February 2023 – Monthly Talk

Speaker: Dr. Mengli Zhang, Research Associate,  Center for Gravity, Electrical, and Magnetic Studies (CGEM), Colorado School of Mines

Title: Efficient geophysical data acquisition using ergodic sampling: Non-linear relationship between information sampling ability (ISA) and number of samples

Date: Thursday February 23, 2022

Time: 4:30pm to 5:30pm PST

Location: Virtual. Zoom link will be distributed via our newsletter in advance of the talk. Contact info@bcgsonline.org if you would like to attend but did not receive the newsletter with link (sent February 21, 2023).


Geophysicists use difference tools such as data display, modeling, and inversion to image subsurface of the earth. The denser the data are, the more details of earth model we can obtain. The price we pay for denser data is of course the higher cost for acquisition, especially for 3D data. We may default to an implicit assumption that the resolution of our earth model is linearly dependent upon the number of samples we can collect for geophysical data. This assumption may be rooted in Nyquist sampling theory. However, Nyquist sampling theory is a sufficient but not necessary condition. We have re-examined the necessity of such dense sampling in geophysical data acquisition and developed an ergodic sampling method and shows that the number of samples has a non-linear relationship with the information sampling ability (ISA). In contrast to Nyquist sampling, which requires a sufficient but larger than necessary sample set, ergodic sampling only acquires the core subset of samples that is both necessary and sufficient to gather the same information. Therefore, ergodic sampling can significantly decrease the number of samples compared with Nyquist sampling. We present our new sampling theory and demonstrate its application in the geophysical data acquisition. Our simulation and field data example show that the cost can be reduced by a factor up to 10. Equivalently, this result also means that it is possible to acquire 10 times more information when the same number of samples used in the traditional equi-spaced sampling is deployed using the ergodic sampling strategy.

Dr. Mengli Zhang is a Research Associate in the Department of Geophysics at Colorado School of Mines. She is a geophysicist specialized in optimization of the exploration cycle from acquisition, interpretation, to discovery by incorporating economic factors. She is also an expert on efficient and economical multi-geophysical data acquisition using ergodic sampling theory. She obtained her BS in Information Engineering and MS degree in Information and Communication Systems from Xi’an Jiaotong University in China. She earned an MS degree in Geoscience from the University of Texas at Dallas, USA, and PhD degree in Geophysics from Colorado School of Mines. She has 10 years of industry experiences, first as a research geophysicist and then as a project manager and as Chief Geophysicist in the eastern Ordos Basin for China National Petroleum Corporation, where she applied information analyses to increase gas reservoir discoveries. She worked closely with geologists to select locations of more than 500 drilled wells, perform post-drilling analyses throughout the life cycle of wells including the production stage, and to improve interpretation and targeting methodology based on drilling and production results. Her current research has applications to the information-based economic geophysical data acquisition, which has the potential to significantly reduce the cost of exploration for energy and metals and to accelerate discoveries.

BCGS 2014 Fall Symposium: Hudbay Lalor Case Study – Registration is Now Open!

The BC Geophysical Society is pleased to announce that registration is officially open for the 2014 Fall Symposium.

Exploration for Deep VMS Ore Bodies: The Hudbay Lalor Case Study

When: October 16 and 17, 2014

Where: BCIT Downtown Campus, Rooms 280, 282, 284, Vancouver, BC,

Cost: Industry $150 / Students $50

Please register in advance by completing and submitting the registration form below. Adobe Reader is required to electronically submit the form by email.

Registration Form BCGS 2014 Symposium_distributed

Payment will be accepted through PayPal. Click on the ‘Buy Now’ button below.

Registration Options:

BCGS Fall Symposium: Oct 16 & 17, 2014

The BC Geophysical Society, in cooperation with Hudbay, is proud to present a two-day symposium on the discovery of the Lalor Deposit. The amount and variety of geophysical work undertaken, coupled with the geological understanding and insight provided by the owner of this prominent deposit, is sure to make this an extremely interesting session.

Additional details including the full speaker schedule will be available this summer. Registration is scheduled to open August 2014.

