Nestor Cuevas

Natural field induced polarization for mapping of deep mineral deposits: A field example from Arizona

A field example of the natural field induced polarization (NFIP) method over a known deep induced polarization (IP) target in Arizona corroborates NFIPs excellent potential as a commercial survey technique for the mineral industry. Data processing and interpretation were successful in extracting the response of the IP target from the natural field signals in the presence of a conductive surface layer. The location of the interpreted IP target correlated well with interpretations from an earlier IP/resistivity dipole-dipole survey. The imaginary component of the ratio of electric field measured along the profile normalized by the electric field at a reference site becomes nonzero over the position of an IP target. The complex ratio of the profiling dipole electric field and fixed-reference-dipole electric field, plotted as a function of frequency and dipole location, is a simple and robust procedure for recovering the IP effect. The NFIP method eliminates the expense and energy requirements of deploying large transmitters and is sensitive to deeper structures (structures that are difficult to resolve using traditional methods). Thus, this method is a valuable mapping tool for identification of IP targets.

Comparison of Sensitivity And Resolution With Two Marine CSEM Exploration Methods

This paper compares the sensitivity to, and resolution of the properties of a resistive target using marine controlled source electromagnetic measurements, with the frequency domain horizontal source-receiver method and the recently introduced vertical source-receiver time domain configuration. The problem is addressed from an analytical stand point, i.e. by analyzing closed form solutions of the 1D spatial and spectral distribution of the fields, and numerically, from 1D inversion of synthetic datasets as well as from 2D simulations of the response of finite lateral extent reservoirs. The 1D analysis demonstrates that the far offset measurement of the standard CSEM has more sensitivity to the presence of the resistive layer than the vertical source-receiver time domain measurement done at close offsets from the source. Closed form solutions derived for the guided mode of the fields yields increasing sensitivity of the standard CSEM configuration for thin resistors and increasing offsets from the source. The image term solution for the fields observed in the near offset vertical source-receiver configuration yields increasing sensitivity with decreasing frequency, i.e. towards the late times of the measurement. For the simplified single layer model a threshold offset is establish beyond which the standard CSEM method is more sensitive. However for a more realistic setting of a finite extent 2D reservoir this report shows that the guided mode driving the far offset sensitivity is only dominant for wide enough targets. The vertical source receiver is more sensitive to smaller targets, where the guided mode does not develop, and it has better resolution to the lateral extent of the reservoirs.

Electrokinetic coupling in hydraulic fracture propagation

Electrokinetic coupling is the most popular mechanism proposed to explain observed electromagnetic (EM) signals associated with the hydraulic fracturing of rocks. Measurements in both, laboratory and in situ conditions show evidence of the phenomenon, however, as far as the authors know, there have been no reports on the description of the source mechanism, its relationship to a propagating crack, nor the electromagnetic field distribution due to such a source advancing through a conductive medium. In this paper it is shown that an electric streaming current density arising on the walls of a fluid driven crack gives rise to the EM fields observed in measurements of streaming potential in hydraulic fracturing experiments. A source function for the current density is established from the fluid pressure profile inside the propagating. Expressions for the EM fields due to such a source are derived for a crack propagating with a constant velocity, in a homogeneous isotropic conducting medium. The spatial and temporal behavior of the fields reasonably agree with measurements performed in laboratory experiments. In situ measurements are qualitatively described only, however it is shown that the magnitude of the fields and their temporal behavior can be well reproduced in a realistic hydraulic fracturing setting.

Near source response of a resistive layer to vertical electric dipole excitation

SUMMARY Analytical expressions are derived for the electromagnetic (EM) fields observed in the vicinity of a vertical dipole source, due to the response of resistive layer embedded in a conductive background. The analytical expressions for the fields observed at zero offset are validated by comparison to the numerical integration of the Bessel-Fourier integral. It is found that the structure of the fields is remarkably different compared to that observed at far offsets, which is described by the residue of the Bessel-Fourier integral at the position of the so called resistive layer pole. Indeed the cylindrical wave front behavior of the far offset fields does not apply anymore at close distances from the source. The derived expressions show that at near offsets it is not possible to identify a preferential radial propagation direction, where furthermore the energy flux appears to change direction with offset. The spectral and spatial distribution of the fields are very similar to that of the image of the primary source, this suggests that the response of the layer can be replaced by a another superimposing image component.

Response induction coil magnetometers to perturbations in orientation

Author(s): Kappler, K; Cuevas, N; Rector, JW | Abstract:

Magnetotelluric imaging of the Kilauea Volcano, Hawaii

A collaborative effort has been undertaken between Lawrence Berkeley National Laboratory, Sandia National Laboratories, the USGS Hawaiian Volcano Observatory, and Electromagnetic Instruments to study the Kilauea volcano in Hawaii using the magnetotelluric (MT) technique. We present results from the first phase of data acquisition, recorded in August 2002, where 33 MT sites over the southwest and east rift zones were acquired. Good to excellent quality data were obtained even in the harshest conditions, such as those encountered on the fresh lava flows of the eastern rift zone, where electrical contact resistances are extremely high. Electrical conductivity anomalies resulting from two dimensional inversion of sites, sorted into profiles, had a high degree of spatial correlation with previously published seismic velocity anomalies observed around the Kilauea caldera. In particular we were able to map the active magma in the East Rift Zone (ERZ) beneath currently active vents, as well as the upper portions of the magma conduit to the mantle. Active fault zones imaged by velocity tomography as low velocity zones tie to low resistivity zones on inverted MT lines. We have also undertaken a three dimensional (3D) MT model study of the volcano, island and ocean, to better access the role these features play in the observed MT measurements. Results from 3D model studies lend confidence to the data sensitivity of the magmatic structure beneath the volcano, and our ability to properly model island topography and coastal effects