Exploring for natural gas and petroleum deposits involves searching for something that is often thousands of feet below the earth’s surface. The search is essentially a crapshoot; there’s no sure formula for success. But over the last 30 years, ingenious new technology and equipment have improved the odds greatly. Seismology (with its roots in the study of earthquakes) is at the heart of the improvement.
The basic concept of seismology is simple. The earth’s crust is composed of different layers, each with its own properties. When energy — in the form of seismic waves — is sent into the ground, it interacts differently with each of these layers. The waves then reflect back toward their source, where they are recorded and analyzed to reveal characteristics of the underground layers. Geophysicists, geologists, and petroleum engineers can interpret these characteristics to make better educated guesses as to where gas and oil deposits exist.
In the early days of seismic exploration, seismic waves were created artificially using dynamite. Charges packed into predrilled holes were detonated on command. Over the past 50 years, as an alternative, mobile seismic vibrators have been developed to generate energy waves mechanically. These machines position themselves at a preselected energy point, create and direct sonic waves into the ground, run their tests, and move on. Seismic vibrators are more efficient, more versatile, and cause less collateral damage than explosives.
Getting into position
As you might guess, the first seismic vibrators were mounted on commercial trucks fitted with a second engine to power the hydraulic system. These units were well suited to over-the-road travel between jobs but were less practical for off-road positioning. Most current production vibrators are purpose-built off-road machines that are trucked from one job to the next. They may be wheeled or tracked, depending on the off-road conditions where they will operate.
The vibrators’ travel and seismic-wave functions are both hydraulically powered. Typically, a single diesel engine drives two sets of pumps: one for the travel circuit, the second for the seismic-wave functions. (Obviously, the vehicle never produces vibrations while traveling.) Because this equipment may operate over a wide range of temperatures, the hydraulic platforms may include heating, cooling, and insulation arrangements.
For off-road travel, a hydraulic motor drives a differential on each vibrator axle so that both are live. The axles are fixed to the vibrator frames, many of which are articulated for maneuverability. The frames have a three-degree-of-freedom joint just behind the operator’s cabin. Hydraulic cylinders between the two frame halves steer the vehicles.
Producing seismic waves
In the field, today’s seismic vibrator is a mobile source of acoustic energy. The vibrators typically operate in synchronized groups of four or more. At the energy point, the vibrator operator lowers a baseplate to the ground. Isolating air springs then apply nearly all the vehicle’s weight to hold the baseplate in place. Next, a hydraulic actuator vibrates the baseplate to produce a coded signal that is the energy source for the reflection seismology.
You could think of a seismic vibrator as a powered subwoofer whose speaker cone is in direct contact with the earth. Many types of coded signals are possible. One example is a sinusoidal sweep (chirp) from 6 to 128 Hz for 14 sec.
The vibrator’s actuator is a large hydraulic cylinder. The cylinder housing serves as a reaction mass for the actuator’s dynamic force. The housing weighs about 5 tons. Another set of air springs counterbalances the reaction mass to improve actuator force. When commanded, a three-stage, four-way servovalve cycles the cylinder to push against the reaction mass while the equal and opposite force drives the baseplate that is coupled to the earth’s surface. This arrangement produces the desired seismic waves to investigate the area surrounding that location.
The vibration circuit’s main variable-displacement pump is pressure compensated to about 3,200 psi. A fixed-displacement pump precharges the closed-loop hydraulic system to 200 psi to prevent cavitation. The circuit is fitted with several accumulators — sized 1 qt to 5 gallons — to maintain pressure stability in both the supply and return lines to the actuator. You might think one pair of large accumulators located at the servovalve on the reaction mass would serve this purpose, but experience has shown that the vibration environment is not conducive to long life for large accumulators.
As with almost every machine today, electronics plays an important role in vibrator performance. Pressure sensors in the lift cylinder circuits must report that adequate hold-down force has been applied before operation can begin. LVDTs provide position feedback to keep the cylinder and servovalve main spool operating in their central regions. Accelerometers mounted on the reaction mass and baseplate feed amplitude and phase data back to enable closed-loop control.
When groups of vibrators are deployed to increase the source energy, a central control — which may be several miles away — sends an encoded tone over VHF radio to signal the actuators when to start. For synchronization within about 1 µsec, the start may be delayed until a GPS receiver marks a second. Each actuator is fitted with a digital sweep generator. Clocks that run the sweep generators are GPS-disciplined for precise long and short-term synchronization.
In one machine, a 425-hp Detroit Series 60 engine — running at 1950 rpm — powers a model PLS-362 vibrator. Two 7-piston Parker Denison pumps are operated in parallel, but in counter-phase —and are geared up to 2730 rpm. This gives the 3300-psi hydraulic supply a ripple frequency of about 650 Hz. The actuator’s rated peak output force is 60,000 lb. The vibrator has a 3-stage, high-response, 240-gpm servovalve with a Moog flapper-nozzle pilot. A speaker-coil-type pilot valve (which is just now entering the marketplace) promises to push the frequency response even higher.
Most of the servovalves on seismic vibrators incorporate differential pressure feedback. Hydraulic pressure feedback serves as an auxiliary servovalve output-spool positioning feedback in addition to its other functions.
Here’s how it works: when the baseplate pushes on the ground, the ground pushes back. That interaction can be sensed in the differential cylinder pressure. If the hydraulic cylinder pressure signal is added to the pilot valve's output, the weighted summation drives the servovalve's main stage. This is a simple and robust method of applying pressure feedback and the pressure loop can be closed to frequencies above 100 Hz.
If the output spool is not in the null position, pressure feedback tends to push it back to null. Differential hydraulic pressure feedback provides a restoring force, a negative feedback -- and thus prevents erratic servovalve behavior in case the primary valve spool position feedback subsystem fails. A servovalve with this feature and with a failed primary valve spool position feedback subsystem tends to return its valve spool to null, gracefully stopping the actuator wherever it may be and preventing erratic and catastrophic motions.
To a significant degree, hydraulic pressure feedback transforms a velocity actuator into a force actuator. This is good because the vibrator's desired output is a force signal. Sensing the earth's reaction force — and using it as a negative feedback — adjusts the actuator's apparent impedance to more closely match the earth's impedance. It dampens the earth/baseplate resonance and makes the open-loop frequency response more linear. For the vibrator controller, it makes hard surfaces look more like soft surfaces, and it makes nonlinear loads look more linear. Phase and amplitude control become simpler, and harmonic distortion is reduced.
Dennis Reust is a consultant at Servo Force, Stillwater, Okla. For more information, contact him at [email protected].