Controlled-source electromagnetic imaging offers a new remote-sensing capability.

Every once in a while, a new technology emerges that dramatically improves our ability to remotely sense the world around us. A familiar example of this type of technological breakthrough in the medical field is Magnetic Resonance Imaging or MRI. Unlike X-ray imaging, which primarily illuminates the contrast between soft tissues and denser structures such as bones, MRI allows soft tissues to be imaged and differentiated from one another, thereby enhancing our ability to detect abnormalities within those tissues. Although X-ray methods such as Computerized Axial Tomography (CAT) scans can provide images with excellent spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable spatial resolution with far superior contrast resolution (the ability to distinguish differences between two arbitrarily similar but non-identical tissue types). This difference in the ability of these two technologies to image and distinguish soft tissues is due to the different physical properties they measure - spin relaxation properties of hydrogen nuclei in the case of MRI and ionizing radiation absorption in the case of X-rays. Because of these differences, MRI has revolutionized medical diagnosis of tumors and other abnormalities by enhancing our ability to detect physiological alterations in living tissue without resorting to exploratory surgery.
In offshore petroleum exploration, Controlled-Source Electromagnetic Imaging (CSEMI) is an example of this type of breakthrough remote sensing technology. Originally developed several decades ago by academic researchers to study hydrothermal and volcanic systems on mid-ocean ridges, the application of the CSEMI technique to petroleum exploration is quite recent, with the first full-scale commercial application of this technology for hydrocarbon detection and delineation carried out in 2000 offshore West Africa. The results of this and other early surveys clearly demonstrated that CSEMI was a powerful new geophysical technology capable of dramatically improving our ability to detect the presence of hydrocarbons in geologic structures beneath the seafloor.
A different measurement
As noted above, MRI makes a fundamentally different type of measurement than X-rays, and in a similar manner CSEMI makes a fundamentally different type of measurement than today's leading geophysical imaging tool - seismic imaging. Whereas seismic technology measures contrasts related to differences in the acoustic properties of geologic structures beneath the earth's surface, CSEMI measures electromagnetic field properties associated with resistivity variations in subsurface formations.
Just as X-rays provide an excellent representation of the skeletal structure of the human body, seismic images provide unrivaled structural representations of subsurface geologic features such as folds, faults and thrusts, as well as stratigraphic sequence boundaries. However, of fundamental importance in petroleum exploration are the fluid properties within these geologic formations, properties that are far harder to constrain with current seismic techniques. Although some techniques can potentially yield information about the fluid properties in subsurface layers, the large impact of even small quantities of free gas on the measured seismic amplitude response has resulted in a number of false positives associated with non-commercial hydrocarbon deposits (e.g., fizz gas). Ideally, we would like to employ a remote sensing technique that allows us to distinguish and delineate commercially viable hydrocarbon accumulations. This is where CSEMI comes in.
Methodology
CSEMI involves transmitting a controlled electromagnetic (EM) signal in close proximity to the seafloor and recording the resultant EM field response with stationary receivers deployed on the ocean bottom (Figure 1). The rate of attenuation of the diffusive EM signals is related to the electrical resistivity of the media through which they travel. Energy transmitted through the conductive water column is therefore rapidly attenuated, and the signal measured by the receivers at ranges of greater than half a mile to a mile is dominated by energy which has propagated through the more resistive earth. The usable range of transmission frequencies is also partly determined by attenuation effects: high-frequency signals are rapidly attenuated even in the more resistive earth, while low frequencies begin to be dominated by environmental noise which has not been screened by the attenuative effects of the water. Consequently, the transmitted signals used for CSEMI are typically focused in a frequency band between 0.01 and 10 Hz. Signals at these frequencies can penetrate to depths of several kilometers beneath the seafloor and are modified as the result of resistivity variations in the subsurface. Not surprisingly, a large component of the research and development activity within the CSEM industry has been focused on developing EM transmitters and receivers (Figures 2 and 3) that allow high-fidelity phase and amplitude information to be recorded despite the highly dispersive nature of EM signal propagation.
