Dynamic 3D Display of Rock Fracturing During Geosequestration of Greenhouse Gas

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"Successful application of the time-of-flight technique requires that we tightly synchronise all data acquisition channels. By using the PXIe-6672 to share a common high-precision clock between all 15 acquisition cards and taking advantage of oversampling (up to 120 MSa/s) and FPGA derived precision on the PXIe-7962R/NI 5734, we synchronised the acquisition windows on all channels to better than 10 ns."

- Mark Trotman, ICON Technologies Pty Ltd

The Challenge:
Developing a high-channel-count Microseismic Data Acquisition System (MDAS) that uses tightly synchronised patterns of ultrasonic excitation and transmission to characterise the dynamics of rock fracturing phenomena in a 3D sample space.

The Solution:
Using the timing and synchronising strengths of PXI Express to drive an array of 60 ultrasonic transducers in a rapid cycle of synchronised excitation and signal acquisition that generates a 3D ultrasonic map able to capture the dynamics of fracture events.

Author(s):
Mark Trotman - ICON Technologies Pty Ltd
Brian Evans - Curtin University, Dept of Petroleum Engineering
Andrew Jupe - Altcom Pty Ltd

Introduction

The National Geosequestration Laboratory engaged NI Alliance Partner ICON Technologies to supply the data acquisition system for a unique new microseismic instrument (the MDAS).

Microseismic systems use ultrasonic transducers as transmitters and receivers in place of real-world seismic sources and geophones to study seismic phenomena in laboratory scale samples. The mathematics of analysing the microseismic ultrasonic signals is derived directly from the mathematics of analysing large-scale seismic signals, albeit at frequencies of 500 KHz to 1.5 KHz rather than 40 Hz.

Microseismic studies of substrate cracking dynamics help us to understand many critically important processes that occur on both global scales, such as the seismic events that precede and follow major earthquakes, and on local scales, such as the fluid transfer mechanisms associated with the geosequestration of greenhouse gases.

System Overview

We based the MDAS on the PXI Express platform. It can acquire up to 60 channels of 16-bit ultrasonic data at sample rates up to 120 MSa/s (10 MSa/s to 20 MSa/s nominal). We distributed the 60 ultrasonic transducers uniformly over the six faces of a cubic rock sample up to 30 cm per side. The samples are held at pressures of up to 70 MPa (10,000 psi), and temperatures of up to 95 C, in a true triaxial pressured stress cell (TTSC). The construction, monitoring, and control of the TTSC is a separate project to the development of the MDAS.

The MDAS is a step-jump evolution of two smaller and earlier microseismic data acquisition systems that ICON Technologies developed for the Curtin University Department of Petroleum Engineering. The MDAS combines all of the experimental procedures and data processing techniques developed and tested in the smaller (<16 channel) systems, adds new dynamic data processing techniques at the forefront of current microseismic research, and extends the capacity of the system to manage up to 60 data acquisition channels on samples up to 300 mm3.

It is unique among world instruments in its capacity to:

• Handle large samples

• Use acid fracturing agents such as CO2

• Generate 3D event maps with significantly increased spatial resolution

• Map the dynamic behaviour of fracturing epicentres in near real time

Curtin University and the Commonwealth Scientific and Industrial Research Organisation invited proposals for MDAS from multiple vendors with experience in microseismic systems. ICON Technologies’ proposal based on the PXI Express platform was the only submission able to address all of the target system specifications. The high channel density, range of I/O types, precise multiboard timing and synchronisation, and overall data throughput that can be achieved using PXI Express were all critical factors in the capacity of the system to meet the challenging specification.

The core of the MDAS is two PXIe-1085 18-slot chassis: the acquisition chassis and the control chassis. The system is driven by an embedded PXIe-8135 controller in the acquisition chassis, with an MXI-Express interface to the control chassis. 

Figure 1. The Acquisition Chassis (Top), Control Chassis (Bottom), and Complete MDAS Rack (Right)

The acquisition chassis houses all the primary time-critical I/O: 15 PXIe-7962R/NI 5734 FlexRIO I/O cards that provide 60 channels of 16-bit ultrasonic signal acquisition at up to 120 MSa/s, a PXIe-6672 timing and synchronisation card, and a PXI-5412 AWG card for generating user-configurable ultrasonic excitation pulses.

