BABBITT- Pulsar Helium continues its exploration of the helium in the Precambrian Duluth Complex rocks southeast of Babbitt, seeking to extend its understanding of the resource. The firm started a seismic campaign last week typical of oil and natural gas exploration but rarely used in the mineral exploration studies in the Iron Range.
The survey started collecting seismic data along the Dunka River Road last week using non-destructive vibroseis trucks to provide the needed sound waves.
“This sort of seismic survey is rarely conducted in northern Minnesota … Our seismic survey is 12.7 miles long,” said Jack Gibbons, a Pulsar geologist. “The data we collect will help us with our upcoming drilling campaign.” Based on the data the firm has gathered already, Pulsar plans to deepen their Jetstream No. 1 exploration well and to drill additional “step-out” wells to exploit the gas in the ground.
Pulsar has conducted multiple geophysical surveys of the area southeast of Babbitt, seeking to characterize the location and quantity of helium present. The company already knows the quality of the sought-after noble gas: it’s the highest helium concentration in North America and may be the richest in the world.
Seismic surveys
Pulsar collected an ambient-noise 3D seismic survey of the region last summer and short vibroseis and vertical seismic surveys in May at the well itself. Seismic surveys use acoustic waves to map what’s underground. The reflected waves are recorded by geophones planted at the surface.
Geophones need the presence of traveling acoustic waves so they can measure the reflections. Those acoustic waves can come from ambient noise, which can be from blasting at the region’s iron mines, or from large earthquakes on the other side of the world. The seismic data gathered by Pulsar last year used ambient noise. Ambient noise seismic surveys are less expensive and non-destructive. Because the survey workers do not control the source of the seismic energy, such surveys are typically less detailed than surveys using artificial sources.
Vibroseis
For detailed seismic studies, an artificial source of acoustic energy is preferable. Before the 1950s, geophysicists used explosives to inject acoustic waves into the ground. Then the Continental Oil Co., better known as Conoco, invented the vibroseis truck. The term “vibroseis” was a Conoco trademark until the company’s patent expired. Over half of the seismic surveys conducted today use the vibroseis method.
A vibroseis truck uses hefty hydraulics and the weight of the truck itself to inject a shaped wave train of energy into the ground. These waves start with low-frequencies better felt than heard. Then a vibroseis truck will increase the frequency over several seconds. The finishing frequencies are usually in the range of tubas and trombones if the waves traveled through the air as sound.
Different features in the subsurface can be sensitive to differences in frequency, so a vibroseis survey can illuminate details in the subsurface that an omnidirectional source like explosives can miss. This is the advantage of the vibroseis method. It’s other advantage, like ambient noise, is that it is non-destructive, thus avoiding the permitting process, safety issues, and damage of using explosives.
The current survey
Pulsar’s existing seismic and other geophysical datasets suggest that the helium resource extends deeper than the current exploration well and may extend beyond its immediate neighborhood. This is why Pulsar is now conducting a detailed multi-mile seismic survey.
About the current seismic campaign, Pulsar President CEO Thomas Abraham-James said, “We are delighted to receive this 2D seismic data, which demonstrates that the helium-bearing zone encountered in the Jetstream No. 1 appraisal well is identifiable and that additional gas-bearing zones are likely at depth. This new data, alongside existing drilling, passive seismic, and gravity information, sets the stage for our upcoming drilling program where we plan to deepen Jetstream No. 1 and drill additional step-out wells.”
The survey is being collected by contractors Nodal Seismic, which is providing the vibroseis trucks, and Evergreen Geophysical of Evergreen, Colo., which is conducting the data collection. The geophysicists have deployed 2,800 geophones planted at 10-meter spaces down the entire length of Dunka River Road. The survey will take approximately a week to collect the data.
Pulsar hopes the new data will help them delineate the seismic anomaly associated with the helium in the exploration well and how it extends regionally. The firm has found a zone of lowered seismic velocity where the 14 percent helium gas has been encountered in well.
The velocity anomaly persists to a depth of approximately 3,773 feet, giving it a vertical thickness of approximately 1,970 feet. The anomaly as currently mapped covers an aerial extent of two square miles. The vertical thickness of the velocity anomaly increases to the north-east to two-thirds of a mile and is connected to another velocity decrease which dips and broadens towards the west.