EXPLORATION

Are wireless seismic systems the wave of the future? Or are they a niche system?

 Someday, all seismic acquisition might be done with wireless systems, but until the batteries and electronics weigh a lot less, the equipment is best applied in areas where the logistics of cabled systems are too difficult. 

Perry Fischer, Editor

There’s been a not-so-quiet “revolution” going on in land-based seismic acquisition equipment over the past few years. It’s wireless acquisition. Actually, it’s been ongoing for about a decade in various ways. Fairfield’s “The Box” was one of the first to have some wireless aspects. Its primary use is for transition zones and it is still in use today.

The term “wireless” can mean many things. As it implies, of course it means a reduction in wires. But power-supply methods and requirements, and data-transfer methods, speeds and protocols all vary considerably across the various technologies.

A typical wireless unit has a battery pack, which may be of several types (e.g., alkaline, lithium ion, metal hydride), a non-volatile flash memory, a micro-processor, a GPS unit and a radio. A short wire connects the geophone sensor to the unit.

Data transmission can occur at radio frequencies but often remains in storage within the field unit, making it more like an autonomous node, except with QA/QC data that are transmitted in the field. Complete data download occurs during retrieval and battery replacement/charging. High channel counts are sometimes offered as a benefit of cableless systems. But recent improvements allow 30k to 50k and higher channel counts with either cable-based or wireless telemetry.

The actual sensors used-whether single or multi-component-are generally the same sensors that are used with cabled systems. However, except when operating in a purely autonomous manner, QC data can be wirelessly transmitted from each sensor package and reveal problems such as noise, coupling or an individual sensor malfunction early in the acquisition process.

Cableless systems allow for recording of individual sensors, but the advantages (and some disadvantages) of array grouping will be sacrificed. However, the individual data records can be combined and stacked in unique ways in the processing lab, albeit at some additional cost and time. In general, the recording of signal (P, PS and S) should be enhanced, but random noise could increase.

Whenever surveys get into the range of tens of thousands of channels, operational logistics, cost and HSE considerations become magnified. Since wireless acquisition in this channel range needs to be deployed and operational for up to a month, perhaps more, the speed at which stations can be laid out becomes very important, as does the type, longevity and weight of the batteries. The ability to reduce or eliminate traditional ground surveys with wireless GPS, as well as continuous field QC, can add to the attractiveness of the system relative to conventional systems.

CONVENTIONAL VS. STATE OF THE ART

Single sensors can remove statics that are sometimes found within arrays. A recent SEG paper1 discussed the noise and array-spacing issue: “Single sensors eliminate intra-array statics … To deliver the same final data quality, surveys with single sensors must have smaller station intervals.” The authors found that single sensor intervals must be spaced at not more than half of the group intervals to achieve the same data quality.

The authors compared the various systems generically.1 Figure 1 shows the approximate weight relationship for three types of recording equipment.

1. Wireless, individual 3C sensors

2. Cabled, state-of-the-art, digital, 3C point receivers

3. Cabled, six-geophone array systems.

Fig. 1. Comparison of three systems in terms of equipment weight vs. receiver spacing (adapted from Lansley, et al., 2007)1.

As common sense would dictate, as the receiver spacing gets closer, equipment weight increases. As Figure 1 shows, for receiver spacings over 30 m, the single-point digital systems have a substantial weight advantage over the six-geophone array systems, while cableless systems have a modest weight advantage over cabled single-point 3C digital systems. However, for receiver spacings 30 m and closer, the cabled system with the single-point 3C digital sensors begins to be the “lightweight” winner.

The authors concluded, “High quality, high density, multi-component 3D surveys are being acquired very cost-effectively using single sensors. As the spatial sampling becomes smaller and the trace density greater, significant operational and recording efficiency benefits are gained using cables.” This assumes, of course, that the terrain/environment will allow the use of cabled systems.

TECHNOLOGY OFFERINGS

Ascend Geo. This company has been offering its wireless acquisition system, Ultra, for the past few years. It is fully commercial, with field experience in many regions, continents and environments, including dense rain forest, desert, highly urbanized areas, farms, plains and forests. The system offers “high channel point array with digital group forming ... and analog 3C.” The system weighs in the range of 1.5-2.0 kg/channel, including internal battery, and uses only one VHF 12.5 kHz channel, although each box is programmable in the range 148-174 MHz, with other options. The system does not transmit to a central tower, but uses up to about 60 transmitters/repeaters in a field, to cover the largest area with the least power. Typically, data downloading and battery recharging occur when the field units are retrieved and plugged into a rack, which is purpose-built and is typically housed in a field trailer.

