A seismic survey is a method used to investigate the subsurface structure of an area through the use of sound waves. This is commonly used for the exploration of hydrocarbon reservoirs, mineral deposits and natural gas. However, it can also be used to investigate areas for building and seismicity events.
The survey is based on the principles of reflection seismology, which is used to acquire and generate seismic data relating to the structure of the subsurface. During the survey, seismic waves are generated using specialised equipment such as vibrator trucks. The waves travel through the subsurface encountering layers of different materials, either passing through or being reflected back to the surface, depending on the acoustic contrast of the layers. The energy on the surface is then measured and recorded by specialised sensors known as receivers (those can be geophones or seismic nodes).
There are various ways you can conduct seismic surveys on land including two dimensional 2D, 3D and 4D methods. These are usually developed and conducted by specialised contractors.
Seismic surveys work by sending a shock wave - also known as a seismic wave - on the surface of the ground. The seismic waves then travel into the earth and are reflected by lithologic layers in the subsurface, returning some of the wave energy to the surface, where it is measured by a sensor. The recording sensor can be an accelerometer or a geophone. The difference between the two is that accelerometers measure acceleration - similar to the piezoelectric sensor used in STRYDE nodes - while geophones measure velocity.
It's crucial to ensure that the seismic survey design is fully optimised if you want to achieve successful exploration or effective development of a project. Some of the main benefits of denser seismic surveys using nodes include:
In a 2D seismic survey, the goal is to acquire a two-dimensional image of the subsurface, called a seismic line. Geophones are spread out along a line at the surface - whether that be on land or water. A seismic source (vibrator) generates waves, at predefined locations along the 2D line, that penetrate the ground. The energy source is moved to the next position on the line and the process is repeated at every source point.
This method involves generating a three-dimensional image, volume, of the subsurface and requires more sensors to be utilised. The sensors are spread out across the surface in an evenly spaced grid design with receiver lines typically running perpendicular or diagonal to source lines.
A 4D seismic survey is the process of using 3D seismic data which is acquired at different time intervals (several months or a few years) over the same area. This is used to assess the changes in a producing hydrocarbon reservoir - changes may include fluid movement and saturation, reservoir pressure and temperature.
2D surveying is still a very popular way of gathering seismic data, due to it being a much more cost-effective and quicker alternative than 3D and 4D methods. 2D seismic is frequently used in the initial stages of regional exploration projects. For context, large scale, high-density 3D seismic surveys can take anywhere from 1-2 years, excluding the time required to process the seismic data into images that can be interpreted by geologists. Even when compared to small 3D surveys, 2D surveys require a lot less permitting, surveying and processing. However, 3D surveys have the advantage of providing a volume of data instead of just a 2D line-image.
Before you set out on developing your survey design, it's important to note that there are a few factors that can influence your survey design including target depth, structural complexity of the subsurface, noise level and required data resolution.
You want to make sure you’re choosing a survey type that is best suited to the geological area and structure that is being targeted. For example, 2D surveys are more appropriate for simple geological areas while 3D surveys are better for complex areas. Other factors like financial and time constraints could determine the type of survey to be chosen.
As with any project, you need to set out clearly defined goals and objectives before thinking about your survey design. In the case of seismic survey design, this may include factors such as why you need the seismic data, the resolution you want, depth you are targeting and data quality. Each of these factors will have an impact on how the survey is conducted and the data acquired, as well as the associated survey costs and time.
It's important to select a seismic source that is relevant to the geological environment and project objectives. For those with a deep target depth, explosives may be better placed, while seismic vibrators may be better for shallow target depths or areas with easy access for the vibrator trucks.
Equally important is the selection of the type of receiver equipment. For many years in land seismic, the geophone receivers, connected with cables, were the only available option. Over the last 10 years, there has been a significant increase in the use of wireless seismic nodes for seismic acquisition. Seismic nodes provide many advantages for seismic acquisition when compared with cable acquisition systems.
Any previous seismic data already acquired in the area should be carefully analysed. It should provide useful information on the required parameters for the new survey. The data analysis should help define the required resolution of the data, the noise level in the area, the required trace density to achieve good data quality and the geometry for the layout of source and receiver lines.
It’s always useful to test the survey design by conducting a pilot survey. This can highlight any potential issues or errors with the design of the survey and gives you the opportunity to rectify these issues before a full survey is conducted.
Generally speaking, what makes data ‘high quality’ is its ability to fulfil its intended purpose which can vary in terms of seismic data. There are three main steps when it comes to seismic data maturity including acquisition, processing and interpretation. The latter two options tend to be the most important stages when it comes to focusing on quality data as the turnaround time is significantly less than the first stage.
At STRYDE, we have a skilled team of land processing geophysicists who use state-of-the-art processing techniques to assure subsurface image excellence and efficient quantitative interpretation.