Exploration for Deep VMS Ore Bodies: The Hudbay Lalor Case Study

When: October 16 and 17, 2014

Where: BCIT Downtown Campus, Vancouver, BC

Cost: Industry $150 / Students $50

Please see the following link to the symposium abstract for additional details.

BCGS Symposium – Deep VMS Exploration- Lalor Case Study



April Technical Talk

BCGS April Technical Talk – Thursday April 17th, 2014

Speaker: Peter Fullagar, Fullagar Geophysics

Title: 3D magnetic modelling and inversion incorporating self-demagnetisation and interactions

Date/Time: Thursday April 17th, 2014 at 4:30pm.

Location: Room 451, 409 Granville St (UK Building at Granville and Hastings), Vancouver


Self-demagnetisation can significantly reduce the amplitude and modify the shape of the response from highly magnetic bodies. The direction of magnetisation rotates in a manner determined by the shape and orientation of the body. Furthermore, when highly magnetic bodies are in close proximity, the magnetisation induced in one body is affected by the magnetisations in all the others. When modelling highly magnetised bodies, it is important to take both self-demagnetisation and interactions into account. Inverting for magnetisation vector has become popular recently. However, this is not a substitute for physically valid magnetic modelling. Magnetisation inversion is highly nonunique, with the result that any particular solution must be interpreted with care. An inverted magnetisation rotated from the ambient field is not necessarily indicative of remanence. Moreover, relating an inverted in situ magnetisation to rock properties is often problematic. In this respect, magnetisation inversion complicates ground truthing. Potential field modelling and inversion software “VPmg” has been upgraded to account for self-demagnetisation within, and interaction between, 3D magnetic bodies. Remanence can be taken into account. The algorithm computes H-field perturbations at the model cell centres in two stages: initialisation and optimisation. During initialisation, a first estimate for the H-field perturbation is derived from the demagnetisation tensor computed for each cell.

During optimisation, the H-field perturbation is refined iteratively via an inversion procedure. The algorithm has been validated for homogeneous spheres, spheroids, slabs, and cylinders. It has also reproduced magnetic interactions between two horizontal cylinders, published by Hjelt (1973). Explicit verification for complex heterogeneous bodies requires a suitable independent algorithm for benchmarking. The application to inversion in highly magnetic environments is illustrated on field data examples.

March Technical Talk

BCGS March Technical Talk – Tuesday March 18th, 2014

Speaker: Sarah Devriese, PhD Student, UBC

Title: Using Electromagnetic Methods to Image SAGD Steam Chambers

Date/Time: Tuesday March 18th, 2014 at 4:30pm.

Location: Room 451, 409 Granville St (UK Building at Granville and Hastings), Vancouver


Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery method, used to produce bitumen from oil sands. The success of this technique is dependent upon the propagation of steam throughout the bitumen layer. However, heterogeneity in the reservoir rock can cause the steam to not always propagate as desired. Monitoring the location and growth of steam chambers provides vital information about the production efforts and has traditionally been done using 4D seismic. While successful in many cases, it may be limited by low sensitivity to changes in fluid content, saturation, and porosity. Alternatively, electrical conductivity is significantly affected by the injection of steam into a bitumen layer. Thus electromagnetic (EM) surveys have greater potential in imaging the steam floods while providing higher resolution compared to traditional 2D DC resistivity surveys. The design of an EM survey for monitoring SAGD is crucial in achieving the highest possible resolution at a minimal cost. This talk will introduce a practical methodology for survey design that addresses location and geometry of electromagnetic transmitters. The technique is applied to a synthetic model that emulates the Athabasca oil sands environment and the results are compared to inversions of 2D ERT data. Two-dimensional inversions of the ERT data produce a low quality image with artifacts but the image is improved by carrying out a 3D inversion. However, a superior image is obtained by working with a modest number of transmitters and inverting multiple-frequency EM data in 3D. The results provide optimism for the potential of using 3D EM for imaging subsurface steam chambers.

POST-TALK: Attendees are encouraged to grab refreshments together at Moose’s Downunder afterwards