Input/Output. This company, now called Ion, is testing its recent (2006) entry into the cableless market. BP wanted to reduce environmental impact at Wamsutter field in Wyoming, and I/O needed a place to test its new FireFly cableless acquisition system. BP is the biggest player in the Wamsutter field, with 950 operated wells producing 135 MMcfgd and an interest in 352,000 leased acres.2 FireFly’s first field trial was acquired by Global Geophysical and comprised about 7,200 shot points of 3C surface seismic data over 28 sq mi-a dense design meant to acquire a multi-azimuth dataset. The sensor is I/O’s well-established MEMS-based Vectorseis unit. It lies at the end of a few-foot-long cable that connects and transmits data to the transmitter/recorder unit, Fig. 2.

Fig. 2. I/O’s (now Ion) Firefly wireless unit.

At Wamsutter, BP flew a LiDAR survey over the survey area. This, combined with GPS, provided enough control for survey planning that conventional survey crews were not needed. Position data is transmitted from the sensor to the recording unit via Bluetooth technology; it’s written onto the headers of the seismic data files. According to Craig Cooper, land seismic project coordinator for the North America Gas Business Unit at BP, “This will not only save time during surveying but, because the positional information is written directly into the trace headers, will mean we won’t have to worry about correcting human-generated errors in the merge step during processing.”3

Deployment for the BP trial consisted of a helicopter flying along a pre-determined route, dropping off 55-60 backpacks. Each backpack holds six complete units, including batteries and sensors. As expected, during the new technology’s first field trial, a number of “bugs” were discovered. These have mostly been resolved.

Building on the lessons learned at Wansutter, Apache Corporation tested the system in North Texas this summer. Apache, PGS and I/O teamed up on the shoot, which comprised 15 million traces in a 77-sq-mi survey area over a 30-day deploy/retrieve cycle.4 A one-week delay occurred due to a firmware bug that was discovered just after deployment of 3,000 stations, but that has been fixed. Ultimately, 7,000-8,000 stations were deployed. Some of the data from these surveys is in the process of being released.

Sercel. This company bought Vibtech last fall, a company that was established in wireless acquisition with its Infinite Telemetry (IT) 3D recording system, which was introduced in 2002. Then, in 2006, the company introduced its UnITe Cellular Seismic system. IT is configured with a four-channel remote acquisition unit and hybrid radio/cable telemetry. The company compares it to a cell phone tower system, with field repeaters to transmit to a Central Control Unit (CCU) within the shot cycle.

John F. Smith, co-founder of Vibtech, described the system:5 “The seismic spread is divided into a number of cells, each of which reports to a cell access node, which is connected by fiber optic cable to the CCU.” UnITe uses a single-channel base unit that is GPS-enabled and real time radio telemetry. “Data are sent back using wireless telemetry and simultaneously stored in the memory within the acquisition unit. Each unit can be remotely programmed to record autonomously, send reduced data sets for quality control, or send back the entire data file within the shot cycle,” Smith said.

CONCLUSIONS

Despite the press and marketing efforts on wireless technologies, their benefits range from non-existent to extraordinary. Simply transmitting data through the air does nothing beneficial, per se; in fact, it can reduce data transmission speeds and potentially introduce noise relative to cabled geophones. Most advantages hinge on equipment weight reduction and efficiency of deployment. However, in terms of point sensor density vs. weight, very high channel counts at dense sensor spacing can be more efficiently acquired using cabled, state-of-the-art, 3C point receivers.

But eliminating the connecting cables can often create cost savings through more efficient deployment, which in turn can enable much greater coverage. Perhaps the most obvious, primary advantage of wireless systems lies in HSE benefits, particularly in accessing difficult terrain, whether in terms of a rugose landscape, urban settings or mitigation of environmental disturbance. In such cases, cableless sensors may be the only way to get the job done.  

LITERATURE CITED

 1 Lansley, M., Laurin, M. and S. Ronen, “Land 3D: Groups or single sensors? Cables or radio? Geophysical and operational considerations,” 77th Annual International Meeting, SEG, Expanded Abstracts, Sept. 23-28, 2007.
2 Williams, M., “Seismic without cables,” Nickle’s New Technology, April/May 2007.
3 Friedemann, C., “Unlocking the potential, of challenging gas reservoirs,” OnPoint, July 2007.
4 Williams, M., “The evolution of FireFly from Wamsutter to the Apache project,” OnPoint, June 2007.
5 Beims, T., “Array of new technologies, improved business conditions transforming land seismic,” The American Oil and Gas Reporter, July 2006.

