Survey

From Previous Work

Previous work on multiphysics velocity modeling and prestack depth migration on pilot 2D lines (Colombo et al., 2012) suggested the presence of deep roots below the wadi where the velocity anomalies associated with the wadi extend for several hundred meters below the surface. These considerations guided the definition of parameters for the HTEM system to enable extended depth penetration without compromising the near-surface resolution (Table 1).

Following encouraging results of 2D multiphysics and joint-inversion studies conducted in the area (Colombo et al., 2012), a comprehensive non-seismic 3D acquisition program was completed in December 2014 consisting of precision gravity on a 200 × 200 m grid, helicopter-borne TEM, and audio-magnetotellurics (AMT) soundings, acquired to extend the depth resolution of the helicopter TEM data in the wadi section.

Helicopter-borne TEM

Among the different non-seismic data sets acquired in the study area, the HTEM (Chen et al., 2014) is the most promising technique for high-resolution characterization of the near surface at an effective cost/benefit ratio. HTEM data were acquired over the survey area with north-south fly lines spaced 100 m apart. The acquisition system was designed to provide adequate depth of penetration by ensuring large transmitted current and dipole moment with suitable recording time (Table 1). The low flight altitude of nominally 30 m and the relatively low flying speed ensured collection of a rich data set corresponding to an output-stacked TEM sounding acquired every 2.7 m along the flight lines for a total of about 1.6 million TEM soundings for the surveyed area. The availability of ground-based measurements from a former 2D multiphysics pilot test (Colombo et al., 2012) enabled comparison of the ground-based with the corresponding helicopter-borne TEM soundings.

Recording time windows	15 ms
Transmitter current	1200 A
Dipole moment	1.7:math:times 10^6; Am^2
Stacked TEM curve spacing	~ 2/7 m (inline)
Transmitter Waveform	4 ms, 30 Hz, half-sine
Fly-Line Spacing	100 m
Total Soundings	~ 1.6 million

Table 1: Helicopter-borne TEM (HTEM) acquisition parameters.

Current HTEM systems can be equipped with dual-moment transmitters allowing coverage of the early times together with the deep time responses to further enhance resolution of very shallow features. The combination of frequency-domain and time-domain helicopter-borne acquisition allows further expansion of the sensitivity to provide even broader bandwidth of electromagnetic responses for virtually no limitations in the resolution by which the near surface can be characterized.