The electrical resistivity of the earth is dominated by fluid phases - seawater, formation water and hydrocarbons - within subsurface formations. As such, CSEMI is ideally suited to studying the electrical properties of fluid-dominated geological systems such as hydrocarbon reservoirs. Because hydrocarbon-saturated formations are typically more resistive than the surrounding water-saturated media, this resistivity contrast can be exploited to detect both the presence and distribution of hydrocarbons in the target reservoir. By carefully studying both the phase and amplitude characteristics of the measured EM field response at the seafloor, it is possible to infer the subsurface resistivity structure that gave rise to that response. A number of advanced data analysis, processing and interpretation techniques are brought to bear in order to develop these images of the earth's subsurface resistivity structure, with the most notable being migration-like imaging techniques and CSEMI inversion. When rendered in depth, these CSEMI inversion results provide unique insight into the physical properties of potential reservoir features (Figure 4).
One particularly powerful aspect of the CSEMI technique lies in the relationship between the size of the measured EM response and the physical property that has the greatest influence on that response - transverse resistance (sometimes referred to as the resistivity-thickness product). The magnitude of the measured electromagnetic response generally increases in the presence of a resistive anomaly, and this property can be exploited to accurately determine the lateral extent of a resistive feature. Once the lateral extent of the anomaly has been established, the magnitude of the EM response at each spatial location is proportional to both the thickness and resistivity of the target reservoir at that location. Thus as reservoir size (in terms of either formation thickness) or hydrocarbon saturation increases, so does the measured EM field response.
By careful modeling and interpretation of the recorded fields, prospective anomalies can therefore be ranked, allowing differentiation between (1)
a larger reservoir thickness or greater hydrocarbon saturation (or both) allowing "sweet spots" within a reservoir to be targeted, and (2) very small reservoirs or large reservoirs with
very low hydrocarbon saturation (e.g., fizz gas).
Industry acceptance
Recent advances in CSEMI have extended this technique, which was originally developed for deepwater applications, into regions with water depths as shallow as 320 ft (100 m). In addition, the use of innovative survey design, data acquisition and data processing methodologies has allowed CSEMI to be successfully applied in relatively complex settings involving multiple resistive anomalies, not all of which are associated with hydrocarbon reservoirs. As a direct result of these and other developments, the adoption curve for CSEMI is on the rise, with this technique being increasingly used by oil companies around the world for a wide range of applications. Projects range from simple yes/no surveys designed to confirm or deny the presence of commercial quantities of hydrocarbons to more complex surveys aimed at mapping the detailed spatial distribution of hydrocarbons, and they span the full range of activities from frontier exploration to detailed reservoir appraisal.
As with any new technology, there are potential pitfalls for the unwary. As previously noted, a number of other conditions can result in significant subsurface resistivity contrasts - for example, carbonates, volcanic intrusions and salt bodies generally exhibit significant resistivity contrasts relative to their surrounding geology. Careful survey design, both in terms of receiver position and transmission frequencies, can help to discriminate between signals generated by a thin resistive layer (such as a reservoir) and a more massive resistor (such as a salt body). Likewise, advanced modeling and interpretation techniques allow clear differentiation between anomalies associated with shallow hydrate layers and those associated with deeper hydrocarbon targets, for example. Nonetheless, in areas where the regional geology is relatively unknown, the presence of a resistive anomaly is not definitive evidence for the presence of hydrocarbons. Yet even in the case of frontier exploration, the EM response is an independent measurement of the earth's subsurface properties that can be used to augment structural information derived from seismic images and thereby enhance the understanding of the geologic setting.
Although CSEMI has not yet reached the level of familiarity of MRI, it is increasingly being recognized as a potent tool for reducing the risk of drilling non-commercial wells, particularly when combined with seismic imaging or other complementary technologies. As this awareness grows, there is little doubt that this game-changing technology will play a vital role in the evolving petroleum exploration paradigm of the 21st century.