Abstract

In present seismic exploration wireless sensor systems with large acquisition channels, it is difficult to achieve a high data rate, high reliability and long distance in wireless data transmission simultaneously. In this paper, a wireless seismic exploration system using a dual-layer network is proposed. The dual-layer network is designed based on Wi-Fi and LTE, so that long-distance high-rate seismic data transmission with a high reliability can be achieved. In the proposed system, the sensor array is composed of two kinds of nodes, the gateway node and the collecting node. Based on the proposed nodes, collecting node positioning, seismic data acquisition, seismic local data storage and quasi real-time remote seismic data transmission can be realized. Reliability mechanisms have been put forward to deal with the exceptions. An experiment was carried out to test the data transmission efficiency of the proposed system. The results show that the seismic exploration wireless sensor system with a dual-layer network structure can achieve quasi real-time remote seismic data transmission with no packet loss.

Keywords:

wireless seismic networking, seismic sensor array, dual-layer network, reliability mechanisms

1. Introduction

Seismic exploration is an important geophysical exploration method. Wireless seismic exploration systems with large acquisition channels (nodes) are the main forms of present seismic exploration equipment. Various wireless seismic exploration systems have been developed successfully and have been used in seismic exploration. The UNITE system, produced by Sercel, and the RT2 system, produced by Wireless SeismicWireless Seismic, are typical wireless seismic exploration systems.

The UNITE system [1,2,3] is a wireless seismic exploration system based on Wi-Fi. It is mainly composed of a recording truck (the central control station), remote acquisition units (RAUs) and cell access nodes (CANs). CAN is the data exchange node. A star-mesh structure is used in the system. The CAN forms a star network with the RAUs. CANs form a mesh network with the recording truck. The communication between the RAUs and a CAN is based on 802.11b. CANs communicate with the recording truck based on 5.8 GHz Wi-Fi. Based on the above network topology, RAUs can collect, store and transmit seismic data to the recording truck according to its instructions. The data transmission of UNITE works in unlicensed bands, so the communication can be used without authorization. The actual short-distance transmission rate, based on 802.11b, is normally less than 1 Mbps in outdoor line-of-sight environments. In order to achieve a transmission distance of 1000 m without a significant reduction in transmission rate, each CAN needs to use a high-power antenna kit, which results in an increase in power consumption.

The RT2 system [4,5,6,7] is composed of a center record system (CRS), cross-station line interface units (LIUs) and wireless remote units (WRUs). A dual-layer multi-hop chain topology structure is used in the RT2 system. The lower layer is a multi-hop chain topological network, a line composed of multiple WRUs and one LIU. The upper layer is a multi-hop chain topological network consisting of the CRS and multiple LIUs. The communication of the lower layer is in the 2.4 GHz band and the communication of the upper layer is in the 5.8 GHz band. Based on the above network topology, WRUs can collect, store and transmit seismic data to CRS. The multi-hop chain topology network makes the system able to achieve a large acquisition channel with a limited number of LIUs. However, congestion is probable near the LIU, since the traffic load rises with the number of hops, which can decline the data rate sharply. Failure of a WRU will probably lead to separation of the downstream WRUs. Failure of a LIU can lead to the separation of a line.

The bucket brigade sensor array [8] proposed by Wireless Seismic has a similar structure to RT2. The main difference is that the even-numbered collecting nodes and odd-numbered collecting nodes form two multi-hop chain networks with the cross-station for the uplink communication of the lower layer. Two uplink multi-hop chain networks in one line reduce the number of hops, so the data transmission is more efficient in the lower layer. To maintain the data transmission of the downstream nodes in the line, network healing was designed for the communication failure of a collecting node. However, when the failure of continuous adjacent collecting nodes or a cross-station occurs, the separation is still probable.

In addition to the above typical systems, some researchers proposed wireless seismic exploration systems with different features.

Savazzi et al. proposed a high-density seismic exploration system using ultra-wide band (UWB) and long-range Wi-Fi [9,10]. In the system, the node array is divided into several subnetworks. A subnetwork includes a gateway node, dozens of cluster-head nodes and hundreds of leaf nodes. The subnetwork is made up of cluster-mesh architecture based on UWB. The lossy data compression is used to reduce the traffic load and enhance the efficiency of data transmission.

Reddy et al. proposed a seismic exploration system using 802.11af in television white space (TVWS) bands [11]. A dual-layer star topology structure is used in the system. The range of communication is 2 km. If the number of nodes in a star network is limited, the data transmission efficiency is high. However, collecting nodes require 1m antennas and gateway nodes require 3m antennas.

The above typical seismic exploration systems adopt dual-layer network architecture; Wi-Fi or UWB high-speed transmission technology is used in the lower layer network, and high-power Wi-Fi technology is used in the upper layer network to realize the on-site transmission of exploration data. A disadvantage is that the increase in transmission distance leads to sacrificing the transmission rate or increasing the power consumption.

Low-power wide-area (LPWA) wireless technologies are effective long-distance wireless data transmission technologies [12]. LoRa and NB-IoT are typical LPWA technologies. Their advantages are long transmission distances and strong penetration. However, because of the low transmission rate