Published in SEG 2013 DAILIES
Seismic data acquisition systems today come in many telemetry variations: cabled (with real-time data return); cableless (blind local recording); cableless (local recording with radio frequency [RF] status return); and cableless (real-time RF telemetry). With the large channel-count active spreads in today’s prospects, it is essential that known good data be continuously acquired. Real-time systems are best able to provide a timely and low cost guarantee of data quality.
It is well documented that cableless systems provide significant savings in total weight of he deployed equipment on the ground, with a corresponding improvement in the HSE footprint of the system. The question has remained, until recently, if cableless RF telemetry could deliver the same high throughput, real-time data as cabled telemetry systems in difficult field conditions. A recent field test by Wireless Seismic, Inc. showed that this question has now been answered in the affirmative.
A test site was secured in the Sam Houston National Forest north of Houston, Texas, to evaluate the effectiveness of Wireless Seismic’s RT System 2 in forest and heavy brush at long station spacing. The basic station interval was 122 meters (400 feet). By skipping stations, the acquisition system was forced to use 244-m (800-ft) or 366-m (1,200-ft) intervals during parts of the test program. The test spread was set up as two lines of 22 channels, each line configured as one part of an “L” on the ground. The two lines together formed a 1.2- by 1.2-km (0.75- by 0.75-mile) square cell that might be used in a large microseismic program.
The test was focused on the ability of the RF solution to work with no loss of data in heavy brush and forest. Another test program was completed last year that showed the ability of RT System 2 to operate at high channel counts (10,000 channels). Logistics dictated the small channel spread for testing in the forest.
The test comprised one hour of continuous recording, in 15-second records. A small weight drop source was used to provide signal to see on the records. The weight drop was not synchronized with the system, so the signal came in at random locations in the record.
The first 30 minutes of continuous recording during the test were carried out using the basic 122-m station spacing. A point on the line was then picked that was known to be in an area of heavy brush, and one station was intentionally skipped, which resulted in a station spacing of 244 meters at that point in the line. Ten more minutes of continuous data were acquired with no loss of data. Another point in the line of heavy brush was then picked, and two stations were disabled. The spread then contained one 244-m station interval, and one 366-m station interval, with all of the other stations at 122 meters. Ten minutes of continuous data were acquired, again, with no loss of data.
A key component of successfully deploying real-time RF systems is the tools provided to monitor the quality of the RF links. RT System 2 provides the observer with real-time measures of the bandwidth utilization on all the radio links, which subsequently lets the observer know if the system was getting close to the point where data was not able to get through the foliage and underbrush at particular locations. The system automatically adjusts the transmit power to maintain a high-quality radio link.
In no cases during the test did the bandwidth utilization on any radio link exceed 43% of capacity while using a 2-millisecond (ms) sample rate. The bandwidth utilization increased to a maximum of 63% when a 1-ms sample rate was tested. In no case was a link oversubscribed, which would have resulted in a delay in collecting data. Since the system has an automatic retry mechanism, even when a radio link is stressed to the maximum, eventually all the data will find its way back to the recorder.
Real-time RF data return systems are now a reality for large channel count and demanding terrain. For further information, visit Wireless Seismic at SEG stand 712. To learn more about the system, visit www.wirelessseismic.com.