Denison 3D Seismic Survey
What is a Seismic Survey?
A seismic survey is a process used to figure out what’s going on underground. It creates a map showing ‘mountains’ and ‘valleys’ and places where oil or gas might be located underground. It helps us choose exactly where to drill a well.
The Denison 3D Seismic Survey
In 2004, ARC Energy (the company who owned the permit area before Mitsui E&P Australia’s predecessor AWE) did a seismic survey. It was the most complicated ever done in Australia at the time. It covered nearly 400 square kilometres and included the town of Dongara as well as three nature reserves and a lot of farms. It covered the land of 280 different owners and was undertaken by a team of 65 people.
The survey needed very careful planning. This involved talking to every one of the 280 landholders to ensure land use activities were understood and considered in the survey plan. Questions from individuals as well as from government departments were answered and potential environmental impacts were also carefully considered.
The survey was a massive undertaking. Good planning and communication was essential.
First things first – a botanical survey
The survey involved laying out a massive grid on the ground by marking out over 2,200 km of gridlines. More than half of that went through vegetation, mainly on private property but also both inside and outside several nature reserves. Since the gridlines were on average 3.5 metres wide, this meant that some bush needed to be removed. To protect important areas, the vegetation had to be properly examined before the seismic survey began.
To kick-off an extensive botanical survey was undertaken to identify and map vegetation types. The grid lines were then mapped out to avoid sensitive areas containing rare flora, rivers, animal habitats and slow growing plants such as grass trees. Areas where erosion might be a problem, or where there were a lot of weeds were also avoided.
Results of the botanical survey were donated to the government agency, CALM (now known as the Department of Biodiversity, Conservation and Attractions) for its use.
Laying out the Grid
The grid consisted of ‘receiver lines’ that were oriented east-west and spaced 240m apart, and ‘source lines’ that were oriented north-south and spaced 480m apart. On the eastern section there were limestone outcrops so the source lines had to be spaced 240 m apart in that area.
It was easy to lay out the grid in cleared areas, but in areas of vegetation the most appropriate method of gaining access had to be chosen to help regrowth occur as soon as possible. Sometimes a bulldozer or a roller could be used to clear a track but at other times the clearing had to be done by hand. Daily toolbox meetings were held to discuss the day ahead, identify if any sensitive environmental areas were involved and if so, the approach.
All staff were trained to stop work if they thought there was a safety or environmental risk.
A total of 328 km was cleared by hand and the survey took six months to complete.
Gathering the data
Vibroseis trucks were used to help determine the composition underground. Sometimes they’re called ‘thumper trucks’ because they have a big steel plate underneath which thumps the ground. It’s a bit like hitting the ground with a sledgehammer.
Here’s a video of vibroseis trucks in action.
The impact of the plate on the earth sends seismic waves downwards. The seismic waves bounce off the rocks underground and some are reflected back to the surface. The returning seismic waves are recorded with sensors. The time it takes for the seismic waves to return helps identify what’s underground.
We also drilled 48 up-holes. These are useful for measuring what’s going on in the top layer of earth, known as the weathering layer.
The holes were up to 200m deep and 6-7 cm across. They were formed with a casing which was removed at the end of the test. After drilling the holes, a technician lowered a geophone into the hole and took measurements at intervals down the hole.
Looking after the crew
A lot of people were involved in the survey. They lived in a specially built camp which contained a large kitchen and dining area, a recreation room, a laundry, and a shower and toilet block and various offices.
There were regular safety and toolbox meetings, and everyone was inducted and given defensive 4WD training. A dozen people completed a one day course about working safely near railway lines, because some cable was laid underneath the tracks.
Crew had to wear long sleeved shirts, covered footwear, sunscreen, and gloves. They were randomly tested for drugs and alcohol. Vehicles were fitted with first aid kits and snake bite kits. Everyone was taught how to correctly treat a person who had just been bitten by a snake.
The Denison 3D survey covered an enormous area and was conducted over the summer, when it can be very hot and dry in the area. The weather provided plenty of challenges to keep the team on their toes:
• Strong winds repeatedly buried cables and other equipment in the sand dunes
• Fire bans put the entire crew on standby on multiple occasions
• Rain destroyed 50 geophones in one day
• A lightning strike destroyed equipment
• Equipment sometimes overheated and had to be turned off to cool down
It wasn’t just the weather creating havoc. Cattle and goats chewed cables, meaning they had to be relaid. Breakdowns caused unexpected delays – for example one day the gearbox in the water truck gave up and had to be taken to Perth for repairs.
The Brand Highway ran right through the survey area so quite often equipment was split between the two sides and cable had to be run across the highway. We had to be careful not to hold up traffic for too long.
Some areas, such as sand dunes, were ‘no vehicle’ zones for environmental reasons. All the equipment had to be carried by hand over the dunes, a job which is described in the Operations Report as ‘a slow and relentless task’.
It was important not to spread weeds like doublegees into the reserves, so hundreds of clean-down stations were placed across the survey area and vehicles had to stop and remove every doublegee from every tyre before proceeding.
Finally, because the survey took so long (several months), the survey team had to work around the farming and social activities of the community. On several occasions all the equipment had to be moved because farmers were about to start ploughing in preparation for seeding, or it was Easter and the town was about to get very busy, or stubble needed to be burned off. Sometimes the depegging team was called in to quickly remove pegs before they were dug into the ground by a farmer.
There was a lot of juggling to be done – of the landowners, the community, the survey crew and the equipment.
Analysing the data
Once ARC Energy had gathered all the data it had to be processed, which is a complex procedure that needs powerful computers, special software and someone who knows what they’re doing. It takes a long time and costs a lot of money.
Once all the data was processed a geophysicist was able to interpret what they saw. It’s a kind of reverse process. The geophysicist knows the properties of various rocks and fluids and he or she can see how long the seismic waves took to arrive at the sensor. From this they can deduce what materials must have been encountered by the seismic waves.
This part of the process is a little bit subjective and two professionals will sometimes come up with two different conclusions.
The data analysis creates a colourful map with red spots indicating the likely areas for finding oil or gas.
Deciding where to drill
It’s not enough just to find gas or oil. We have to be able to get it out in a way that makes economic sense. Sometimes we’ll find what looks like quite a good supply but it’s nowhere near any of our processing plants or pipes to transport it to market. In that case it’s earmarked for another time.
When we find a reserve that’s well positioned in relation to our existing infrastructure we drill a test hole to see what’s down there. Is it enough to be worthwhile? Is it flowing fast enough?
It’s like finding a needle in a haystack. We cast our net wide then gradually eliminate potential points until we arrive at one that’s suitable for further investigation or perhaps production.
Cleaning up after ourselves
It was important that the tracks we made for the seismic survey didn’t start being used by other people, so within two weeks of finishing with any grid line we closed it off and started the process of rehabilitation.
Rehabilitating is a long term process and it’s important that it’s done properly. The goal is to make it impossible to tell that we were ever there. We employed specialists for a period of three years to ensure that rehabilitation was carried out correctly.
All but two of the seismic lines were successfully rehabilitated. These lines are in the Beekeepers Nature Reserve and although they were blocked off to stop them from being used by vehicles, and all the revegetation indicators were good, unauthorised traffic caused ongoing disruption, In recognition of this, the regulators have approved the completion.
The Final Outcome
As a direct result of conducting the Denison 3D Seismic Survey four wells were drilled. Two showed oil and two were dry.
Most importantly however, the survey has added to our knowledge of the Perth Basin so that good decisions can continue to be made into the future.