When it comes to seismic survey design, it’s widespread practice for seismic acquisition contractors to design the survey acquisition during the bidding process, which often comes with a few challenges. These challenges can impact the overall success and feasibility of the seismic survey design process.
Smaller or independent operators, alongside those who have experienced budget cuts due to the volatile oil prices, may find it challenging to allocate adequate budget and resources to develop detailed technical specifications which are required in order for seismic contractors to design a successful survey acquisition plan. In the absence of these technical details, the operational and logistical risks after awarding the contract can increase, resulting in heightened commercial risk exposure for the operator who commissioned the survey.
Survey design has a significant impact on the success, risk, and costs associated with land seismic acquisition. To help set your next seismic survey up for success, consider these 3 points:
This will pay dividends when it comes to the acquisition contractors designing an accurate survey acquisition plan and commercial model that will minimise project and commercial risks. As a minimum, the following information should be available to the contractor within this document:
Survey location (plus available GIS data)
HSE risks and requirements
A summary of the local knowledge of the area, key restriction zones, environmental challenges, land compensation levels, permitting challenges
Survey size and limits
Identification of weather or acquisition windows and expected completion date
Acquisition program summary and parameters (inclusive of design, receiver and source parameters, offsets and azimuths, recording parameters, noise levels etc)
A good contractor should conduct a Scouting of the environment and topography of the seismic survey area. The Scouting is important to fully understand and account for the landscape, logistical considerations, operational efficiencies, commercial contingencies, the number of people required, equipment needed, etc in their bid.
“GIS Modelling” is a critical success factor to this process and should always be used as a technique to accurately convert the real world to the digitally and logically represented spatial objects, consisting of the attributes and geometry.
It is good practice to request the contractor to show evidence of their “Scouting Report”, with the GIS Modelling results included, to demonstrate they are familiar with the survey area. They should have a strategy in place to minimise all potential challenges that could occur during the acquisition. This should be inclusive of, but not limited to:
Means of access
Main and fly-camp locations
Potential or known access restrictions
Landowners in agricultural areas
Very often, further efficiencies are unlocked post-contract award, where a second on site Scouting exercise or deep dive has been conducted. The objective is to fully investigate landowner requirements and additional restrictions to determine further opportunities to improve operational efficiency. This exercise should be considered for surveys that are located on complex terrains, where there are likely to be more challenges during acquisition.
Nodal seismic receiver technology is beginning to penetrate the marketplace due to advances in technology and the industry’s need for lower cost, and more environmentally friendly seismic solutions. However, cabled-based receiver equipment is still a solution of choice for some countries and companies.
There could be some reluctance to adopt Nodal technology due to a few reasons:
With operators increasingly requiring denser surveys, at a lower price, with reduced environmental footprint and less HSE risk, it is crucial for companies to remove specifications related to the seismic receiver equipment in the tender documentation. Leverage the experience of the seismic contractors and open up opportunities for them to propose alternative solutions. The objective is to get a better understanding of the available options to acquire seismic in the most efficient and cost-effective way.
Small, light autonomous nodes are proven to be highly reliable seismic equipment that can be deployed with ease in all terrains, can free the survey design from spread restrictions and inefficient receiver roll limitations, allowing, in some scenarios, to achieve super dense surveys when the sources are as unconstrained, and in other scenarios, to compensate for the constraints or limitations imposed on source access and density.
Lower cost nodes are also becoming more accessible to small acquisition contractors, who have greater cash flow challenges compared to multi-national companies. Managing cash flow has always been a challenge for land seismic contractors. Using systems that are cheaper to lease, easier to mobilise, require less people and support infrastructure, will be a big benefit to contractor’s cash flow modelling, and will ultimately offer operators a more cost-effective solution. This will also allow operators who mandate surveys to have a wider vendor list and support localisation initiatives, where the use of local providers and labour is given a higher priority.
Don’t rule nodes out from the onset! They will reduce your exploration risks and costs and the ability to deliver higher density, better subsurface imaging.
Concerns over the reliability of getting the desired results from fully autonomous nodes
The need to conduct live QC of the data being harvested
The desire to utilise existing stocks of cheap, depreciated cable systems instead of purchasing or leasing new nodal systems – meaning adoption of new nodal systems are often seen as not economically viable
A mindset of “don’t fix something that’s not broken”
Proud creators of the worlds smallest, lightest and most affordable seismic nodes, we are here to help companies seeking to explore the subsurface in high-definition to:
Lower the costs associated with subsurface exploration
Reduce the environmental impact associated with land seismic surveys
Minimise HSE risk
Increase land seismic efficiency and reduce exploration cycle times
A seismic survey is a geophysical exploration technique that involves the use of sound waves to produce images of the subsurface structure of the earth.
Seismic surveys are a critical component in oil and gas exploration as the subsurface images produced during seismic methods can be used to identify potential reservoirs for drilling.