The control chassis includes eight PXIe-2529 matrix switch cards and a PXIe-2527 multiplexor card to enable the system’s two operating modes, four PXI-6509 digital I/O cards and a PXIe-6341 multifunction I/O card to provide software configurable gain control of external amplifiers, and an MXI Express interface to an HDD-8266 24TB RAID enclosure for archival data storage.

The balance of the system hardware includes 60 Femto DHPCA-100 single-channel amplifiers for ultrasonic signal input and a Krohn-Hite 7602M wideband amplifier for ultrasonic signal output.

ICON Technologies worked with UK companies Altcom Pty Ltd and VSProwess to integrate a custom version of their commercial 3D seismic analysis and display engines into the system’s LabVIEW user interface.

System Operation

The MDAS has two modes of operation: passive and active. We interfaced all 60 ultrasonic transducers to the MDAS through the PXIe-2527 and PXIe-2529 switches to allow transition between the two modes.

Figure 2. The schematic of the acquisition and control chassis shows signal flow to switch between passive and active operating modes for eight ultrasonic transducers. The pattern is replicated for the balance of 52 transducers (not shown).

In passive mode, all 60 transducers act as receivers (inputs). They listen for a threshold trigger condition initiated by either a natural or induced fracture event in the sample.  Detection of the trigger at any one transducer initiates a synchronised capture of pre- and post-trigger data from all transducers. The Analysis Engine uses a time-of-flight algorithm to locate and display the event in 3D space. The system then typically returns to passive mode operation, waiting for the next event.

However, if the Analysis Engine detects a correlated pattern of events that indicate the formation and propagation of a significant crack, the MDAS will transition to active mode operation. In active mode, each of the 60 transducers in turn is switched to act as a transmitter (output), and pulsed with a user-configurable excitation waveform from the PXI-5412. The other 59 transducers act as receivers and are triggered to capture a synchronised window of data in response to the pulsed transmitter. A single pass through all 60 transducers as transmitters takes around two seconds, and the cycle is repeated indefinitely until the operator or a specific system-generated stop trigger terminates it. The process is similar in concept to an actively “pinging” submarine sonar, with the event epicentre being tracked by reflection and refraction of the source pulses from each transducer.

Figure 3. 3D Sample Space Showing a Typical Passive Mode Display of Induced Events

The Analysis Engine uses one of three user-selectable time-of-flight processing algorithms to generate a dynamic map of the propagation of the feature in 3D space in near real time, with less than two seconds latency. Each processing algorithm is based on a different model of interpreting time-of-flight signals, and highlights different dynamic characteristics.

The sample rate, pre- and post-trigger window size, and trigger threshold are all user configurable for both operating modes. The 3D display can be rotated as required to optimise the view and interpretation of the fracture system.

A Unique Combination of Challenges

The client specified a unique instrument that is at the cutting edge of microseismic research.  Designing and engineering the system to meet the specification presented many challenges.

Successful application of the time-of-flight technique requires that we tightly synchronise all data acquisition channels. By using the PXIe-6672 to share a common high-precision clock between all 15 acquisition cards and taking advantage of oversampling (up to 120 MSa/s) and FPGA derived precision on the PXIe-7962R/NI 5734, we synchronised the acquisition windows on all channels to better than 10 ns.

The active data processing algorithms are based on the interpretation of weaker secondary peaks from reflection and diffraction of the initiating source signal. The ability of the system to acquire high-speed data at 16-bit resolution is critical to the capture of this fine structure detail.

Finally, the flexibility of the LabVIEW environment was a major factor in our ability to integrate the high-level hardware drivers, intensive data preprocessing, and communications to external tools (in this case, the third-party Analysis Engine) were essential to delivering the system under a unified software interface.

In ICON Technologies’ experience, PXI/PXI Express remains unchallenged as the best platform solution for addressing challenging high-channel-count mixed-signal I/O problems.

 

Author Information:
Mark Trotman
ICON Technologies Pty Ltd

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