J.B. Stoll, Mobile Geophysical Technologies GmbH, Celler Str. 13, 29229 Celle, Germany
Summary
This paper describes some potential trends in geophysical exploration. In particular, the increasing depth of deposits and the reduction of cost are driving forces to adapt the geophysical methods in a way that address these requirements.
Particularly the design and performance of rotory wing drones are ideal to carry light weight geophysical sensors and deploy them in terrains which would not allow the access by foot or collect geophysical data with a manned airplane/helicopter.
The use of drones is now accepted in geophysical exploration. There are numerous small enterprises which offer drone-borne geophysical services to the industry, mainly geomagnetics. The development of further geophysical measurement methods, especially electromagnetics, is necessary and will open new application fields for drone geophysics.
Introduction
The efficacy of exploration techniques, teams and strategies has been under fire for several years. The discovery rates paint a picture of a failing system. Exploration is becoming a challenging subject: grades are lower, virgin ground is increasingly scarce or difficult to access, and the industry is progressively making discoveries under deeper cover. These concerns affect greenfields and brownfields exploration equally.
The variety of minerals being used in highly developed countries has increased to a stage that makes use of almost all chemical elements of the periodic table. In many cases the desired commodity or mineralized target has a physical property that permits indirect discovery only by geophysical measurements. Many base metal deposits are discovered by virtue of the physical properties of an associated non- economic mineral. Many minerals needed in the high tech industry elude direct discovery by geophysical methods because geophysical signatures are absent or these are too weak to be resolvable.
Drone borne geophysics becomes feasible through the advent of various key technologies that have reached market maturity in the past 10 years. Drone technology has become more reliable over the past 10 years. Drone regulations in Europe and North America have become harmonized over the years, which makes it easier to apply for a flight permit. Flight operations beyond line of sight (BLOS) are possible and allows to expand the size of the survey area to a scales that make it a very efficient economic proposition.
Drones are flying robots that operate autonomously relying on a worldwide network of GPS satellites and sophisticated onboard navigation technology. In the last decades geophysics has benefited greatly from the advances in electrical engineering, in particular from the introduction of ultra-mobile PC technology, and ever-increasing computational power.
All light-weight geophysical sensors and instruments benefit greatly from the availability of reliable drone technology. High-performance data acquisition systems became available in the early 2000s which are small size and light weight at low power consumption. Fig. 1 displays the time line of important developments of geophysical instruments, methodologies and the increase of computational power and the miniaturization of instruments through advances in micro electronic engineering. Drone geophysics benefits from this trend. Due to limited payload capacity and the limited flight time, drone geophysics relies on the ability to make the sensors lightweight. All rotary wing UAV allow flight times up to 30 min and payloads between 1kg and 10kg. In the future a swarm of drones will enable novel sensor configurations that will yield new resolution enhancement properties through multi-source- multi-receiver combinations.
UAVs have great potential to support exploration tasks in unstructured environments. These are agile, small and lightweight which can incorporate many sensors that are suitable for detecting an object of interest across various terrains. UAVs are perfectly suited for object targeting. However, these are vulnerable when operated alone. A swarm of drones can cover a larger area in a short period through resembling the flocking nature of birds. These drones are connected by an AI-based system that controls these drones and achieve the goal that is assigned to it, e.g. to sweeping an area flying a certain pattern. The drones communicate in the swarm wirelessly creating a Flying Network. For navigation, this network utilizes GPS technology
Future trends in geophysical exploration
Numerous economic reasons exist for the predicament of water scarcity and the need for energy and natural resources. As population grows water demand increases.
While the human population has more than doubled in the last 50 years, there has been a corresponding growth in industrialization and economic development which increases water usage, transformation of water ecosystems, and a huge loss of biodiversity. The problem will intensify by the expected society transfer from poverty into the middle class. Societies in developing countries will consume more resources and energy.
Africa is the world's second fastest-growing region. For the current period until 2022, Africa's real GDP is projected to grow at 3.9% annually (Fig. 2)
It is assumed that the primary production of minerals doubles in the next 25 years. Fig. 3 shows the demand of copper for the next 25 years.
A third trend is a country's demand for specific natural resources. A country's demand and needs depend on its level of development. Figure 4 shows the development of the demand for different regions and commodities.
Discoveries are being made under progressively increasing depth of cover (Schodde, 2020) (Figure 5). The rate of change appears to be accelerating. The tools and techniques used to make discoveries have evolved over time. The role and importance for geophysics varies by scale, location and commodity. Geophysics is the most important tool in deciding where to locate a drill hole.
Exploration success has declined in modern time with fewer quality discoveries and increasing costs on a per unit metal basis (Schodde, 2020). Increasingly the minerals industry is focused on mature districts or areas which are remote, covered or have high political risk. Exploration is becoming increasingly technically challenging and more expensive. Therefore, more effective targeting across a range of scales is essential to increase success rates and potentially reverse or slow the trend of increased discovery costs. The decision to develop a mine integrate a trade-off between the relative inputs of prediction and detection technologies and the concomitant escalation of expenditure with decreasing scale.
Exploration targeting is undertaken over a range of scales, which focuses people, time and money to increase the probability of exploration success. The prospect and project scale of exploration can be expensive and time consuming. At this scale appropriate decisions are needed to continue to advancing the exploration. Geophysics using drones is most suitable to perform geophysical exploration at scales between 100 sqkm down to several hectares at significantly lower cost compared to traditional airborne geophysics or ground geophysics. In addition, drone geophysics fills a gap between airborne and ground geophysics that has been difficult to explore.
