EM Survey

We used the frequency-domain EM method to measure apparent electrical conductivity at shallow depths along the Mustang Island State Park and Port Aransas transects. Frequency-domain EM methods employ a changing primary magnetic field created around a transmitter coil to induce current to flow in the ground, which in turn creates a secondary magnetic field that is sensed by the receiver coil (Parasnis, 1973; Frischknecht and others, 1991; West and Macnae, 1991). The strength of the secondary field is a complex function of EM frequency and ground conductivity (McNeill, 1980b), but generally increases with ground conductivity at constant frequency.

We used a hand-held Geonics EM38 ground conductivity meter to measure the apparent conductivity of the ground. This instrument operates at a primary frequency of 14.6 kHz, measuring apparent conductivity to a depth of about 0.8 m (horizontal dipole orientation) and 1.5 m (vertical dipole orientation). The instrument has a useful conductivity range of less than 1 mS/m to more than 1,000 mS/m.

We acquired ground conductivity measurements at 234 sites on Mustang Island between December 3 and 5, 2003. For the Mustang Island State Park and Port Aransas transects, we measured apparent conductivity in the horizontal and vertical dipole mode at stations spaced 20 m apart from the gulf beach to the bay shore or its associated tidal flats. We supplemented regularly spaced measurements with additional readings within distinct environments or at boundaries between environments along each transect. We measured the apparent conductivity of the ground at 234 sites

Where the apparent conductivity of the ground was within the instrument's range, we recorded measurements with the instrument on the ground. In areas where apparent conductivity approached or exceeded the upper limit of the instruments range, we made one set of measurements with the instrument on the ground (which in some cases exceeded the range of the instrument) and another set with the instrument at a fixed height of 0.6 m above the ground. We then corrected the out-of-range values by extrapolating the lower apparent conductivities recorded with the instrument at a fixed height according to the empirical relationship observed between the ground-based and fixed-height measurements made over ground having lower apparent conductivities. These corrected values were used for comparison with transect elevation and vegetation surveys.

In the horizontal dipole orientation, we determined an empirical, statistical relationship between ground-level measurements and raised-instrument measurements using 22 data pairs that had apparent conductivities at ground level of less than 1400 mS/m. The relationship,

(sigma g) = 4.03 x (sigma r) - 85.5,

where sigma g is the apparent conductivity at the ground surface and sigma r is the apparent conductivity with the instrument 0.6 m above the ground surface, gives an r squared value of 0.97. We used this relationship to extrapolate a corrected ground-level apparent conductivity in the horizontal dipole orientation from the raised-instrument conductivity where the measured conductivity at ground level exceeded 1400 mS/m, the instrument's maximum linear limit in this orientation.

In the vertical dipole orientation, we determined a similar relationship between ground-level measurements and raised-instrument measurements using 24 data pairs that had apparent conductivities at ground level of less than 1300 mS/m. This relationship,

(sigma g) = 1.89 x (sigma r) + 34.7,

gives an r squared value of 0.95. We used this formula to extrapolate a corrected ground-level apparent conductivity in the vertical dipole orientation from the raised-instrument conductivity where the measured conductivity at ground level exceeded 1300 mS/m, the instrument's maximum linear limit in this orientation.