Schematic layout of a PGS Multitransient EM onshore survey. (Images courtesy of PGS)

Electromagnetic (EM) methods offer the ability to detect resistive targets that may correspond to hydrocarbons, and they are increasingly being used for exploration and delineation of discoveries. The geophysical study of EM in the shallow earth can identify the spatial location of resistive zones. Hydrocarbon-bearing rocks exhibit increased resistivity relative to surrounding water-bearing rocks, and thus detection of highly resistive zones may indicate the presence of hydrocarbons.

In that context, resistivity profiling has the potential to discriminate between commercial and non-commercial hydrocarbon saturation within a reservoir. Of course, not all resistors are hydrocarbons. Salt, impermeable carbonates, coals, igneous intrusions and volcanic layers are all significant resistors. So interpretation of the apparent resistivity profiles should be constrained by all other available data.

The PGS Multitransient EM (MTEM) method can be considered as a class of Controlled Source EM (CSEM); however, the implementation is notably different from, and overcomes the limitations of, frequency domain methods, resulting in a massively increased signal-to-noise ratio over other time domain techniques. EM exploration for hydrocarbons is usually conducted with a source current injected across dipole electrodes. In a simple step transient mode, when the primary source is switched off only the secondary field remains, from which deductions can be made about the conductivity of the subsurface. In a frequency domain CSEM system, the secondary field (of same frequency as the source) is measured in the presence of the primary, which limits the detection of the secondary signal.

MTEM, on the other hand, uses a transient signal. The source waveform used is a pseudo-random binary sequence (PRBS) that has a frequency range from close to zero frequency up to the Nyquist frequency of the receiver unit. The input current is measured and a patented deconvolution method used to yield the earth’s impulse response. The PRBS source enables the frequency content to be tuned to the depth of interest: a shallow target would employ a higher frequency sequence, while deeper targets use lower frequencies and longer source receiver offsets.

The layout of an onshore MTEM survey resembles that of a 2-D seismic reflection survey (Figure 1). In the onshore EM case, the response includes an airwave that travels at about the speed of light, arriving at the receivers almost instantaneously. After deconvolution the airwave appears as a sharp impulse that precedes the earth’s impulse response. The airwave and impulse response are therefore separable in time, and onshore MTEM can be considered to have no airwave problem.

For offshore surveying, MTEM uses mature, reliable ocean bottom cable (OBC) technology in water depths ranging from 30 to 1,600+ ft (10 to 500+ m). The OBC is deployed in a linear configuration on the seabed, where it remains stationary. Thus, the MTEM approach automatically measures the inline component of the electric field; has much denser (inline) receiver spatial sampling than sparsely deployed CSEM nodes; and is operationally much faster to deploy, relocate and retrieve than CSEM nodes.

METM typically uses a PRBS source function to efficiently inject a broad frequency spectrum into the ground (Figure 2). The frequency range of the source function is modified for each source-receiver offset, so unnecessarily high frequencies are not wasted. Horizontal resolution is half the receiver interval (like reflection seismic), and existing resistivity inversion methods can resolve the top of a resistor to about 10% of the depth. However, MTEM generally resolves the transverse resistance of a reservoir — the product of thickness and resistivity. If seismic data can constrain the target thickness, or well logs can constrain the resistivity, then the other quantity is much better determined by MTEM.

CSEM surveys derive a resistivity profile of the earth by frequency domain inversion or forward modeling of the amplitudes and phase derived at each receiver location. In contrast to MTEM surveys, CSEM surveys cannot recover the earth’s impulse response because of the limited source frequencies available. At least 32 frequencies would be required to recover the impulse response from a CSEM survey, which operationally would require repeatedly surveying the same line with a different switching frequency and exceptional source-receiver geometry repeatability.

Case studies

PGS acquired 13 miles (21.6 km) of MTEM data in water depths of 443 ft (135 m). Survey duration was less than five days, with the delivery of data products about a week later. Observe in Figure 3 that seismic amplitudes at the target level of 4,708 ft (1,435 m) increase to the left of the figure, away from an existing dry well. In contrast, a strong resistivity anomaly increases in amplitude in the opposite direction, towards the well location.

In another example, MTEM acquired 9 miles (14.8 km) of MTEM data in water depth of 330 ft (100 m). Survey duration was less than three days, with the delivery of data products about a week later. A known producing interval occurs at 5,577 to 6,234 ft (1,700 to 1,900 m) depth. Observe in Figure 4 that the MTEM amplitude anomaly correlates with the producing well location and is absent below the dry well location. Additional 2-D surveying and 3-D processing and inversion is recommended to properly delineate the MTEM anomaly.

Both North Sea case examples above (acquired in late 2007) verify the resolution and quality of MTEM data in very shallow water.

Summary

MTEM is a proven remote sensing method for hydrocarbon detection and delineation. Profiles are rapidly derived to describe the resistivity of the earth and can be spatially correlated with the structural information provided by seismic data. Hydrocarbon fluids and gas can thus be discriminated in terms of location and saturation — prior to any drilling decisions. MTEM is equally applicable to land, transition zone and marine, providing greater penetration depth and higher resolution that other EM methods. The methodology is intuitively analogous to the seismic method, recording offset and depth-variant resistivity information from the earth. An extensive suite of applications includes exploration, exploitation and production, and 4-D production monitoring.