Novel Geophysical Instruments transported by drones
Drones open a huge potential in wide area scanning in many fields of geophysical application.
Effectiveness is defined by the capability of producing a desired result or the ability to produce desired output. When something is deemed effective, it means it has an intended outcome. Therefore, there are important criteria that drone geophysics must meet to be more effective against traditional measuring platforms. These are: sample rate, spatial resolution, drape flight capability, air-ground separation, sensor sensitivity and resolution, depth of investigation and productivity rate.
Exploration is becoming increasingly technically challenging and more expensive. Therefore, more effective targeting across a range of scales is essential to increase success rates and potentially reverse or slow the trend of increased discovery costs. The view here is, that drone geophysics is an interesting tool to improve the effectivity of exploration targeting. But it is an essential requirement, that the geophysical methodologies used on drones are designed to achieving the same quality as measurements on ground.
The following developments shall be made aiming to improve the effectiveness of drone geophysics:
Tensor Magnetic Gradiometry
Magnetic surveys form an integral part of exploration programs and are now routinely undertaken before geological mapping programs.
UAV-borne surveys for UXO are capable of providing data for characterizing potential UXO contamination at a site at considerably lower cost than ground-based systems (Stoll et.al, 2020b).
Vector magnetometer surveys, where the direct measurement of vector components has been attempted, have met with mixed success. The accuracy of direct measurement of the field vector is largely governed by orientation errors, which for airborne platforms are so large that the theoretical derivation of the components from the TMI is actually preferable. For this reason it is desirable to measure the field gradient rather than the field vector.
Gradient measurements are relatively insensitive to orientation. This is because gradients arise largely from anomalous sources, and the background gradient is low. Gradient measurements are therefore most appropriate for airborne applications.
Undersampling is common perpendicular to flight lines in airborne surveys, is usual in ground surveys, and always applies in downhole surveys. The next breakthrough in magnetic exploration is likely to be the measurement of the gradient tensor.
Tensor gradiometry will prove useful for aeromagnetic surveys with wide line spacings, environmental surveys, defense applications and unexploded ordnance detection, downhole magnetics, and for marine surveys. Any substantial improvement in this technology will have enormous benefits in terms of new discoveries and lower exploration costs.
The crucial difference between full-tensor gradiometry and total field gradiometry is the production of more detailed and quantitatively interpretable maps and 3D models, rather than scalar anomaly detection. Magnetics is still the cheapest and most widely used geophysical mapping tool in hard rock environments, with increasing importance and potential for further growth in mineral exploration.
In contrast to total-field intensity measurements, gradient data have the advantage that they are less affected by common-mode noise and may carry more useful information about subsurface targets. Multiple tensor components also enable the extraction of a coherent common signal to further increase the signal-to-noise ratio.
The increased data quality opens new avenues for developing novel approaches to UXO detection and discrimination.
Detection and discrimination of buried unexploded ordnance (UXO) with magnetic data is faced with a number of challenges. Among them is the limitation of total-field data imposed by the noise level. Currently available total-field magnetic data are only accurate enough for detecting and modeling UXO targets as dipole sources. Thus we are unable to extract any shape related information for these metallic objects, except in rare cases where high signal-to-noise ratio data are acquired by repeated measurements. This has forced magnetometry in UXO to focus on determining primarily dipole moment.
Acquiring magnetic data with traverse gradient measurements is equivalent to reducing the effective line spacing. Thus, acquiring full tensor gradiometry data at the commonly used line spacing is equivalent to increasing the line density of total field magnetic surveys and it provides a cost-effective way for area coverage as well as for detecting small targets buried at shallow depths.
Detection and discrimination of buried unexploded ordnance (UXO) with magnetic data is faced with a number of challenges. Among them is the limitation of total-field data imposed by the noise level. Currently available total-field magnetic data are only accurate enough for detecting and modeling UXO targets as dipole sources. Thus we are unable to extract any shape related information for these metallic objects, except in rare cases where high signal-to-noise ratio data are acquired by repeated measurements. This has forced magnetometry in UXO to focus on determining primarily dipole moment.
Multicopter-borne Electromagnetics
Airborne Electromagnetic methods (AEM) are an efficient way to map geology and the implementation of innovative technologies for and are widely used in mineral exploration and environmental monitoring. Most EM methods currently used in mineral exploration are of the moving source type; i.e., the primary field source is moved simultaneously and in a fixed configuration with respect to the receiver.
Here we present an alternative configuration of AEM measurements, where a fixed transmitter is established on the ground and the receiver is flown on an unmanned aerial vehicle (UAV). This configuration is called the semiairborne electromagnetic method. The use of unmanned aircraft in airborne electromagnetics is new. Conventional measurement devices were too heavy to be lifted by small UAVs. Great efforts have been made in the last 10 years to make EM instruments light weight enough for use on small sized UAVs.
Here, we describe an octocopter based semi-airborne electromagnetic data acquisition system and show the results of a field test in Northern Germany (Stoll et.al., 2020a). This test affirmed that the UAV enabled semi-airborne electromagnetic method very well suited to conduct geophysical surveys on prospect scale.