
A final harvesting advantage is that Sigma is not limited to just 2 or 4 gigabytes internal memory. SAUs come with 8 GB which may be extended to the largest sizes available in CF memory (currently 64 GB for industrial grade cards) while actual memory address space limit is 2 TB.

Flexible data retrieval makes or breaks a cableless recording system. Only Sigma offers this range of harvesting options.

Jerry Blaxton (Sigma user, June 2011): "The most impressive aspect of the experience processing data from Sigma is the inherent improvement in data quality by reducing human errors. This is accomplished by encoding all surface location data into the "headers" of the seismic traces. There are no "geometry" errors. Geometry errors are significant in that they cascade errors through the processing flow. Remove the errors and improve data quality. Furthermore, I have processed many lines with far less critical information available through a traditional OB report. This system significantly raises the confidence level for data integrity. The ease of inputting the individual recordings into my processing system was a welcome, pleasant phase. Sometimes, simply getting the data from field into a processing computer is fraught with troubles, not here, thankfully."