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Petroleum exploration activities
Under an exploration permit there are certain kinds of scientific studies that can be undertaken, including literature reviews, data analysis and computer modelling, laboratory experiments, seismic surveys, gravity and magnetic surveys, and the drilling of exploratory wells.
Exploration drilling operations
Seismic Surveys
Why do we use seismic surveys?
Seismic surveys are conducted in order to image and understand the rock properties and their structure. This is done to help further define an area likely to contain commercially viable accumulations of hydrocarbons (oil and/or gas).
Seismic surveys allow explorers to test for oil and gas without physically going under the earth’s surface.
A seismic survey measures the earth’s physical properties (geophysics) by measuring the elastic response of water and rock layers to an elastic disturbance, i.e. how sound waves/vibrations travel through the different rock layers and how they are reflected. This is very similar to how an ultra sound works.
In order to do this you need a controlled elastic disturbance or seismic source, an instrument to measure the disturbance, a Geophone or Hydrophone, and a recording device.
How do they work?
A seismic survey can be conducted on land or offshore, and can be used to collect data in two dimensions (2D seismic) or three dimensions (3D).
A typical land survey in New Zealand consists of several hundred seismic sources being individually activated, detected and recorded. The seismic sources (known as shot) and recording equipment are laid out in lines that can be several kilometres long.
These lines need to be carefully and accurately surveyed and measured prior to being laid out. This is known as a 2D land seismic survey. If they are laid out in a close grid formation instead, then the data collected will be imaged in 3D.
The instrument used to measure the reflected waves is known as a geophone. This device turns the vibration into an electric signal by exploiting the movement of a magnetic mass inside a coil. The geophone has a small spike, usually around 10 cm long, which is placed into the ground, holding it in place and allowing for 'cleaner' vibration detection.
During a seismic survey geophones are laid out in long lines and all connected together with a cable. The cable is linked to a recording station (usually a 'recording truck') where the data is filtered, recorded and placed on magnetic tapes or large hard drives. At this stage the data is known as Raw Field data and will need to be processed further after the survey to get an image for the geologist to interpret.
A shot in New Zealand is usually created by drilling a slim hole down to the bed rock (the depth being dependant on the area). The hole then has a small amount of explosives placed at the bottom of it, with a detonator and ignition wire leading to the surface.
The hole is then backfilled to contain the explosion and to try and keep as much energy in the ground as possible. Shots are only set off when all personal and livestock are at a safe distance. In some cases the shot can be a vibrating truck or weight drop.
The seismic waves generated by the shot are reflected from the layers of rock within the earth and recorded with the geophones. The information gathered tells explorers about the structure of the rocks.
Surveys undertaken offshore follow the same basic principles, with a vessel towing a sound wave source and recording devices. The photo below shows the Bergen Resolution undertaking a 2D seismic survey off Cape Palliser, Southern Wairarapa.
What is the environmental impact of offshore seismic surveys?
While petroleum exploration activities prior to drilling are much lower in impact than drilling, potential exists for seismic operations at sea to have an adverse impact on marine mammals through acoustic disturbance.
Therefore, there are guidelines, administered by the Department of Conservation (DoC), for minimising acoustic disturbance to marine mammals from seismic survey operations. These guidelines can be found on the DoC website.
The guidelines apply to all marine mammal species (including Hector's and Maui's dolphins, seals, whales - and any other species recommended for inclusion).
In brief, operators are required to:
- Have present an independent trained marine mammal observer
- Record all observations/sightings of marine mammals before and during operations
- Have regard to the 1.5 km radius for species of concern and 200 m for all other species (operations must stop if there are mammals within radius)
- Use the lowest practical power levels throughout the survey;
- Make a gradual start regarding the number of air guns used initially
- Record all observations.
The Petroleum Exploration and Production Association of New Zealand (PEPANZ) and the Department of Conservation are currently reviewing these guidelines to ensure that New Zealand meets the world's best practice in seismic operations when operating near marine mammals. This is being done in consultation with various NGOs and relevant Government departments.
Gravity surveys
Gravity surveys can be done by air or on land. The earth's gravity field is affected by the density of different kinds of rocks. Surveys to map these differences can be used by mineral explorers and developers to help locate certain rock formations.
Measurements of the gravitational field are done using a very sensitive spring and weight system. A weight is attached to a beam and a spring and as gravity increases, the weight is forced downwards, stretching the spring.
The beam is then brought back to a horizontal position, the amount of movement required to do so is proportional to the gravitational force. This information is recorded and later analysed.
Magnetic surveys
These are commonly done by air using 'magnetometers', which measure small changes in the earth's magnetic field. Finding where these changes occur can be used by mineral explorers to help locate where different kinds of rocks lie under the earth.
